darwin-origin.txt

ON THE ORIGIN OF SPECIES.

INTRODUCTION.

When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South America,
and in the geological relations of the present to the past inhabitants
of that continent. These facts seemed to me to throw some light on the
origin of species--that mystery of mysteries, as it has been called by
one of our greatest philosophers. On my return home, it occurred to me,
in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years' work I allowed myself
to speculate on the subject, and drew up some short notes; these I
enlarged in 1844 into a sketch of the conclusions, which then seemed to
me probable: from that period to the present day I have steadily pursued
the same object. I hope that I may be excused for entering on these
personal details, as I give them to show that I have not been hasty in
coming to a decision.

My work is now nearly finished; but as it will take me two or three more
years to complete it, and as my health is far from strong, I have been
urged to publish this Abstract. I have more especially been induced to
do this, as Mr. Wallace, who is now studying the natural history of
the Malay archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. Last year he sent to
me a memoir on this subject, with a request that I would forward it
to Sir Charles Lyell, who sent it to the Linnean Society, and it is
published in the third volume of the Journal of that Society. Sir C.
Lyell and Dr. Hooker, who both knew of my work--the latter having read
my sketch of 1844--honoured me by thinking it advisable to publish, with
Mr. Wallace's excellent memoir, some brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors will have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only
the general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my conclusions have been
grounded; and I hope in a future work to do this. For I am well aware
that scarcely a single point is discussed in this volume on which facts
cannot be adduced, often apparently leading to conclusions directly
opposite to those at which I have arrived. A fair result can be obtained
only by fully stating and balancing the facts and arguments on both
sides of each question; and this cannot possibly be here done.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who for the last fifteen years has aided me
in every possible way by his large stores of knowledge and his excellent
judgment.

In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings,
on their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that each species had not been independently created, but
had descended, like varieties, from other species. Nevertheless, such
a conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world
have been modified, so as to acquire that perfection of structure and
coadaptation which most justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one very limited sense,
as we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance,
of the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
misseltoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally preposterous
to account for the structure of this parasite, with its relations to
several distinct organic beings, by the effects of external conditions,
or of habit, or of the volition of the plant itself.

The author of the 'Vestiges of Creation' would, I presume, say that,
after a certain unknown number of generations, some bird had given birth
to a woodpecker, and some plant to the misseltoe, and that these had
been produced perfect as we now see them; but this assumption seems to
me to be no explanation, for it leaves the case of the coadaptations of
organic beings to each other and to their physical conditions of life,
untouched and unexplained.

It is, therefore, of the highest importance to gain a clear insight into
the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been disappointed;
in this and in all other perplexing cases I have invariably found that
our knowledge, imperfect though it be, of variation under domestication,
afforded the best and safest clue. I may venture to express my
conviction of the high value of such studies, although they have been
very commonly neglected by naturalists.

From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible, and, what
is equally or more important, we shall see how great is the power of man
in accumulating by his Selection successive slight variations. I will
then pass on to the variability of species in a state of nature; but
I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues of
facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the Struggle
for Existence amongst all organic beings throughout the world, which
inevitably follows from their high geometrical powers of increase, will
be treated of. This is the doctrine of Malthus, applied to the whole
animal and vegetable kingdoms. As many more individuals of each species
are born than can possibly survive; and as, consequently, there is a
frequently recurring struggle for existence, it follows that any being,
if it vary however slightly in any manner profitable to itself, under
the complex and sometimes varying conditions of life, will have a better
chance of surviving, and thus be NATURALLY SELECTED. From the strong
principle of inheritance, any selected variety will tend to propagate
its new and modified form.

This fundamental subject of Natural Selection will be treated at
some length in the fourth chapter; and we shall then see how Natural
Selection almost inevitably causes much Extinction of the less improved
forms of life and induces what I have called Divergence of Character.
In the next chapter I shall discuss the complex and little known laws of
variation and of correlation of growth. In the four succeeding chapters,
the most apparent and gravest difficulties on the theory will be given:
namely, first, the difficulties of transitions, or in understanding how
a simple being or a simple organ can be changed and perfected into a
highly developed being or elaborately constructed organ; secondly
the subject of Instinct, or the mental powers of animals, thirdly,
Hybridism, or the infertility of species and the fertility of varieties
when intercrossed; and fourthly, the imperfection of the Geological
Record. In the next chapter I shall consider the geological succession
of organic beings throughout time; in the eleventh and twelfth, their
geographical distribution throughout space; in the thirteenth, their
classification or mutual affinities, both when mature and in an
embryonic condition. In the last chapter I shall give a brief
recapitulation of the whole work, and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due allowance
for our profound ignorance in regard to the mutual relations of all
the beings which live around us. Who can explain why one species ranges
widely and is very numerous, and why another allied species has a narrow
range and is rare? Yet these relations are of the highest importance,
for they determine the present welfare, and, as I believe, the future
success and modification of every inhabitant of this world. Still less
do we know of the mutual relations of the innumerable inhabitants of the
world during the many past geological epochs in its history. Although
much remains obscure, and will long remain obscure, I can entertain no
doubt, after the most deliberate study and dispassionate judgment of
which I am capable, that the view which most naturalists entertain,
and which I formerly entertained--namely, that each species has been
independently created--is erroneous. I am fully convinced that species
are not immutable; but that those belonging to what are called the
same genera are lineal descendants of some other and generally extinct
species, in the same manner as the acknowledged varieties of any one
species are the descendants of that species. Furthermore, I am convinced
that Natural Selection has been the main but not exclusive means of
modification.




1. VARIATION UNDER DOMESTICATION.

Causes of Variability. Effects of Habit. Correlation of Growth.
Inheritance. Character of Domestic Varieties. Difficulty of
distinguishing between Varieties and Species. Origin of Domestic
Varieties from one or more Species. Domestic Pigeons, their Differences
and Origin. Principle of Selection anciently followed, its Effects.
Methodical and Unconscious Selection. Unknown Origin of our Domestic
Productions. Circumstances favourable to Man's power of Selection.

When we look to the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us, is, that they generally differ much more from each other,
than do the individuals of any one species or variety in a state of
nature. When we reflect on the vast diversity of the plants and animals
which have been cultivated, and which have varied during all ages under
the most different climates and treatment, I think we are driven to
conclude that this greater variability is simply due to our domestic
productions having been raised under conditions of life not so uniform
as, and somewhat different from, those to which the parent-species have
been exposed under nature. There is, also, I think, some probability
in the view propounded by Andrew Knight, that this variability may be
partly connected with excess of food. It seems pretty clear that organic
beings must be exposed during several generations to the new conditions
of life to cause any appreciable amount of variation; and that when the
organisation has once begun to vary, it generally continues to vary for
many generations. No case is on record of a variable being ceasing to be
variable under cultivation. Our oldest cultivated plants, such as wheat,
still often yield new varieties: our oldest domesticated animals are
still capable of rapid improvement or modification.

It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by
any clear line of distinction from mere variations. But I am strongly
inclined to suspect that the most frequent cause of variability may
be attributed to the male and female reproductive elements having been
affected prior to the act of conception. Several reasons make me believe
in this; but the chief one is the remarkable effect which confinement or
cultivation has on the functions of the reproductive system; this
system appearing to be far more susceptible than any other part of the
organisation, to the action of any change in the conditions of life.
Nothing is more easy than to tame an animal, and few things more
difficult than to get it to breed freely under confinement, even in the
many cases when the male and female unite. How many animals there
are which will not breed, though living long under not very close
confinement in their native country! This is generally attributed to
vitiated instincts; but how many cultivated plants display the utmost
vigour, and yet rarely or never seed! In some few such cases it has
been found out that very trifling changes, such as a little more or less
water at some particular period of growth, will determine whether or not
the plant sets a seed. I cannot here enter on the copious details which
I have collected on this curious subject; but to show how singular the
laws are which determine the reproduction of animals under confinement,
I may just mention that carnivorous animals, even from the tropics,
breed in this country pretty freely under confinement, with the
exception of the plantigrades or bear family; whereas, carnivorous
birds, with the rarest exceptions, hardly ever lay fertile eggs. Many
exotic plants have pollen utterly worthless, in the same exact
condition as in the most sterile hybrids. When, on the one hand, we
see domesticated animals and plants, though often weak and sickly, yet
breeding quite freely under confinement; and when, on the other hand,
we see individuals, though taken young from a state of nature,
perfectly tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail in acting, we need not be surprised
at this system, when it does act under confinement, acting not quite
regularly, and producing offspring not perfectly like their parents or
variable.

Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility; and
variability is the source of all the choicest productions of the garden.
I may add, that as some organisms will breed most freely under the
most unnatural conditions (for instance, the rabbit and ferret kept
in hutches), showing that their reproductive system has not been thus
affected; so will some animals and plants withstand domestication or
cultivation, and vary very slightly--perhaps hardly more than in a state
of nature.

A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant.
Such buds can be propagated by grafting, etc., and sometimes by seed.
These "sports" are extremely rare under nature, but far from rare under
cultivation; and in this case we see that the treatment of the parent
has affected a bud or offset, and not the ovules or pollen. But it is
the opinion of most physiologists that there is no essential difference
between a bud and an ovule in their earliest stages of formation; so
that, in fact, "sports" support my view, that variability may be largely
attributed to the ovules or pollen, or to both, having been affected by
the treatment of the parent prior to the act of conception. These cases
anyhow show that variation is not necessarily connected, as some authors
have supposed, with the act of generation.

Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Muller has remarked, have apparently been exposed to
exactly the same conditions of life; and this shows how unimportant the
direct effects of the conditions of life are in comparison with the laws
of reproduction, and of growth, and of inheritance; for had the action
of the conditions been direct, if any of the young had varied, all would
probably have varied in the same manner. To judge how much, in the case
of any variation, we should attribute to the direct action of heat,
moisture, light, food, etc., is most difficult: my impression is, that
with animals such agencies have produced very little direct effect,
though apparently more in the case of plants. Under this point of view,
Mr. Buckman's recent experiments on plants seem extremely valuable.
When all or nearly all the individuals exposed to certain conditions are
affected in the same way, the change at first appears to be directly
due to such conditions; but in some cases it can be shown that quite
opposite conditions produce similar changes of structure. Nevertheless
some slight amount of change may, I think, be attributed to the direct
action of the conditions of life--as, in some cases, increased size from
amount of food, colour from particular kinds of food and from light, and
perhaps the thickness of fur from climate.

Habit also has a decided influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has
a more marked effect; for instance, I find in the domestic duck that
the bones of the wing weigh less and the bones of the leg more,
in proportion to the whole skeleton, than do the same bones in the
wild-duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild
parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison with
the state of these organs in other countries, is another instance of the
effect of use. Not a single domestic animal can be named which has not
in some country drooping ears; and the view suggested by some authors,
that the drooping is due to the disuse of the muscles of the ear, from
the animals not being much alarmed by danger, seems probable.

There are many laws regulating variation, some few of which can be dimly
seen, and will be hereafter briefly mentioned. I will here only allude
to what may be called correlation of growth. Any change in the embryo
or larva will almost certainly entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts are very
curious; and many instances are given in Isidore Geoffroy St. Hilaire's
great work on this subject. Breeders believe that long limbs are almost
always accompanied by an elongated head. Some instances of correlation
are quite whimsical; thus cats with blue eyes are invariably deaf;
colour and constitutional peculiarities go together, of which many
remarkable cases could be given amongst animals and plants. From the
facts collected by Heusinger, it appears that white sheep and pigs are
differently affected from coloured individuals by certain vegetable
poisons. Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many
horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence, if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly unconsciously modify other parts
of the structure, owing to the mysterious laws of the correlation of
growth.

The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth while
carefully to study the several treatises published on some of our old
cultivated plants, as on the hyacinth, potato, even the dahlia, etc.;
and it is really surprising to note the endless points in structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic,
and tends to depart in some small degree from that of the parental type.

Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance,
is endless. Dr. Prosper Lucas's treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong
is the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical writers
alone. When a deviation appears not unfrequently, and we see it in the
father and child, we cannot tell whether it may not be due to the same
original cause acting on both; but when amongst individuals, apparently
exposed to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent--say,
once amongst several million individuals--and it reappears in the
child, the mere doctrine of chances almost compels us to attribute
its reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, etc., appearing in several members
of the same family. If strange and rare deviations of structure are
truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the
whole subject, would be, to look at the inheritance of every character
whatever as the rule, and non-inheritance as the anomaly.

The laws governing inheritance are quite unknown; no one can say why the
same peculiarity in different individuals of the same species, and in
individuals of different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characters to its
grandfather or grandmother or other much more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes or to one
sex alone, more commonly but not exclusively to the like sex. It is a
fact of some little importance to us, that peculiarities appearing
in the males of our domestic breeds are often transmitted either
exclusively, or in a much greater degree, to males alone. A much more
important rule, which I think may be trusted, is that, at whatever
period of life a peculiarity first appears, it tends to appear in the
offspring at a corresponding age, though sometimes earlier. In many
cases this could not be otherwise: thus the inherited peculiarities
in the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider extension,
and that when there is no apparent reason why a peculiarity should
appear at any particular age, yet that it does tend to appear in the
offspring at the same period at which it first appeared in the parent. I
believe this rule to be of the highest importance in explaining the
laws of embryology. These remarks are of course confined to the first
APPEARANCE of the peculiarity, and not to its primary cause, which may
have acted on the ovules or male element; in nearly the same manner as
in the crossed offspring from a short-horned cow by a long-horned bull,
the greater length of horn, though appearing late in life, is clearly
due to the male element.

Having alluded to the subject of reversion, I may here refer to
a statement often made by naturalists--namely, that our domestic
varieties, when run wild, gradually but certainly revert in character to
their aboriginal stocks. Hence it has been argued that no deductions can
be drawn from domestic races to species in a state of nature. I have in
vain endeavoured to discover on what decisive facts the above statement
has so often and so boldly been made. There would be great difficulty
in proving its truth: we may safely conclude that very many of the most
strongly-marked domestic varieties could not possibly live in a wild
state. In many cases we do not know what the aboriginal stock was, and
so could not tell whether or not nearly perfect reversion had ensued.
It would be quite necessary, in order to prevent the effects of
intercrossing, that only a single variety should be turned loose in
its new home. Nevertheless, as our varieties certainly do occasionally
revert in some of their characters to ancestral forms, it seems to me
not improbable, that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance,
of the cabbage, in very poor soil (in which case, however, some effect
would have to be attributed to the direct action of the poor soil),
that they would to a large extent, or even wholly, revert to the wild
aboriginal stock. Whether or not the experiment would succeed, is not of
great importance for our line of argument; for by the experiment itself
the conditions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to reversion,--that
is, to lose their acquired characters, whilst kept under unchanged
conditions, and whilst kept in a considerable body, so that free
intercrossing might check, by blending together, any slight deviations
of structure, in such case, I grant that we could deduce nothing from
domestic varieties in regard to species. But there is not a shadow of
evidence in favour of this view: to assert that we could not breed
our cart and race-horses, long and short-horned cattle, and poultry of
various breeds, and esculent vegetables, for an almost infinite number
of generations, would be opposed to all experience. I may add, that when
under nature the conditions of life do change, variations and reversions
of character probably do occur; but natural selection, as will hereafter
be explained, will determine how far the new characters thus arising
shall be preserved.

When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already
remarked, less uniformity of character than in true species. Domestic
races of the same species, also, often have a somewhat monstrous
character; by which I mean, that, although differing from each other,
and from the other species of the same genus, in several trifling
respects, they often differ in an extreme degree in some one part, both
when compared one with another, and more especially when compared with
all the species in nature to which they are nearest allied. With these
exceptions (and with that of the perfect fertility of varieties when
crossed,--a subject hereafter to be discussed), domestic races of the
same species differ from each other in the same manner as, only in most
cases in a lesser degree than, do closely-allied species of the same
genus in a state of nature. I think this must be admitted, when we find
that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by some competent judges as
mere varieties, and by other competent judges as the descendants of
aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not
differ from each other in characters of generic value. I think it could
be shown that this statement is hardly correct; but naturalists differ
most widely in determining what characters are of generic value; all
such valuations being at present empirical. Moreover, on the view of
the origin of genera which I shall presently give, we have no right
to expect often to meet with generic differences in our domesticated
productions.

When we attempt to estimate the amount of structural difference between
the domestic races of the same species, we are soon involved in doubt,
from not knowing whether they have descended from one or several
parent-species. This point, if it could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know propagate
their kind so truly, were the offspring of any single species, then such
facts would have great weight in making us doubt about the immutability
of the many very closely allied and natural species--for instance, of
the many foxes--inhabiting different quarters of the world. I do not
believe, as we shall presently see, that all our dogs have descended
from any one wild species; but, in the case of some other domestic
races, there is presumptive, or even strong, evidence in favour of this
view.

It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates? Has the little variability of the ass or
guinea-fowl, or the small power of endurance of warmth by the rein-deer,
or of cold by the common camel, prevented their domestication? I
cannot doubt that if other animals and plants, equal in number to our
domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to breed
for an equal number of generations under domestication, they would
vary on an average as largely as the parent species of our existing
domesticated productions have varied.

In the case of most of our anciently domesticated animals and plants, I
do not think it is possible to come to any definite conclusion, whether
they have descended from one or several species. The argument mainly
relied on by those who believe in the multiple origin of our domestic
animals is, that we find in the most ancient records, more especially on
the monuments of Egypt, much diversity in the breeds; and that some of
the breeds closely resemble, perhaps are identical with, those still
existing. Even if this latter fact were found more strictly and
generally true than seems to me to be the case, what does it show, but
that some of our breeds originated there, four or five thousand years
ago? But Mr. Horner's researches have rendered it in some degree
probable that man sufficiently civilized to have manufactured pottery
existed in the valley of the Nile thirteen or fourteen thousand years
ago; and who will pretend to say how long before these ancient periods,
savages, like those of Tierra del Fuego or Australia, who possess a
semi-domestic dog, may not have existed in Egypt?

The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical and
other considerations, I think it highly probable that our domestic dogs
have descended from several wild species. In regard to sheep and goats
I can form no opinion. I should think, from facts communicated to me by
Mr. Blyth, on the habits, voice, and constitution, etc., of the humped
Indian cattle, that these had descended from a different aboriginal
stock from our European cattle; and several competent judges believe
that these latter have had more than one wild parent. With respect to
horses, from reasons which I cannot give here, I am doubtfully inclined
to believe, in opposition to several authors, that all the races have
descended from one wild stock. Mr. Blyth, whose opinion, from his large
and varied stores of knowledge, I should value more than that of almost
any one, thinks that all the breeds of poultry have proceeded from
the common wild Indian fowl (Gallus bankiva). In regard to ducks and
rabbits, the breeds of which differ considerably from each other in
structure, I do not doubt that they all have descended from the common
wild duck and rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of wild
cattle, as many sheep, and several goats in Europe alone, and several
even within Great Britain. One author believes that there formerly
existed in Great Britain eleven wild species of sheep peculiar to it!
When we bear in mind that Britain has now hardly one peculiar mammal,
and France but few distinct from those of Germany and conversely, and
so with Hungary, Spain, etc., but that each of these kingdoms possesses
several peculiar breeds of cattle, sheep, etc., we must admit that many
domestic breeds have originated in Europe; for whence could they have
been derived, as these several countries do not possess a number of
peculiar species as distinct parent-stocks? So it is in India. Even in
the case of the domestic dogs of the whole world, which I fully admit
have probably descended from several wild species, I cannot doubt that
there has been an immense amount of inherited variation. Who can believe
that animals closely resembling the Italian greyhound, the bloodhound,
the bull-dog, or Blenheim spaniel, etc.--so unlike all wild
Canidae--ever existed freely in a state of nature? It has often been
loosely said that all our races of dogs have been produced by the
crossing of a few aboriginal species; but by crossing we can get only
forms in some degree intermediate between their parents; and if we
account for our several domestic races by this process, we must
admit the former existence of the most extreme forms, as the Italian
greyhound, bloodhound, bull-dog, etc., in the wild state. Moreover,
the possibility of making distinct races by crossing has been greatly
exaggerated. There can be no doubt that a race may be modified
by occasional crosses, if aided by the careful selection of those
individual mongrels, which present any desired character; but that
a race could be obtained nearly intermediate between two extremely
different races or species, I can hardly believe. Sir J. Sebright
expressly experimentised for this object, and failed. The offspring from
the first cross between two pure breeds is tolerably and sometimes (as I
have found with pigeons) extremely uniform, and everything seems simple
enough; but when these mongrels are crossed one with another for several
generations, hardly two of them will be alike, and then the extreme
difficulty, or rather utter hopelessness, of the task becomes apparent.
Certainly, a breed intermediate between TWO VERY DISTINCT breeds could
not be got without extreme care and long-continued selection; nor can
I find a single case on record of a permanent race having been thus
formed.

ON THE BREEDS OF THE DOMESTIC PIGEON.

Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured
with skins from several quarters of the world, more especially by the
Honourable W. Elliot from India, and by the Honourable C. Murray from
Persia. Many treatises in different languages have been published on
pigeons, and some of them are very important, as being of considerable
antiquity. I have associated with several eminent fanciers, and have
been permitted to join two of the London Pigeon Clubs. The diversity of
the breeds is something astonishing. Compare the English carrier and the
short-faced tumbler, and see the wonderful difference in their beaks,
entailing corresponding differences in their skulls. The carrier,
more especially the male bird, is also remarkable from the wonderful
development of the carunculated skin about the head, and this is
accompanied by greatly elongated eyelids, very large external orifices
to the nostrils, and a wide gape of mouth. The short-faced tumbler has a
beak in outline almost like that of a finch; and the common tumbler has
the singular and strictly inherited habit of flying at a great height in
a compact flock, and tumbling in the air head over heels. The runt is a
bird of great size, with long, massive beak and large feet; some of the
sub-breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the carrier,
but, instead of a very long beak, has a very short and very broad
one. The pouter has a much elongated body, wings, and legs; and its
enormously developed crop, which it glories in inflating, may well
excite astonishment and even laughter. The turbit has a very short and
conical beak, with a line of reversed feathers down the breast; and it
has the habit of continually expanding slightly the upper part of the
oesophagus. The Jacobin has the feathers so much reversed along the back
of the neck that they form a hood, and it has, proportionally to its
size, much elongated wing and tail feathers. The trumpeter and laugher,
as their names express, utter a very different coo from the other
breeds. The fantail has thirty or even forty tail-feathers, instead of
twelve or fourteen, the normal number in all members of the great pigeon
family; and these feathers are kept expanded, and are carried so erect
that in good birds the head and tail touch; the oil-gland is quite
aborted. Several other less distinct breeds might have been specified.

In the skeletons of the several breeds, the development of the bones
of the face in length and breadth and curvature differs enormously. The
shape, as well as the breadth and length of the ramus of the lower
jaw, varies in a highly remarkable manner. The number of the caudal and
sacral vertebrae vary; as does the number of the ribs, together with
their relative breadth and the presence of processes. The size and shape
of the apertures in the sternum are highly variable; so is the degree
of divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in
strict correlation with the length of beak), the size of the crop and
of the upper part of the oesophagus; the development and abortion of
the oil-gland; the number of the primary wing and caudal feathers; the
relative length of wing and tail to each other and to the body; the
relative length of leg and of the feet; the number of scutellae on
the toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage is
acquired varies, as does the state of the down with which the nestling
birds are clothed when hatched. The shape and size of the eggs vary. The
manner of flight differs remarkably; as does in some breeds the voice
and disposition. Lastly, in certain breeds, the males and females have
come to differ to a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which if shown
to an ornithologist, and he were told that they were wild birds, would
certainly, I think, be ranked by him as well-defined species. Moreover,
I do not believe that any ornithologist would place the English carrier,
the short-faced tumbler, the runt, the barb, pouter, and fantail in
the same genus; more especially as in each of these breeds several
truly-inherited sub-breeds, or species as he might have called them,
could be shown him.

Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely,
that all have descended from the rock-pigeon (Columba livia), including
under this term several geographical races or sub-species, which differ
from each other in the most trifling respects. As several of the reasons
which have led me to this belief are in some degree applicable in other
cases, I will here briefly give them. If the several breeds are not
varieties, and have not proceeded from the rock-pigeon, they must have
descended from at least seven or eight aboriginal stocks; for it is
impossible to make the present domestic breeds by the crossing of any
lesser number: how, for instance, could a pouter be produced by crossing
two breeds unless one of the parent-stocks possessed the characteristic
enormous crop? The supposed aboriginal stocks must all have been
rock-pigeons, that is, not breeding or willingly perching on trees. But
besides C. livia, with its geographical sub-species, only two or three
other species of rock-pigeons are known; and these have not any of the
characters of the domestic breeds. Hence the supposed aboriginal stocks
must either still exist in the countries where they were originally
domesticated, and yet be unknown to ornithologists; and this,
considering their size, habits, and remarkable characters, seems very
improbable; or they must have become extinct in the wild state. But
birds breeding on precipices, and good fliers, are unlikely to be
exterminated; and the common rock-pigeon, which has the same habits with
the domestic breeds, has not been exterminated even on several of the
smaller British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with the
rock-pigeon seems to me a very rash assumption. Moreover, the several
above-named domesticated breeds have been transported to all parts of
the world, and, therefore, some of them must have been carried back
again into their native country; but not one has ever become wild or
feral, though the dovecot-pigeon, which is the rock-pigeon in a very
slightly altered state, has become feral in several places. Again, all
recent experience shows that it is most difficult to get any wild
animal to breed freely under domestication; yet on the hypothesis of the
multiple origin of our pigeons, it must be assumed that at least seven
or eight species were so thoroughly domesticated in ancient times by
half-civilized man, as to be quite prolific under confinement.

An argument, as it seems to me, of great weight, and applicable in
several other cases, is, that the above-specified breeds, though
agreeing generally in constitution, habits, voice, colouring, and
in most parts of their structure, with the wild rock-pigeon, yet are
certainly highly abnormal in other parts of their structure: we may look
in vain throughout the whole great family of Columbidae for a beak like
that of the English carrier, or that of the short-faced tumbler, or
barb; for reversed feathers like those of the jacobin; for a crop like
that of the pouter; for tail-feathers like those of the fantail.
Hence it must be assumed not only that half-civilized man succeeded in
thoroughly domesticating several species, but that he intentionally or
by chance picked out extraordinarily abnormal species; and further, that
these very species have since all become extinct or unknown. So many
strange contingencies seem to me improbable in the highest degree.

Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white rump
(the Indian sub-species, C. intermedia of Strickland, having it bluish);
the tail has a terminal dark bar, with the bases of the outer feathers
externally edged with white; the wings have two black bars; some
semi-domestic breeds and some apparently truly wild breeds have, besides
the two black bars, the wings chequered with black. These several marks
do not occur together in any other species of the whole family. Now, in
every one of the domestic breeds, taking thoroughly well-bred birds, all
the above marks, even to the white edging of the outer tail-feathers,
sometimes concur perfectly developed. Moreover, when two birds belonging
to two distinct breeds are crossed, neither of which is blue or has
any of the above-specified marks, the mongrel offspring are very apt
suddenly to acquire these characters; for instance, I crossed some
uniformly white fantails with some uniformly black barbs, and they
produced mottled brown and black birds; these I again crossed together,
and one grandchild of the pure white fantail and pure black barb was of
as beautiful a blue colour, with the white rump, double black wing-bar,
and barred and white-edged tail-feathers, as any wild rock-pigeon! We
can understand these facts, on the well-known principle of reversion to
ancestral characters, if all the domestic breeds have descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, firstly, that all the
several imagined aboriginal stocks were coloured and marked like the
rock-pigeon, although no other existing species is thus coloured and
marked, so that in each separate breed there might be a tendency to
revert to the very same colours and markings. Or, secondly, that each
breed, even the purest, has within a dozen or, at most, within a score
of generations, been crossed by the rock-pigeon: I say within a dozen or
twenty generations, for we know of no fact countenancing the belief that
the child ever reverts to some one ancestor, removed by a greater number
of generations. In a breed which has been crossed only once with some
distinct breed, the tendency to reversion to any character derived from
such cross will naturally become less and less, as in each succeeding
generation there will be less of the foreign blood; but when there has
been no cross with a distinct breed, and there is a tendency in both
parents to revert to a character, which has been lost during some former
generation, this tendency, for all that we can see to the contrary, may
be transmitted undiminished for an indefinite number of generations.
These two distinct cases are often confounded in treatises on
inheritance.

Lastly, the hybrids or mongrels from between all the domestic breeds
of pigeons are perfectly fertile. I can state this from my own
observations, purposely made on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals CLEARLY DISTINCT being themselves perfectly
fertile. Some authors believe that long-continued domestication
eliminates this strong tendency to sterility: from the history of the
dog I think there is some probability in this hypothesis, if applied to
species closely related together, though it is unsupported by a single
experiment. But to extend the hypothesis so far as to suppose that
species, aboriginally as distinct as carriers, tumblers, pouters, and
fantails now are, should yield offspring perfectly fertile, inter se,
seems to me rash in the extreme.

From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed species of pigeons to breed freely
under domestication; these supposed species being quite unknown in a
wild state, and their becoming nowhere feral; these species having very
abnormal characters in certain respects, as compared with all other
Columbidae, though so like in most other respects to the rock-pigeon;
the blue colour and various marks occasionally appearing in all the
breeds, both when kept pure and when crossed; the mongrel offspring
being perfectly fertile;--from these several reasons, taken together, I
can feel no doubt that all our domestic breeds have descended from the
Columba livia with its geographical sub-species.

In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
breeds, more especially those brought from distant countries, we
can make an almost perfect series between the extremes of structure.
Thirdly, those characters which are mainly distinctive of each breed,
for instance the wattle and length of beak of the carrier, the shortness
of that of the tumbler, and the number of tail-feathers in the fantail,
are in each breed eminently variable; and the explanation of this fact
will be obvious when we come to treat of selection. Fourthly, pigeons
have been watched, and tended with the utmost care, and loved by many
people. They have been domesticated for thousands of years in several
quarters of the world; the earliest known record of pigeons is in the
fifth Aegyptian dynasty, about 3000 B.C., as was pointed out to me by
Professor Lepsius; but Mr. Birch informs me that pigeons are given in a
bill of fare in the previous dynasty. In the time of the Romans, as we
hear from Pliny, immense prices were given for pigeons; "nay, they are
come to this pass, that they can reckon up their pedigree and race."
Pigeons were much valued by Akber Khan in India, about the year 1600;
never less than 20,000 pigeons were taken with the court. "The monarchs
of Iran and Turan sent him some very rare birds;" and, continues the
courtly historian, "His Majesty by crossing the breeds, which method
was never practised before, has improved them astonishingly." About
this same period the Dutch were as eager about pigeons as were the old
Romans. The paramount importance of these considerations in explaining
the immense amount of variation which pigeons have undergone, will be
obvious when we treat of Selection. We shall then, also, see how it is
that the breeds so often have a somewhat monstrous character. It is also
a most favourable circumstance for the production of distinct breeds,
that male and female pigeons can be easily mated for life; and thus
different breeds can be kept together in the same aviary.

I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons and
watched the several kinds, knowing well how true they bred, I felt fully
as much difficulty in believing that they could ever have descended
from a common parent, as any naturalist could in coming to a similar
conclusion in regard to the many species of finches, or other large
groups of birds, in nature. One circumstance has struck me much;
namely, that all the breeders of the various domestic animals and
the cultivators of plants, with whom I have ever conversed, or whose
treatises I have read, are firmly convinced that the several breeds
to which each has attended, are descended from so many aboriginally
distinct species. Ask, as I have asked, a celebrated raiser of Hereford
cattle, whether his cattle might not have descended from long horns, and
he will laugh you to scorn. I have never met a pigeon, or poultry, or
duck, or rabbit fancier, who was not fully convinced that each main
breed was descended from a distinct species. Van Mons, in his treatise
on pears and apples, shows how utterly he disbelieves that the several
sorts, for instance a Ribston-pippin or Codlin-apple, could ever have
proceeded from the seeds of the same tree. Innumerable other examples
could be given. The explanation, I think, is simple: from long-continued
study they are strongly impressed with the differences between the
several races; and though they well know that each race varies slightly,
for they win their prizes by selecting such slight differences, yet they
ignore all general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May not
those naturalists who, knowing far less of the laws of inheritance than
does the breeder, and knowing no more than he does of the intermediate
links in the long lines of descent, yet admit that many of our domestic
races have descended from the same parents--may they not learn a lesson
of caution, when they deride the idea of species in a state of nature
being lineal descendants of other species?

SELECTION.

Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some little
effect may, perhaps, be attributed to the direct action of the external
conditions of life, and some little to habit; but he would be a bold
man who would account by such agencies for the differences of a dray and
race horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
One of the most remarkable features in our domesticated races is that we
see in them adaptation, not indeed to the animal's or plant's own good,
but to man's use or fancy. Some variations useful to him have probably
arisen suddenly, or by one step; many botanists, for instance, believe
that the fuller's teazle, with its hooks, which cannot be rivalled by
any mechanical contrivance, is only a variety of the wild Dipsacus; and
this amount of change may have suddenly arisen in a seedling. So it has
probably been with the turnspit dog; and this is known to have been
the case with the ancon sheep. But when we compare the dray-horse and
race-horse, the dromedary and camel, the various breeds of sheep fitted
either for cultivated land or mountain pasture, with the wool of one
breed good for one purpose, and that of another breed for another
purpose; when we compare the many breeds of dogs, each good for man in
very different ways; when we compare the game-cock, so pertinacious
in battle, with other breeds so little quarrelsome, with "everlasting
layers" which never desire to sit, and with the bantam so small and
elegant; when we compare the host of agricultural, culinary, orchard,
and flower-garden races of plants, most useful to man at different
seasons and for different purposes, or so beautiful in his eyes, we
must, I think, look further than to mere variability. We cannot suppose
that all the breeds were suddenly produced as perfect and as useful as
we now see them; indeed, in several cases, we know that this has not
been their history. The key is man's power of accumulative selection:
nature gives successive variations; man adds them up in certain
directions useful to him. In this sense he may be said to make for
himself useful breeds.

The great power of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within a
single lifetime, modified to a large extent some breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this subject,
and to inspect the animals. Breeders habitually speak of an animal's
organisation as something quite plastic, which they can model almost
as they please. If I had space I could quote numerous passages to this
effect from highly competent authorities. Youatt, who was probably
better acquainted with the works of agriculturalists than almost any
other individual, and who was himself a very good judge of an animal,
speaks of the principle of selection as "that which enables the
agriculturist, not only to modify the character of his flock, but to
change it altogether. It is the magician's wand, by means of which
he may summon into life whatever form and mould he pleases." Lord
Somerville, speaking of what breeders have done for sheep, says:--"It
would seem as if they had chalked out upon a wall a form perfect in
itself, and then had given it existence." That most skilful breeder,
Sir John Sebright, used to say, with respect to pigeons, that "he would
produce any given feather in three years, but it would take him
six years to obtain head and beak." In Saxony the importance of the
principle of selection in regard to merino sheep is so fully recognised,
that men follow it as a trade: the sheep are placed on a table and are
studied, like a picture by a connoisseur; this is done three times at
intervals of months, and the sheep are each time marked and classed, so
that the very best may ultimately be selected for breeding.

What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; all the best breeders
are strongly opposed to this practice, except sometimes amongst closely
allied sub-breeds. And when a cross has been made, the closest selection
is far more indispensable even than in ordinary cases. If selection
consisted merely in separating some very distinct variety, and breeding
from it, the principle would be so obvious as hardly to be worth
notice; but its importance consists in the great effect produced by
the accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye--differences
which I for one have vainly attempted to appreciate. Not one man in
a thousand has accuracy of eye and judgment sufficient to become an
eminent breeder. If gifted with these qualities, and he studies his
subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if he
wants any of these qualities, he will assuredly fail. Few would readily
believe in the natural capacity and years of practice requisite to
become even a skilful pigeon-fancier.

The same principles are followed by horticulturists; but the variations
are here often more abrupt. No one supposes that our choicest
productions have been produced by a single variation from the aboriginal
stock. We have proofs that this is not so in some cases, in which exact
records have been kept; thus, to give a very trifling instance, the
steadily-increasing size of the common gooseberry may be quoted. We see
an astonishing improvement in many florists' flowers, when the flowers
of the present day are compared with drawings made only twenty or thirty
years ago. When a race of plants is once pretty well established, the
seed-raisers do not pick out the best plants, but merely go over their
seed-beds, and pull up the "rogues," as they call the plants that
deviate from the proper standard. With animals this kind of selection
is, in fact, also followed; for hardly any one is so careless as to
allow his worst animals to breed.

In regard to plants, there is another means of observing the accumulated
effects of selection--namely, by comparing the diversity of flowers in
the different varieties of the same species in the flower-garden; the
diversity of leaves, pods, or tubers, or whatever part is valued, in the
kitchen-garden, in comparison with the flowers of the same varieties;
and the diversity of fruit of the same species in the orchard, in
comparison with the leaves and flowers of the same set of varieties. See
how different the leaves of the cabbage are, and how extremely alike the
flowers; how unlike the flowers of the heartsease are, and how alike the
leaves; how much the fruit of the different kinds of gooseberries differ
in size, colour, shape, and hairiness, and yet the flowers present very
slight differences. It is not that the varieties which differ largely
in some one point do not differ at all in other points; this is hardly
ever, perhaps never, the case. The laws of correlation of growth,
the importance of which should never be overlooked, will ensure some
differences; but, as a general rule, I cannot doubt that the continued
selection of slight variations, either in the leaves, the flowers, or
the fruit, will produce races differing from each other chiefly in these
characters.

It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century;
it has certainly been more attended to of late years, and many treatises
have been published on the subject; and the result, I may add, has been,
in a corresponding degree, rapid and important. But it is very far from
true that the principle is a modern discovery. I could give several
references to the full acknowledgment of the importance of the principle
in works of high antiquity. In rude and barbarous periods of English
history choice animals were often imported, and laws were passed to
prevent their exportation: the destruction of horses under a certain
size was ordered, and this may be compared to the "roguing" of plants
by nurserymen. The principle of selection I find distinctly given in an
ancient Chinese encyclopaedia. Explicit rules are laid down by some of
the Roman classical writers. From passages in Genesis, it is clear that
the colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to
improve the breed, and they formerly did so, as is attested by passages
in Pliny. The savages in South Africa match their draught cattle by
colour, as do some of the Esquimaux their teams of dogs. Livingstone
shows how much good domestic breeds are valued by the negroes of the
interior of Africa who have not associated with Europeans. Some of these
facts do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is now
attended to by the lowest savages. It would, indeed, have been a strange
fact, had attention not been paid to breeding, for the inheritance of
good and bad qualities is so obvious.

At the present time, eminent breeders try by methodical selection, with
a distinct object in view, to make a new strain or sub-breed, superior
to anything existing in the country. But, for our purpose, a kind of
Selection, which may be called Unconscious, and which results from every
one trying to possess and breed from the best individual animals, is
more important. Thus, a man who intends keeping pointers naturally tries
to get as good dogs as he can, and afterwards breeds from his own best
dogs, but he has no wish or expectation of permanently altering the
breed. Nevertheless I cannot doubt that this process, continued during
centuries, would improve and modify any breed, in the same way as
Bakewell, Collins, etc., by this very same process, only carried on more
methodically, did greatly modify, even during their own lifetimes, the
forms and qualities of their cattle. Slow and insensible changes of this
kind could never be recognised unless actual measurements or careful
drawings of the breeds in question had been made long ago, which might
serve for comparison. In some cases, however, unchanged or but little
changed individuals of the same breed may be found in less civilised
districts, where the breed has been less improved. There is reason to
believe that King Charles's spaniel has been unconsciously modified to
a large extent since the time of that monarch. Some highly competent
authorities are convinced that the setter is directly derived from the
spaniel, and has probably been slowly altered from it. It is known that
the English pointer has been greatly changed within the last century,
and in this case the change has, it is believed, been chiefly effected
by crosses with the fox-hound; but what concerns us is, that the change
has been effected unconsciously and gradually, and yet so effectually,
that, though the old Spanish pointer certainly came from Spain, Mr.
Borrow has not seen, as I am informed by him, any native dog in Spain
like our pointer.

By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size
the parent Arab stock, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights they carry. Lord Spencer and
others have shown how the cattle of England have increased in weight
and in early maturity, compared with the stock formerly kept in this
country. By comparing the accounts given in old pigeon treatises of
carriers and tumblers with these breeds as now existing in Britain,
India, and Persia, we can, I think, clearly trace the stages through
which they have insensibly passed, and come to differ so greatly from
the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so far
that the breeders could never have expected or even have wished to have
produced the result which ensued--namely, the production of two distinct
strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr.
Burgess, as Mr. Youatt remarks, "have been purely bred from the original
stock of Mr. Bakewell for upwards of fifty years. There is not a
suspicion existing in the mind of any one at all acquainted with
the subject that the owner of either of them has deviated in any one
instance from the pure blood of Mr. Bakewell's flock, and yet the
difference between the sheep possessed by these two gentlemen is so
great that they have the appearance of being quite different varieties."

If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are
so liable, and such choice animals would thus generally leave more
offspring than the inferior ones; so that in this case there would be a
kind of unconscious selection going on. We see the value set on animals
even by the barbarians of Tierra del Fuego, by their killing and
devouring their old women, in times of dearth, as of less value than
their dogs.

In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked at their first appearance as distinct
varieties, and whether or not two or more species or races have become
blended together by crossing, may plainly be recognised in the increased
size and beauty which we now see in the varieties of the heartsease,
rose, pelargonium, dahlia, and other plants, when compared with the
older varieties or with their parent-stocks. No one would ever expect to
get a first-rate heartsease or dahlia from the seed of a wild plant. No
one would expect to raise a first-rate melting pear from the seed of a
wild pear, though he might succeed from a poor seedling growing wild,
if it had come from a garden-stock. The pear, though cultivated in
classical times, appears, from Pliny's description, to have been a
fruit of very inferior quality. I have seen great surprise expressed
in horticultural works at the wonderful skill of gardeners, in having
produced such splendid results from such poor materials; but the art,
I cannot doubt, has been simple, and, as far as the final result is
concerned, has been followed almost unconsciously. It has consisted in
always cultivating the best known variety, sowing its seeds, and, when
a slightly better variety has chanced to appear, selecting it, and so
onwards. But the gardeners of the classical period, who cultivated
the best pear they could procure, never thought what splendid fruit we
should eat; though we owe our excellent fruit, in some small degree,
to their having naturally chosen and preserved the best varieties they
could anywhere find.

A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known fact,
that in a vast number of cases we cannot recognise, and therefore do
not know, the wild parent-stocks of the plants which have been longest
cultivated in our flower and kitchen gardens. If it has taken centuries
or thousands of years to improve or modify most of our plants up to
their present standard of usefulness to man, we can understand how it
is that neither Australia, the Cape of Good Hope, nor any other region
inhabited by quite uncivilised man, has afforded us a single plant worth
culture. It is not that these countries, so rich in species, do not by
a strange chance possess the aboriginal stocks of any useful plants, but
that the native plants have not been improved by continued selection up
to a standard of perfection comparable with that given to the plants in
countries anciently civilised.

In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
"natural selection," as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains what has been
remarked by some authors, namely, that the varieties kept by savages
have more of the character of species than the varieties kept in
civilised countries.

On the view here given of the all-important part which selection by
man has played, it becomes at once obvious, how it is that our domestic
races show adaptation in their structure or in their habits to man's
wants or fancies. We can, I think, further understand the frequently
abnormal character of our domestic races, and likewise their differences
being so great in external characters and relatively so slight in
internal parts or organs. Man can hardly select, or only with much
difficulty, any deviation of structure excepting such as is externally
visible; and indeed he rarely cares for what is internal. He can never
act by selection, excepting on variations which are first given to
him in some slight degree by nature. No man would ever try to make
a fantail, till he saw a pigeon with a tail developed in some slight
degree in an unusual manner, or a pouter till he saw a pigeon with a
crop of somewhat unusual size; and the more abnormal or unusual any
character was when it first appeared, the more likely it would be to
catch his attention. But to use such an expression as trying to make a
fantail, is, I have no doubt, in most cases, utterly incorrect. The man
who first selected a pigeon with a slightly larger tail, never dreamed
what the descendants of that pigeon would become through long-continued,
partly unconscious and partly methodical selection. Perhaps the parent
bird of all fantails had only fourteen tail-feathers somewhat expanded,
like the present Java fantail, or like individuals of other and distinct
breeds, in which as many as seventeen tail-feathers have been counted.
Perhaps the first pouter-pigeon did not inflate its crop much more than
the turbit now does the upper part of its oesophagus,--a habit which
is disregarded by all fanciers, as it is not one of the points of the
breed.

Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly
be set on any slight differences in the individuals of the same species,
be judged of by the value which would now be set on them, after several
breeds have once fairly been established. Many slight differences might,
and indeed do now, arise amongst pigeons, which are rejected as faults
or deviations from the standard of perfection of each breed. The common
goose has not given rise to any marked varieties; hence the Thoulouse
and the common breed, which differ only in colour, that most fleeting of
characters, have lately been exhibited as distinct at our poultry-shows.

I think these views further explain what has sometimes been
noticed--namely that we know nothing about the origin or history of
any of our domestic breeds. But, in fact, a breed, like a dialect of
a language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals
and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a distinct
name, and from being only slightly valued, their history will be
disregarded. When further improved by the same slow and gradual process,
they will spread more widely, and will get recognised as something
distinct and valuable, and will then probably first receive a provincial
name. In semi-civilised countries, with little free communication, the
spreading and knowledge of any new sub-breed will be a slow process.
As soon as the points of value of the new sub-breed are once fully
acknowledged, the principle, as I have called it, of unconscious
selection will always tend,--perhaps more at one period than at another,
as the breed rises or falls in fashion,--perhaps more in one district
than in another, according to the state of civilisation of the
inhabitants--slowly to add to the characteristic features of the breed,
whatever they may be. But the chance will be infinitely small of any
record having been preserved of such slow, varying, and insensible
changes.

I must now say a few words on the circumstances, favourable, or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply sufficient,
with extreme care, to allow of the accumulation of a large amount
of modification in almost any desired direction. But as variations
manifestly useful or pleasing to man appear only occasionally, the
chance of their appearance will be much increased by a large number
of individuals being kept; and hence this comes to be of the highest
importance to success. On this principle Marshall has remarked, with
respect to the sheep of parts of Yorkshire, that "as they generally
belong to poor people, and are mostly IN SMALL LOTS, they never can be
improved." On the other hand, nurserymen, from raising large stocks
of the same plants, are generally far more successful than amateurs in
getting new and valuable varieties. The keeping of a large number of
individuals of a species in any country requires that the species should
be placed under favourable conditions of life, so as to breed freely in
that country. When the individuals of any species are scanty, all the
individuals, whatever their quality may be, will generally be allowed
to breed, and this will effectually prevent selection. But probably the
most important point of all, is, that the animal or plant should be
so highly useful to man, or so much valued by him, that the closest
attention should be paid to even the slightest deviation in the
qualities or structure of each individual. Unless such attention be paid
nothing can be effected. I have seen it gravely remarked, that it was
most fortunate that the strawberry began to vary just when gardeners
began to attend closely to this plant. No doubt the strawberry had
always varied since it was cultivated, but the slight varieties had been
neglected. As soon, however, as gardeners picked out individual plants
with slightly larger, earlier, or better fruit, and raised seedlings
from them, and again picked out the best seedlings and bred from them,
then, there appeared (aided by some crossing with distinct species)
those many admirable varieties of the strawberry which have been raised
during the last thirty or forty years.

In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races,--at least, in a country which is already stocked with other
races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one
breed of the same species. Pigeons can be mated for life, and this is a
great convenience to the fancier, for thus many races may be kept true,
though mingled in the same aviary; and this circumstance must have
largely favoured the improvement and formation of new breeds. Pigeons,
I may add, can be propagated in great numbers and at a very quick rate,
and inferior birds may be freely rejected, as when killed they serve
for food. On the other hand, cats, from their nocturnal rambling habits,
cannot be matched, and, although so much valued by women and children,
we hardly ever see a distinct breed kept up; such breeds as we do
sometimes see are almost always imported from some other country, often
from islands. Although I do not doubt that some domestic animals vary
less than others, yet the rarity or absence of distinct breeds of the
cat, the donkey, peacock, goose, etc., may be attributed in main part
to selection not having been brought into play: in cats, from the
difficulty in pairing them; in donkeys, from only a few being kept by
poor people, and little attention paid to their breeding; in peacocks,
from not being very easily reared and a large stock not kept; in geese,
from being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of distinct
breeds.

To sum up on the origin of our Domestic Races of animals and plants.
I believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and
necessary contingency, under all circumstances, with all organic beings,
as some authors have thought. The effects of variability are modified by
various degrees of inheritance and of reversion. Variability is governed
by many unknown laws, more especially by that of correlation of growth.
Something may be attributed to the direct action of the conditions of
life. Something must be attributed to use and disuse. The final result
is thus rendered infinitely complex. In some cases, I do not doubt
that the intercrossing of species, aboriginally distinct, has played an
important part in the origin of our domestic productions. When in
any country several domestic breeds have once been established, their
occasional intercrossing, with the aid of selection, has, no doubt,
largely aided in the formation of new sub-breeds; but the importance of
the crossing of varieties has, I believe, been greatly exaggerated, both
in regard to animals and to those plants which are propagated by seed.
In plants which are temporarily propagated by cuttings, buds, etc., the
importance of the crossing both of distinct species and of varieties
is immense; for the cultivator here quite disregards the extreme
variability both of hybrids and mongrels, and the frequent sterility of
hybrids; but the cases of plants not propagated by seed are of little
importance to us, for their endurance is only temporary. Over all
these causes of Change I am convinced that the accumulative action
of Selection, whether applied methodically and more quickly, or
unconsciously and more slowly, but more efficiently, is by far the
predominant Power.




2. VARIATION UNDER NATURE.

Variability. Individual differences. Doubtful species. Wide ranging,
much diffused, and common species vary most. Species of the larger
genera in any country vary more than the species of the smaller genera.
Many of the species of the larger genera resemble varieties in being
very closely, but unequally, related to each other, and in having
restricted ranges.

Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these
latter are subject to any variation. To treat this subject at all
properly, a long catalogue of dry facts should be given; but these I
shall reserve for my future work. Nor shall I here discuss the various
definitions which have been given of the term species. No one definition
has as yet satisfied all naturalists; yet every naturalist knows vaguely
what he means when he speaks of a species. Generally the term includes
the unknown element of a distinct act of creation. The term "variety"
is almost equally difficult to define; but here community of descent is
almost universally implied, though it can rarely be proved. We have also
what are called monstrosities; but they graduate into varieties. By a
monstrosity I presume is meant some considerable deviation of structure
in one part, either injurious to or not useful to the species, and
not generally propagated. Some authors use the term "variation" in a
technical sense, as implying a modification directly due to the physical
conditions of life; and "variations" in this sense are supposed not to
be inherited: but who can say that the dwarfed condition of shells in
the brackish waters of the Baltic, or dwarfed plants on Alpine summits,
or the thicker fur of an animal from far northwards, would not in some
cases be inherited for at least some few generations? and in this case I
presume that the form would be called a variety.

Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring
from the same parents, or which may be presumed to have thus arisen,
from being frequently observed in the individuals of the same species
inhabiting the same confined locality. No one supposes that all the
individuals of the same species are cast in the very same mould. These
individual differences are highly important for us, as they afford
materials for natural selection to accumulate, in the same manner as
man can accumulate in any given direction individual differences in his
domesticated productions. These individual differences generally affect
what naturalists consider unimportant parts; but I could show by a long
catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the
most experienced naturalist would be surprised at the number of the
cases of variability, even in important parts of structure, which he
could collect on good authority, as I have collected, during a course of
years. It should be remembered that systematists are far from pleased at
finding variability in important characters, and that there are not
many men who will laboriously examine internal and important organs, and
compare them in many specimens of the same species. I should never
have expected that the branching of the main nerves close to the great
central ganglion of an insect would have been variable in the same
species; I should have expected that changes of this nature could have
been effected only by slow degrees: yet quite recently Mr. Lubbock has
shown a degree of variability in these main nerves in Coccus, which may
almost be compared to the irregular branching of the stem of a tree.
This philosophical naturalist, I may add, has also quite recently shown
that the muscles in the larvae of certain insects are very far from
uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically rank
that character as important (as some few naturalists have honestly
confessed) which does not vary; and, under this point of view, no
instance of an important part varying will ever be found: but under any
other point of view many instances assuredly can be given.

There is one point connected with individual differences, which seems
to me extremely perplexing: I refer to those genera which have sometimes
been called "protean" or "polymorphic," in which the species present
an inordinate amount of variation; and hardly two naturalists can agree
which forms to rank as species and which as varieties. We may instance
Rubus, Rosa, and Hieracium amongst plants, several genera of insects,
and several genera of Brachiopod shells. In most polymorphic genera
some of the species have fixed and definite characters. Genera which
are polymorphic in one country seem to be, with some few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts seem to be very
perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that we
see in these polymorphic genera variations in points of structure which
are of no service or disservice to the species, and which consequently
have not been seized on and rendered definite by natural selection, as
hereafter will be explained.

Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of
these doubtful and closely-allied forms have permanently retained their
characters in their own country for a long time; for as long, as far as
we know, as have good and true species. Practically, when a naturalist
can unite two forms together by others having intermediate characters,
he treats the one as a variety of the other, ranking the most common,
but sometimes the one first described, as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes occur in deciding whether or not to rank one
form as a variety of another, even when they are closely connected by
intermediate links; nor will the commonly-assumed hybrid nature of the
intermediate links always remove the difficulty. In very many cases,
however, one form is ranked as a variety of another, not because the
intermediate links have actually been found, but because analogy leads
the observer to suppose either that they do now somewhere exist, or may
formerly have existed; and here a wide door for the entry of doubt and
conjecture is opened.

Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges.

That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or
of the United States, drawn up by different botanists, and see what
a surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H. C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties, but
which have all been ranked by botanists as species; and in making this
list he has omitted many trifling varieties, but which nevertheless have
been ranked by some botanists as species, and he has entirely omitted
several highly polymorphic genera. Under genera, including the most
polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham
gives only 112,--a difference of 139 doubtful forms! Amongst animals
which unite for each birth, and which are highly locomotive, doubtful
forms, ranked by one zoologist as a species and by another as a variety,
can rarely be found within the same country, but are common in separated
areas. How many of those birds and insects in North America and Europe,
which differ very slightly from each other, have been ranked by one
eminent naturalist as undoubted species, and by another as varieties,
or, as they are often called, as geographical races! Many years ago,
when comparing, and seeing others compare, the birds from the separate
islands of the Galapagos Archipelago, both one with another, and with
those from the American mainland, I was much struck how entirely vague
and arbitrary is the distinction between species and varieties. On the
islets of the little Madeira group there are many insects which are
characterized as varieties in Mr. Wollaston's admirable work, but
which it cannot be doubted would be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded
as varieties, but which have been ranked as species by some zoologists.
Several most experienced ornithologists consider our British red grouse
as only a strongly-marked race of a Norwegian species, whereas the
greater number rank it as an undoubted species peculiar to Great
Britain. A wide distance between the homes of two doubtful forms leads
many naturalists to rank both as distinct species; but what distance, it
has been well asked, will suffice? if that between America and Europe
is ample, will that between the Continent and the Azores, or Madeira, or
the Canaries, or Ireland, be sufficient? It must be admitted that many
forms, considered by highly-competent judges as varieties, have so
perfectly the character of species that they are ranked by other
highly-competent judges as good and true species. But to discuss whether
they are rightly called species or varieties, before any definition of
these terms has been generally accepted, is vainly to beat the air.

Many of the cases of strongly-marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have
been brought to bear on the attempt to determine their rank. I will here
give only a single instance,--the well-known one of the primrose and
cowslip, or Primula veris and elatior. These plants differ considerably
in appearance; they have a different flavour and emit a different
odour; they flower at slightly different periods; they grow in somewhat
different stations; they ascend mountains to different heights; they
have different geographical ranges; and lastly, according to very
numerous experiments made during several years by that most careful
observer Gartner, they can be crossed only with much difficulty.
We could hardly wish for better evidence of the two forms being
specifically distinct. On the other hand, they are united by many
intermediate links, and it is very doubtful whether these links are
hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents,
and consequently must be ranked as varieties.

Close investigation, in most cases, will bring naturalists to an
agreement how to rank doubtful forms. Yet it must be confessed, that it
is in the best-known countries that we find the greatest number of forms
of doubtful value. I have been struck with the fact, that if any animal
or plant in a state of nature be highly useful to man, or from any cause
closely attract his attention, varieties of it will almost universally
be found recorded. These varieties, moreover, will be often ranked by
some authors as species. Look at the common oak, how closely it has
been studied; yet a German author makes more than a dozen species out
of forms, which are very generally considered as varieties; and in
this country the highest botanical authorities and practical men can be
quoted to show that the sessile and pedunculated oaks are either good
and distinct species or mere varieties.

When a young naturalist commences the study of a group of organisms
quite unknown to him, he is at first much perplexed to determine what
differences to consider as specific, and what as varieties; for he
knows nothing of the amount and kind of variation to which the group
is subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country, he will soon make up his mind how to rank most of the doubtful
forms. His general tendency will be to make many species, for he will
become impressed, just like the pigeon or poultry-fancier before alluded
to, with the amount of difference in the forms which he is continually
studying; and he has little general knowledge of analogical variation
in other groups and in other countries, by which to correct his first
impressions. As he extends the range of his observations, he will meet
with more cases of difficulty; for he will encounter a greater number
of closely-allied forms. But if his observations be widely extended, he
will in the end generally be enabled to make up his own mind which to
call varieties and which species; but he will succeed in this at the
expense of admitting much variation,--and the truth of this admission
will often be disputed by other naturalists. When, moreover, he comes to
study allied forms brought from countries not now continuous, in which
case he can hardly hope to find the intermediate links between his
doubtful forms, he will have to trust almost entirely to analogy, and
his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other in an insensible series; and a series impresses
the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to
the systematist, as of high importance for us, as being the first step
towards such slight varieties as are barely thought worth recording
in works on natural history. And I look at varieties which are in any
degree more distinct and permanent, as steps leading to more strongly
marked and more permanent varieties; and at these latter, as leading to
sub-species, and to species. The passage from one stage of difference
to another and higher stage may be, in some cases, due merely to the
long-continued action of different physical conditions in two different
regions; but I have not much faith in this view; and I attribute the
passage of a variety, from a state in which it differs very slightly
from its parent to one in which it differs more, to the action of
natural selection in accumulating (as will hereafter be more fully
explained) differences of structure in certain definite directions.
Hence I believe a well-marked variety may be justly called an incipient
species; but whether this belief be justifiable must be judged of by
the general weight of the several facts and views given throughout this
work.

It need not be supposed that all varieties or incipient species
necessarily attain the rank of species. They may whilst in this
incipient state become extinct, or they may endure as varieties for very
long periods, as has been shown to be the case by Mr. Wollaston with the
varieties of certain fossil land-shells in Madeira. If a variety were
to flourish so as to exceed in numbers the parent species, it would then
rank as the species, and the species as the variety; or it might come to
supplant and exterminate the parent species; or both might co-exist, and
both rank as independent species. But we shall hereafter have to return
to this subject.

From these remarks it will be seen that I look at the term species,
as one arbitrarily given for the sake of convenience to a set of
individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again,
in comparison with mere individual differences, is also applied
arbitrarily, and for mere convenience sake.

Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H. C.
Watson, to whom I am much indebted for valuable advice and assistance
on this subject, soon convinced me that there were many difficulties, as
did subsequently Dr. Hooker, even in stronger terms. I shall reserve
for my future work the discussion of these difficulties, and the tables
themselves of the proportional numbers of the varying species.
Dr. Hooker permits me to add, that after having carefully read my
manuscript, and examined the tables, he thinks that the following
statements are fairly well established. The whole subject, however,
treated as it necessarily here is with much brevity, is rather
perplexing, and allusions cannot be avoided to the "struggle for
existence," "divergence of character," and other questions, hereafter to
be discussed.

Alph. De Candolle and others have shown that plants which have very wide
ranges generally present varieties; and this might have been expected,
as they become exposed to diverse physical conditions, and as they
come into competition (which, as we shall hereafter see, is a far more
important circumstance) with different sets of organic beings. But my
tables further show that, in any limited country, the species which are
most common, that is abound most in individuals, and the species
which are most widely diffused within their own country (and this is a
different consideration from wide range, and to a certain extent from
commonness), often give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most flourishing,
or, as they may be called, the dominant species,--those which range
widely over the world, are the most diffused in their own country,
and are the most numerous in individuals,--which oftenest produce
well-marked varieties, or, as I consider them, incipient species. And
this, perhaps, might have been anticipated; for, as varieties, in order
to become in any degree permanent, necessarily have to struggle with the
other inhabitants of the country, the species which are already dominant
will be the most likely to yield offspring which, though in some slight
degree modified, will still inherit those advantages that enabled their
parents to become dominant over their compatriots.

If the plants inhabiting a country and described in any Flora be divided
into two equal masses, all those in the larger genera being placed
on one side, and all those in the smaller genera on the other side, a
somewhat larger number of the very common and much diffused or dominant
species will be found on the side of the larger genera. This, again,
might have been anticipated; for the mere fact of many species of the
same genus inhabiting any country, shows that there is something in the
organic or inorganic conditions of that country favourable to the genus;
and, consequently, we might have expected to have found in the larger
genera, or those including many species, a large proportional number of
dominant species. But so many causes tend to obscure this result, that
I am surprised that my tables show even a small majority on the side of
the larger genera. I will here allude to only two causes of obscurity.
Fresh-water and salt-loving plants have generally very wide ranges and
are much diffused, but this seems to be connected with the nature of the
stations inhabited by them, and has little or no relation to the size of
the genera to which the species belong. Again, plants low in the scale
of organisation are generally much more widely diffused than plants
higher in the scale; and here again there is no close relation to the
size of the genera. The cause of lowly-organised plants ranging widely
will be discussed in our chapter on geographical distribution.

From looking at species as only strongly-marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of the
smaller genera; for wherever many closely related species (i.e. species
of the same genus) have been formed, many varieties or incipient species
ought, as a general rule, to be now forming. Where many large trees
grow, we expect to find saplings. Where many species of a genus have
been formed through variation, circumstances have been favourable
for variation; and hence we might expect that the circumstances would
generally be still favourable to variation. On the other hand, if we
look at each species as a special act of creation, there is no apparent
reason why more varieties should occur in a group having many species,
than in one having few.

To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the
side of the larger genera present varieties, than on the side of the
smaller genera. Moreover, the species of the large genera which present
any varieties, invariably present a larger average number of varieties
than do the species of the small genera. Both these results follow when
another division is made, and when all the smallest genera, with from
only one to four species, are absolutely excluded from the tables.
These facts are of plain signification on the view that species are only
strongly marked and permanent varieties; for wherever many species of
the same genus have been formed, or where, if we may use the expression,
the manufactory of species has been active, we ought generally to find
the manufactory still in action, more especially as we have every reason
to believe the process of manufacturing new species to be a slow one.
And this certainly is the case, if varieties be looked at as incipient
species; for my tables clearly show as a general rule that, wherever
many species of a genus have been formed, the species of that genus
present a number of varieties, that is of incipient species, beyond the
average. It is not that all large genera are now varying much, and are
thus increasing in the number of their species, or that no small genera
are now varying and increasing; for if this had been so, it would have
been fatal to my theory; inasmuch as geology plainly tells us that small
genera have in the lapse of time often increased greatly in size;
and that large genera have often come to their maxima, declined, and
disappeared. All that we want to show is, that where many species of a
genus have been formed, on an average many are still forming; and this
holds good.

There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been
found between doubtful forms, naturalists are compelled to come to
a determination by the amount of difference between them, judging by
analogy whether or not the amount suffices to raise one or both to the
rank of species. Hence the amount of difference is one very important
criterion in settling whether two forms should be ranked as species or
varieties. Now Fries has remarked in regard to plants, and Westwood in
regard to insects, that in large genera the amount of difference between
the species is often exceedingly small. I have endeavoured to test this
numerically by averages, and, as far as my imperfect results go, they
always confirm the view. I have also consulted some sagacious and most
experienced observers, and, after deliberation, they concur in this
view. In this respect, therefore, the species of the larger genera
resemble varieties, more than do the species of the smaller genera.
Or the case may be put in another way, and it may be said, that in
the larger genera, in which a number of varieties or incipient species
greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for
they differ from each other by a less than usual amount of difference.

Moreover, the species of the large genera are related to each other, in
the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
certain other species. And what are varieties but groups of forms,
unequally related to each other, and clustered round certain forms--that
is, round their parent-species? Undoubtedly there is one most important
point of difference between varieties and species; namely, that the
amount of difference between varieties, when compared with each other or
with their parent-species, is much less than that between the species of
the same genus. But when we come to discuss the principle, as I call it,
of Divergence of Character, we shall see how this may be explained, and
how the lesser differences between varieties will tend to increase into
the greater differences between species.

There is one other point which seems to me worth notice. Varieties
generally have much restricted ranges: this statement is indeed scarcely
more than a truism, for if a variety were found to have a wider range
than that of its supposed parent-species, their denominations ought to
be reversed. But there is also reason to believe, that those species
which are very closely allied to other species, and in so far resemble
varieties, often have much restricted ranges. For instance, Mr. H. C.
Watson has marked for me in the well-sifted London Catalogue of plants
(4th edition) 63 plants which are therein ranked as species, but which
he considers as so closely allied to other species as to be of doubtful
value: these 63 reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this
same catalogue, 53 acknowledged varieties are recorded, and these range
over 7.7 provinces; whereas, the species to which these varieties belong
range over 14.3 provinces. So that the acknowledged varieties have very
nearly the same restricted average range, as have those very closely
allied forms, marked for me by Mr. Watson as doubtful species, but which
are almost universally ranked by British botanists as good and true
species.

Finally, then, varieties have the same general characters as species,
for they cannot be distinguished from species,--except, firstly, by
the discovery of intermediate linking forms, and the occurrence of
such links cannot affect the actual characters of the forms which they
connect; and except, secondly, by a certain amount of difference, for
two forms, if differing very little, are generally ranked as varieties,
notwithstanding that intermediate linking forms have not been
discovered; but the amount of difference considered necessary to give to
two forms the rank of species is quite indefinite. In genera having more
than the average number of species in any country, the species of these
genera have more than the average number of varieties. In large genera
the species are apt to be closely, but unequally, allied together,
forming little clusters round certain species. Species very closely
allied to other species apparently have restricted ranges. In all these
several respects the species of large genera present a strong analogy
with varieties. And we can clearly understand these analogies, if
species have once existed as varieties, and have thus originated:
whereas, these analogies are utterly inexplicable if each species has
been independently created.

We have, also, seen that it is the most flourishing and dominant species
of the larger genera which on an average vary most; and varieties, as
we shall hereafter see, tend to become converted into new and distinct
species. The larger genera thus tend to become larger; and throughout
nature the forms of life which are now dominant tend to become still
more dominant by leaving many modified and dominant descendants. But by
steps hereafter to be explained, the larger genera also tend to break up
into smaller genera. And thus, the forms of life throughout the universe
become divided into groups subordinate to groups.




3. STRUGGLE FOR EXISTENCE.

Bears on natural selection. The term used in a wide sense. Geometrical
powers of increase. Rapid increase of naturalised animals and plants.
Nature of the checks to increase. Competition universal. Effects of
climate. Protection from the number of individuals. Complex relations of
all animals and plants throughout nature. Struggle for life most severe
between individuals and varieties of the same species; often severe
between species of the same genus. The relation of organism to organism
the most important of all relations.

Before entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on
Natural Selection. It has been seen in the last chapter that
amongst organic beings in a state of nature there is some individual
variability; indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be called
species or sub-species or varieties; what rank, for instance, the two or
three hundred doubtful forms of British plants are entitled to hold,
if the existence of any well-marked varieties be admitted. But the
mere existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us but
little in understanding how species arise in nature. How have all those
exquisite adaptations of one part of the organisation to another part,
and to the conditions of life, and of one distinct organic being to
another being, been perfected? We see these beautiful co-adaptations
most plainly in the woodpecker and missletoe; and only a little
less plainly in the humblest parasite which clings to the hairs of a
quadruped or feathers of a bird; in the structure of the beetle which
dives through the water; in the plumed seed which is wafted by the
gentlest breeze; in short, we see beautiful adaptations everywhere and
in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far
more than do the varieties of the same species? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow inevitably from the struggle for life. Owing to this
struggle for life, any variation, however slight and from whatever cause
proceeding, if it be in any degree profitable to an individual of any
species, in its infinitely complex relations to other organic beings and
to external nature, will tend to the preservation of that individual,
and will generally be inherited by its offspring. The offspring,
also, will thus have a better chance of surviving, for, of the many
individuals of any species which are periodically born, but a small
number can survive. I have called this principle, by which each slight
variation, if useful, is preserved, by the term of Natural Selection,
in order to mark its relation to man's power of selection. We have seen
that man by selection can certainly produce great results, and can adapt
organic beings to his own uses, through the accumulation of slight
but useful variations, given to him by the hand of Nature. But Natural
Selection, as we shall hereafter see, is a power incessantly ready for
action, and is as immeasurably superior to man's feeble efforts, as the
works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for existence.
In my future work this subject shall be treated, as it well deserves,
at much greater length. The elder De Candolle and Lyell have largely
and philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently
the result of his great horticultural knowledge. Nothing is easier than
to admit in words the truth of the universal struggle for life, or more
difficult--at least I have found it so--than constantly to bear this
conclusion in mind. Yet unless it be thoroughly engrained in the mind,
I am convinced that the whole economy of nature, with every fact on
distribution, rarity, abundance, extinction, and variation, will be
dimly seen or quite misunderstood. We behold the face of nature bright
with gladness, we often see superabundance of food; we do not see, or
we forget, that the birds which are idly singing round us mostly live on
insects or seeds, and are thus constantly destroying life; or we forget
how largely these songsters, or their eggs, or their nestlings, are
destroyed by birds and beasts of prey; we do not always bear in mind,
that though food may be now superabundant, it is not so at all seasons
of each recurring year.

I should premise that I use the term Struggle for Existence in a large
and metaphorical sense, including dependence of one being on another,
and including (which is more important) not only the life of the
individual, but success in leaving progeny. Two canine animals in a time
of dearth, may be truly said to struggle with each other which shall get
food and live. But a plant on the edge of a desert is said to struggle
for life against the drought, though more properly it should be said to
be dependent on the moisture. A plant which annually produces a thousand
seeds, of which on an average only one comes to maturity, may be more
truly said to struggle with the plants of the same and other kinds which
already clothe the ground. The missletoe is dependent on the apple and a
few other trees, but can only in a far-fetched sense be said to struggle
with these trees, for if too many of these parasites grow on the same
tree, it will languish and die. But several seedling missletoes, growing
close together on the same branch, may more truly be said to struggle
with each other. As the missletoe is disseminated by birds, its
existence depends on birds; and it may metaphorically be said to
struggle with other fruit-bearing plants, in order to tempt birds to
devour and thus disseminate its seeds rather than those of other
plants. In these several senses, which pass into each other, I use for
convenience sake the general term of struggle for existence.

A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer destruction
during some period of its life, and during some season or occasional
year, otherwise, on the principle of geometrical increase, its numbers
would quickly become so inordinately great that no country could support
the product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either
one individual with another of the same species, or with the individuals
of distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase
of food, and no prudential restraint from marriage. Although some
species may be now increasing, more or less rapidly, in numbers, all
cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man has
doubled in twenty-five years, and at this rate, in a few thousand years,
there would literally not be standing room for his progeny. Linnaeus has
calculated that if an annual plant produced only two seeds--and there is
no plant so unproductive as this--and their seedlings next year produced
two, and so on, then in twenty years there would be a million plants.
The elephant is reckoned to be the slowest breeder of all known animals,
and I have taken some pains to estimate its probable minimum rate of
natural increase: it will be under the mark to assume that it breeds
when thirty years old, and goes on breeding till ninety years old,
bringing forth three pair of young in this interval; if this be so,
at the end of the fifth century there would be alive fifteen million
elephants, descended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three following
seasons. Still more striking is the evidence from our domestic animals
of many kinds which have run wild in several parts of the world: if the
statements of the rate of increase of slow-breeding cattle and horses
in South America, and latterly in Australia, had not been well
authenticated, they would have been quite incredible. So it is with
plants: cases could be given of introduced plants which have become
common throughout whole islands in a period of less than ten years.
Several of the plants now most numerous over the wide plains of La
Plata, clothing square leagues of surface almost to the exclusion of
all other plants, have been introduced from Europe; and there are plants
which now range in India, as I hear from Dr. Falconer, from Cape
Comorin to the Himalaya, which have been imported from America since its
discovery. In such cases, and endless instances could be given, no one
supposes that the fertility of these animals or plants has been
suddenly and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very favourable,
and that there has consequently been less destruction of the old and
young, and that nearly all the young have been enabled to breed. In such
cases the geometrical ratio of increase, the result of which never fails
to be surprising, simply explains the extraordinarily rapid increase and
wide diffusion of naturalised productions in their new homes.

In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to increase
at a geometrical ratio, that all would most rapidly stock every station
in which they could any how exist, and that the geometrical tendency
to increase must be checked by destruction at some period of life. Our
familiarity with the larger domestic animals tends, I think, to mislead
us: we see no great destruction falling on them, and we forget that
thousands are annually slaughtered for food, and that in a state of
nature an equal number would have somehow to be disposed of.

The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is,
that the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two: the Fulmar
petrel lays but one egg, yet it is believed to be the most numerous bird
in the world. One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one; but this difference does not determine how
many individuals of the two species can be supported in a district.
A large number of eggs is of some importance to those species, which
depend on a rapidly fluctuating amount of food, for it allows them
rapidly to increase in number. But the real importance of a large number
of eggs or seeds is to make up for much destruction at some period of
life; and this period in the great majority of cases is an early one. If
an animal can in any way protect its own eggs or young, a small number
may be produced, and yet the average stock be fully kept up; but if many
eggs or young are destroyed, many must be produced, or the species will
become extinct. It would suffice to keep up the full number of a tree,
which lived on an average for a thousand years, if a single seed were
produced once in a thousand years, supposing that this seed were never
destroyed, and could be ensured to germinate in a fitting place. So that
in all cases, the average number of any animal or plant depends only
indirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single organic
being around us may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life; that
heavy destruction inevitably falls either on the young or old, during
each generation or at recurrent intervals. Lighten any check, mitigate
the destruction ever so little, and the number of the species will
almost instantaneously increase to any amount. The face of Nature may
be compared to a yielding surface, with ten thousand sharp wedges packed
close together and driven inwards by incessant blows, sometimes one
wedge being struck, and then another with greater force.

What checks the natural tendency of each species to increase in number
is most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will its tendency to increase be still
further increased. We know not exactly what the checks are in even
one single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, so incomparably
better known than any other animal. This subject has been ably treated
by several authors, and I shall, in my future work, discuss some of the
checks at considerable length, more especially in regard to the feral
animals of South America. Here I will make only a few remarks, just to
recall to the reader's mind some of the chief points. Eggs or very young
animals seem generally to suffer most, but this is not invariably the
case. With plants there is a vast destruction of seeds, but, from some
observations which I have made, I believe that it is the seedlings which
suffer most from germinating in ground already thickly stocked with
other plants. Seedlings, also, are destroyed in vast numbers by various
enemies; for instance, on a piece of ground three feet long and two
wide, dug and cleared, and where there could be no choking from other
plants, I marked all the seedlings of our native weeds as they came up,
and out of the 357 no less than 295 were destroyed, chiefly by slugs
and insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown,
plants: thus out of twenty species growing on a little plot of turf
(three feet by four) nine species perished from the other species being
allowed to grow up freely.

The amount of food for each species of course gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average numbers of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually killed. On the other hand, in some cases, as with the elephant
and rhinoceros, none are destroyed by beasts of prey: even the tiger in
India most rarely dares to attack a young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought, I believe
to be the most effective of all checks. I estimated that the winter of
1854-55 destroyed four-fifths of the birds in my own grounds; and this
is a tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it
brings on the most severe struggle between the individuals, whether of
the same or of distinct species, which subsist on the same kind of food.
Even when climate, for instance extreme cold, acts directly, it will
be the least vigorous, or those which have got least food through the
advancing winter, which will suffer most. When we travel from south to
north, or from a damp region to a dry, we invariably see some species
gradually getting rarer and rarer, and finally disappearing; and the
change of climate being conspicuous, we are tempted to attribute the
whole effect to its direct action. But this is a very false view: we
forget that each species, even where it most abounds, is constantly
suffering enormous destruction at some period of its life, from enemies
or from competitors for the same place and food; and if these enemies
or competitors be in the least degree favoured by any slight change of
climate, they will increase in numbers, and, as each area is already
fully stocked with inhabitants, the other species will decrease. When
we travel southward and see a species decreasing in numbers, we may feel
sure that the cause lies quite as much in other species being favoured,
as in this one being hurt. So it is when we travel northward, but in
a somewhat lesser degree, for the number of species of all kinds,
and therefore of competitors, decreases northwards; hence in going
northward, or in ascending a mountain, we far oftener meet with stunted
forms, due to the DIRECTLY injurious action of climate, than we do in
proceeding southwards or in descending a mountain. When we reach
the Arctic regions, or snow-capped summits, or absolute deserts, the
struggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species, we
may clearly see in the prodigious number of plants in our gardens
which can perfectly well endure our climate, but which never become
naturalised, for they cannot compete with our native plants, nor resist
destruction by our native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this
seems generally to occur with our game animals--often ensue: and here
we have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of
diffusion amongst the crowded animals, been disproportionably favoured:
and here comes in a sort of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during winter: but any one who has tried, knows how
troublesome it is to get seed from a few wheat or other such plants in
a garden; I have in this case lost every single seed. This view of the
necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such as that of very
rare plants being sometimes extremely abundant in the few spots where
they do occur; and that of some social plants being social, that is,
abounding in individuals, even on the extreme confines of their range.
For in such cases, we may believe, that a plant could exist only where
the conditions of its life were so favourable that many could exist
together, and thus save each other from utter destruction. I should add
that the good effects of frequent intercrossing, and the ill effects of
close interbreeding, probably come into play in some of these cases; but
on this intricate subject I will not here enlarge.

Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance, which,
though a simple one, has interested me. In Staffordshire, on the estate
of a relation where I had ample means of investigation, there was a
large and extremely barren heath, which had never been touched by the
hand of man; but several hundred acres of exactly the same nature had
been enclosed twenty-five years previously and planted with Scotch fir.
The change in the native vegetation of the planted part of the heath was
most remarkable, more than is generally seen in passing from one quite
different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not
counting grasses and carices) flourished in the plantations, which could
not be found on the heath. The effect on the insects must have been
still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see how
potent has been the effect of the introduction of a single tree, nothing
whatever else having been done, with the exception that the land had
been enclosed, so that cattle could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on
the distant hill-tops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live.

When I ascertained that these young trees had not been sown or planted,
I was so much surprised at their numbers that I went to several points
of view, whence I could examine hundreds of acres of the unenclosed
heath, and literally I could not see a single Scotch fir, except the old
planted clumps. But on looking closely between the stems of the heath,
I found a multitude of seedlings and little trees, which had been
perpetually browsed down by the cattle. In one square yard, at a
point some hundred yards distant from one of the old clumps, I counted
thirty-two little trees; and one of them, judging from the rings of
growth, had during twenty-six years tried to raise its head above the
stems of the heath, and had failed. No wonder that, as soon as the land
was enclosed, it became thickly clothed with vigorously growing young
firs. Yet the heath was so extremely barren and so extensive that no
one would ever have imagined that cattle would have so closely and
effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence
of cattle. Perhaps Paraguay offers the most curious instance of this;
for here neither cattle nor horses nor dogs have ever run wild, though
they swarm southward and northward in a feral state; and Azara and
Rengger have shown that this is caused by the greater number in Paraguay
of a certain fly, which lays its eggs in the navels of these animals
when first born. The increase of these flies, numerous as they are,
must be habitually checked by some means, probably by birds. Hence, if
certain insectivorous birds (whose numbers are probably regulated by
hawks or beasts of prey) were to increase in Paraguay, the flies would
decrease--then cattle and horses would become feral, and this would
certainly greatly alter (as indeed I have observed in parts of South
America) the vegetation: this again would largely affect the insects;
and this, as we just have seen in Staffordshire, the insectivorous
birds, and so onwards in ever-increasing circles of complexity. We began
this series by insectivorous birds, and we have ended with them. Not
that in nature the relations can ever be as simple as this. Battle
within battle must ever be recurring with varying success; and yet in
the long-run the forces are so nicely balanced, that the face of nature
remains uniform for long periods of time, though assuredly the merest
trifle would often give the victory to one organic being over another.
Nevertheless so profound is our ignorance, and so high our presumption,
that we marvel when we hear of the extinction of an organic being; and
as we do not see the cause, we invoke cataclysms to desolate the world,
or invent laws on the duration of the forms of life!

I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of
complex relations. I shall hereafter have occasion to show that the
exotic Lobelia fulgens, in this part of England, is never visited by
insects, and consequently, from its peculiar structure, never can set a
seed. Many of our orchidaceous plants absolutely require the visits of
moths to remove their pollen-masses and thus to fertilise them. I
have, also, reason to believe that humble-bees are indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not
visit this flower. From experiments which I have tried, I have found
that the visits of bees, if not indispensable, are at least highly
beneficial to the fertilisation of our clovers; but humble-bees alone
visit the common red clover (Trifolium pratense), as other bees cannot
reach the nectar. Hence I have very little doubt, that if the whole
genus of humble-bees became extinct or very rare in England, the
heartsease and red clover would become very rare, or wholly disappear.
The number of humble-bees in any district depends in a great degree on
the number of field-mice, which destroy their combs and nests; and Mr.
H. Newman, who has long attended to the habits of humble-bees, believes
that "more than two thirds of them are thus destroyed all over England."
Now the number of mice is largely dependent, as every one knows, on the
number of cats; and Mr. Newman says, "Near villages and small towns I
have found the nests of humble-bees more numerous than elsewhere, which
I attribute to the number of cats that destroy the mice." Hence it is
quite credible that the presence of a feline animal in large numbers in
a district might determine, through the intervention first of mice and
then of bees, the frequency of certain flowers in that district!

In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come
into play; some one check or some few being generally the most potent,
but all concurring in determining the average number or even
the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what we
call chance. But how false a view is this! Every one has heard that when
an American forest is cut down, a very different vegetation springs
up; but it has been observed that the trees now growing on the ancient
Indian mounds, in the Southern United States, display the same beautiful
diversity and proportion of kinds as in the surrounding virgin forests.
What a struggle between the several kinds of trees must here have gone
on during long centuries, each annually scattering its seeds by the
thousand; what war between insect and insect--between insects, snails,
and other animals with birds and beasts of prey--all striving to
increase, and all feeding on each other or on the trees or their seeds
and seedlings, or on the other plants which first clothed the ground and
thus checked the growth of the trees! Throw up a handful of feathers,
and all must fall to the ground according to definite laws; but how
simple is this problem compared to the action and reaction of the
innumerable plants and animals which have determined, in the course of
centuries, the proportional numbers and kinds of trees now growing on
the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This
is often the case with those which may strictly be said to struggle with
each other for existence, as in the case of locusts and grass-feeding
quadrupeds. But the struggle almost invariably will be most severe
between the individuals of the same species, for they frequent the same
districts, require the same food, and are exposed to the same dangers.
In the case of varieties of the same species, the struggle will
generally be almost equally severe, and we sometimes see the contest
soon decided: for instance, if several varieties of wheat be sown
together, and the mixed seed be resown, some of the varieties which best
suit the soil or climate, or are naturally the most fertile, will beat
the others and so yield more seed, and will consequently in a few years
quite supplant the other varieties. To keep up a mixed stock of even
such extremely close varieties as the variously coloured sweet-peas,
they must be each year harvested separately, and the seed then mixed
in due proportion, otherwise the weaker kinds will steadily decrease in
numbers and disappear. So again with the varieties of sheep: it has
been asserted that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same
result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any
one of our domestic plants or animals have so exactly the same strength,
habits, and constitution, that the original proportions of a mixed stock
could be kept up for half a dozen generations, if they were allowed to
struggle together, like beings in a state of nature, and if the seed or
young were not annually sorted.

As species of the same genus have usually, though by no means
invariably, some similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between species
of the same genus, when they come into competition with each other, than
between species of distinct genera. We see this in the recent extension
over parts of the United States of one species of swallow having
caused the decrease of another species. The recent increase of the
missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia
the small Asiatic cockroach has everywhere driven before it its great
congener. One species of charlock will supplant another, and so in
other cases. We can dimly see why the competition should be most severe
between allied forms, which fill nearly the same place in the economy
of nature; but probably in no one case could we precisely say why one
species has been victorious over another in the great battle of life.

A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other
organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the tiger; and in
that of the legs and claws of the parasite which clings to the hair on
the tiger's body. But in the beautifully plumed seed of the dandelion,
and in the flattened and fringed legs of the water-beetle, the relation
seems at first confined to the elements of air and water. Yet the
advantage of plumed seeds no doubt stands in the closest relation to the
land being already thickly clothed by other plants; so that the
seeds may be widely distributed and fall on unoccupied ground. In the
water-beetle, the structure of its legs, so well adapted for diving,
allows it to compete with other aquatic insects, to hunt for its own
prey, and to escape serving as prey to other animals.

The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds (as peas and
beans), when sown in the midst of long grass, I suspect that the chief
use of the nutriment in the seed is to favour the growth of the young
seedling, whilst struggling with other plants growing vigorously all
around.

Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wished in imagination to give the plant
the power of increasing in number, we should have to give it some
advantage over its competitors, or over the animals which preyed on it.
On the confines of its geographical range, a change of constitution with
respect to climate would clearly be an advantage to our plant; but we
have reason to believe that only a few plants or animals range so far,
that they are destroyed by the rigour of the climate alone. Not until
we reach the extreme confines of life, in the arctic regions or on the
borders of an utter desert, will competition cease. The land may be
extremely cold or dry, yet there will be competition between some few
species, or between the individuals of the same species, for the warmest
or dampest spots.

Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly
the same as in its former home, yet the conditions of its life will
generally be changed in an essential manner. If we wished to increase
its average numbers in its new home, we should have to modify it in a
different way to what we should have done in its native country; for
we should have to give it some advantage over a different set of
competitors or enemies.

It is good thus to try in our imagination to give any form some
advantage over another. Probably in no single instance should we know
what to do, so as to succeed. It will convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,
as it seems to be difficult to acquire. All that we can do, is to keep
steadily in mind that each organic being is striving to increase at a
geometrical ratio; that each at some period of its life, during some
season of the year, during each generation or at intervals, has to
struggle for life, and to suffer great destruction. When we reflect on
this struggle, we may console ourselves with the full belief, that the
war of nature is not incessant, that no fear is felt, that death is
generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.




4.

NATURAL SELECTION.

Natural Selection: its power compared with man's selection, its power
on characters of trifling importance, its power at all ages and on
both sexes. Sexual Selection. On the generality of intercrosses
between individuals of the same species. Circumstances favourable and
unfavourable to Natural Selection, namely, intercrossing, isolation,
number of individuals. Slow action. Extinction caused by Natural
Selection. Divergence of Character, related to the diversity of
inhabitants of any small area, and to naturalisation. Action of Natural
Selection, through Divergence of Character and Extinction, on the
descendants from a common parent. Explains the Grouping of all organic
beings.

How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply in nature? I
think we shall see that it can act most effectually. Let it be borne
in mind in what an endless number of strange peculiarities our domestic
productions, and, in a lesser degree, those under nature, vary; and how
strong the hereditary tendency is. Under domestication, it may be truly
said that the whole organisation becomes in some degree plastic. Let it
be borne in mind how infinitely complex and close-fitting are the mutual
relations of all organic beings to each other and to their physical
conditions of life. Can it, then, be thought improbable, seeing
that variations useful to man have undoubtedly occurred, that other
variations useful in some way to each being in the great and complex
battle of life, should sometimes occur in the course of thousands of
generations? If such do occur, can we doubt (remembering that many more
individuals are born than can possibly survive) that individuals having
any advantage, however slight, over others, would have the best chance
of surviving and of procreating their kind? On the other hand, we may
feel sure that any variation in the least degree injurious would be
rigidly destroyed. This preservation of favourable variations and the
rejection of injurious variations, I call Natural Selection. Variations
neither useful nor injurious would not be affected by natural selection,
and would be left a fluctuating element, as perhaps we see in the
species called polymorphic.

We shall best understand the probable course of natural selection
by taking the case of a country undergoing some physical change, for
instance, of climate. The proportional numbers of its inhabitants would
almost immediately undergo a change, and some species might become
extinct. We may conclude, from what we have seen of the intimate and
complex manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of some of the
inhabitants, independently of the change of climate itself, would most
seriously affect many of the others. If the country were open on its
borders, new forms would certainly immigrate, and this also would
seriously disturb the relations of some of the former inhabitants. Let
it be remembered how powerful the influence of a single introduced tree
or mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better adapted
forms could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up, if some of the
original inhabitants were in some manner modified; for, had the area
been open to immigration, these same places would have been seized on by
intruders. In such case, every slight modification, which in the course
of ages chanced to arise, and which in any way favoured the individuals
of any of the species, by better adapting them to their altered
conditions, would tend to be preserved; and natural selection would thus
have free scope for the work of improvement.

We have reason to believe, as stated in the first chapter, that a change
in the conditions of life, by specially acting on the reproductive
system, causes or increases variability; and in the foregoing case the
conditions of life are supposed to have undergone a change, and this
would manifestly be favourable to natural selection, by giving a
better chance of profitable variations occurring; and unless profitable
variations do occur, natural selection can do nothing. Not that, as
I believe, any extreme amount of variability is necessary; as man can
certainly produce great results by adding up in any given direction
mere individual differences, so could Nature, but far more easily, from
having incomparably longer time at her disposal. Nor do I believe that
any great physical change, as of climate, or any unusual degree of
isolation to check immigration, is actually necessary to produce new
and unoccupied places for natural selection to fill up by modifying and
improving some of the varying inhabitants. For as all the inhabitants
of each country are struggling together with nicely balanced forces,
extremely slight modifications in the structure or habits of one
inhabitant would often give it an advantage over others; and still
further modifications of the same kind would often still further
increase the advantage. No country can be named in which all the native
inhabitants are now so perfectly adapted to each other and to the
physical conditions under which they live, that none of them could
anyhow be improved; for in all countries, the natives have been so far
conquered by naturalised productions, that they have allowed foreigners
to take firm possession of the land. And as foreigners have thus
everywhere beaten some of the natives, we may safely conclude that the
natives might have been modified with advantage, so as to have better
resisted such intruders.

As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature
effect? Man can act only on external and visible characters: nature
cares nothing for appearances, except in so far as they may be useful
to any being. She can act on every internal organ, on every shade of
constitutional difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being which she
tends. Every selected character is fully exercised by her; and the being
is placed under well-suited conditions of life. Man keeps the natives
of many climates in the same country; he seldom exercises each selected
character in some peculiar and fitting manner; he feeds a long and a
short beaked pigeon on the same food; he does not exercise a long-backed
or long-legged quadruped in any peculiar manner; he exposes sheep with
long and short wool to the same climate. He does not allow the most
vigorous males to struggle for the females. He does not rigidly destroy
all inferior animals, but protects during each varying season, as far as
lies in his power, all his productions. He often begins his selection
by some half-monstrous form; or at least by some modification prominent
enough to catch his eye, or to be plainly useful to him. Under nature,
the slightest difference of structure or constitution may well turn the
nicely-balanced scale in the struggle for life, and so be preserved.
How fleeting are the wishes and efforts of man! how short his time!
and consequently how poor will his products be, compared with those
accumulated by nature during whole geological periods. Can we wonder,
then, that nature's productions should be far "truer" in character than
man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of
far higher workmanship?

It may be said that natural selection is daily and hourly scrutinising,
throughout the world, every variation, even the slightest; rejecting
that which is bad, preserving and adding up all that is good; silently
and insensibly working, whenever and wherever opportunity offers, at
the improvement of each organic being in relation to its organic and
inorganic conditions of life. We see nothing of these slow changes in
progress, until the hand of time has marked the long lapse of ages, and
then so imperfect is our view into long past geological ages, that
we only see that the forms of life are now different from what they
formerly were.

Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white
in winter, the red-grouse the colour of heather, and the black-grouse
that of peaty earth, we must believe that these tints are of service to
these birds and insects in preserving them from danger. Grouse, if not
destroyed at some period of their lives, would increase in countless
numbers; they are known to suffer largely from birds of prey; and hawks
are guided by eyesight to their prey,--so much so, that on parts of the
Continent persons are warned not to keep white pigeons, as being the
most liable to destruction. Hence I can see no reason to doubt that
natural selection might be most effective in giving the proper colour
to each kind of grouse, and in keeping that colour, when once acquired,
true and constant. Nor ought we to think that the occasional destruction
of an animal of any particular colour would produce little effect: we
should remember how essential it is in a flock of white sheep to destroy
every lamb with the faintest trace of black. In plants the down on
the fruit and the colour of the flesh are considered by botanists
as characters of the most trifling importance: yet we hear from
an excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a curculio, than
those with down; that purple plums suffer far more from a certain
disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh.
If, with all the aids of art, these slight differences make a great
difference in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other trees and
with a host of enemies, such differences would effectually settle which
variety, whether a smooth or downy, a yellow or purple fleshed fruit,
should succeed.

In looking at many small points of difference between species, which, as
far as our ignorance permits us to judge, seem to be quite unimportant,
we must not forget that climate, food, etc., probably produce some
slight and direct effect. It is, however, far more necessary to bear in
mind that there are many unknown laws of correlation of growth, which,
when one part of the organisation is modified through variation, and the
modifications are accumulated by natural selection for the good of the
being, will cause other modifications, often of the most unexpected
nature.

As we see that those variations which under domestication appear at any
particular period of life, tend to reappear in the offspring at the
same period;--for instance, in the seeds of the many varieties of our
culinary and agricultural plants; in the caterpillar and cocoon stages
of the varieties of the silkworm; in the eggs of poultry, and in the
colour of the down of their chickens; in the horns of our sheep and
cattle when nearly adult;--so in a state of nature, natural selection
will be enabled to act on and modify organic beings at any age, by
the accumulation of profitable variations at that age, and by their
inheritance at a corresponding age. If it profit a plant to have its
seeds more and more widely disseminated by the wind, I can see no
greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify
and adapt the larva of an insect to a score of contingencies,
wholly different from those which concern the mature insect. These
modifications will no doubt affect, through the laws of correlation, the
structure of the adult; and probably in the case of those insects which
live only for a few hours, and which never feed, a large part of their
structure is merely the correlated result of successive changes in the
structure of their larvae. So, conversely, modifications in the adult
will probably often affect the structure of the larva; but in all cases
natural selection will ensure that modifications consequent on other
modifications at a different period of life, shall not be in the least
degree injurious: for if they became so, they would cause the extinction
of the species.

Natural selection will modify the structure of the young in relation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the community; if each in consequence profits by the selected change.
What natural selection cannot do, is to modify the structure of one
species, without giving it any advantage, for the good of another
species; and though statements to this effect may be found in works of
natural history, I cannot find one case which will bear investigation.
A structure used only once in an animal's whole life, if of high
importance to it, might be modified to any extent by natural selection;
for instance, the great jaws possessed by certain insects, and used
exclusively for opening the cocoon--or the hard tip to the beak of
nestling birds, used for breaking the egg. It has been asserted, that
of the best short-beaked tumbler-pigeons more perish in the egg than are
able to get out of it; so that fanciers assist in the act of hatching.
Now, if nature had to make the beak of a full-grown pigeon very short
for the bird's own advantage, the process of modification would be very
slow, and there would be simultaneously the most rigorous selection of
the young birds within the egg, which had the most powerful and hardest
beaks, for all with weak beaks would inevitably perish: or, more
delicate and more easily broken shells might be selected, the thickness
of the shell being known to vary like every other structure.

SEXUAL SELECTION.

Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, the same fact probably
occurs under nature, and if so, natural selection will be able to modify
one sex in its functional relations to the other sex, or in relation to
wholly different habits of life in the two sexes, as is sometimes the
case with insects. And this leads me to say a few words on what I call
Sexual Selection. This depends, not on a struggle for existence, but on
a struggle between the males for possession of the females; the result
is not death to the unsuccessful competitor, but few or no offspring.
Sexual selection is, therefore, less rigorous than natural selection.
Generally, the most vigorous males, those which are best fitted for
their places in nature, will leave most progeny. But in many cases,
victory will depend not on general vigour, but on having special
weapons, confined to the male sex. A hornless stag or spurless cock
would have a poor chance of leaving offspring. Sexual selection by
always allowing the victor to breed might surely give indomitable
courage, length to the spur, and strength to the wing to strike in the
spurred leg, as well as the brutal cock-fighter, who knows well that he
can improve his breed by careful selection of the best cocks. How low
in the scale of nature this law of battle descends, I know not; male
alligators have been described as fighting, bellowing, and whirling
round, like Indians in a war-dance, for the possession of the females;
male salmons have been seen fighting all day long; male stag-beetles
often bear wounds from the huge mandibles of other males. The war is,
perhaps, severest between the males of polygamous animals, and these
seem oftenest provided with special weapons. The males of carnivorous
animals are already well armed; though to them and to others, special
means of defence may be given through means of sexual selection, as the
mane to the lion, the shoulder-pad to the boar, and the hooked jaw to
the male salmon; for the shield may be as important for victory, as the
sword or spear.

Amongst birds, the contest is often of a more peaceful character.
All those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract by singing
the females. The rock-thrush of Guiana, birds of Paradise, and some
others, congregate; and successive males display their gorgeous plumage
and perform strange antics before the females, which standing by as
spectators, at last choose the most attractive partner. Those who have
closely attended to birds in confinement well know that they often take
individual preferences and dislikes: thus Sir R. Heron has described how
one pied peacock was eminently attractive to all his hen birds. It may
appear childish to attribute any effect to such apparently weak means: I
cannot here enter on the details necessary to support this view; but if
man can in a short time give elegant carriage and beauty to his bantams,
according to his standard of beauty, I can see no good reason to doubt
that female birds, by selecting, during thousands of generations,
the most melodious or beautiful males, according to their standard of
beauty, might produce a marked effect. I strongly suspect that some
well-known laws with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can be explained on the view
of plumage having been chiefly modified by sexual selection, acting when
the birds have come to the breeding age or during the breeding season;
the modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I
have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour,
or ornament, such differences have been mainly caused by sexual
selection; that is, individual males have had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; and have transmitted these advantages to
their male offspring. Yet, I would not wish to attribute all such
sexual differences to this agency: for we see peculiarities arising and
becoming attached to the male sex in our domestic animals (as the wattle
in male carriers, horn-like protuberances in the cocks of certain fowls,
etc.), which we cannot believe to be either useful to the males in
battle, or attractive to the females. We see analogous cases under
nature, for instance, the tuft of hair on the breast of the turkey-cock,
which can hardly be either useful or ornamental to this bird;--indeed,
had the tuft appeared under domestication, it would have been called a
monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased in
numbers, during that season of the year when the wolf is hardest pressed
for food. I can under such circumstances see no reason to doubt that the
swiftest and slimmest wolves would have the best chance of surviving,
and so be preserved or selected,--provided always that they retained
strength to master their prey at this or at some other period of the
year, when they might be compelled to prey on other animals. I can see
no more reason to doubt this, than that man can improve the fleetness
of his greyhounds by careful and methodical selection, or by that
unconscious selection which results from each man trying to keep the
best dogs without any thought of modifying the breed.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats
rather than mice is known to be inherited. Now, if any slight innate
change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its
young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent-form of wolf. Or, again,
the wolves inhabiting a mountainous district, and those frequenting the
lowlands, would naturally be forced to hunt different prey; and from the
continued preservation of the individuals best fitted for the two sites,
two varieties might slowly be formed. These varieties would cross and
blend where they met; but to this subject of intercrossing we shall soon
have to return. I may add, that, according to Mr. Pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains in the United
States, one with a light greyhound-like form, which pursues deer, and
the other more bulky, with shorter legs, which more frequently attacks
the shepherd's flocks.

Let us now take a more complex case. Certain plants excrete a sweet
juice, apparently for the sake of eliminating something injurious from
their sap: this is effected by glands at the base of the stipules in
some Leguminosae, and at the back of the leaf of the common laurel. This
juice, though small in quantity, is greedily sought by insects. Let us
now suppose a little sweet juice or nectar to be excreted by the inner
bases of the petals of a flower. In this case insects in seeking the
nectar would get dusted with pollen, and would certainly often transport
the pollen from one flower to the stigma of another flower. The flowers
of two distinct individuals of the same species would thus get crossed;
and the act of crossing, we have good reason to believe (as will
hereafter be more fully alluded to), would produce very vigorous
seedlings, which consequently would have the best chance of flourishing
and surviving. Some of these seedlings would probably inherit the
nectar-excreting power. Those individual flowers which had the largest
glands or nectaries, and which excreted most nectar, would be oftenest
visited by insects, and would be oftenest crossed; and so in the
long-run would gain the upper hand. Those flowers, also, which had their
stamens and pistils placed, in relation to the size and habits of the
particular insects which visited them, so as to favour in any degree
the transportal of their pollen from flower to flower, would likewise be
favoured or selected. We might have taken the case of insects visiting
flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole object of fertilisation, its destruction
appears a simple loss to the plant; yet if a little pollen were carried,
at first occasionally and then habitually, by the pollen-devouring
insects from flower to flower, and a cross thus effected, although
nine-tenths of the pollen were destroyed, it might still be a great gain
to the plant; and those individuals which produced more and more pollen,
and had larger and larger anthers, would be selected.

When our plant, by this process of the continued preservation or natural
selection of more and more attractive flowers, had been rendered highly
attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they can most
effectually do this, I could easily show by many striking instances.
I will give only one--not as a very striking case, but as likewise
illustrating one step in the separation of the sexes of plants,
presently to be alluded to. Some holly-trees bear only male flowers,
which have four stamens producing rather a small quantity of pollen, and
a rudimentary pistil; other holly-trees bear only female flowers; these
have a full-sized pistil, and four stamens with shrivelled anthers, in
which not a grain of pollen can be detected. Having found a female
tree exactly sixty yards from a male tree, I put the stigmas of twenty
flowers, taken from different branches, under the microscope, and
on all, without exception, there were pollen-grains, and on some a
profusion of pollen. As the wind had set for several days from the
female to the male tree, the pollen could not thus have been carried.
The weather had been cold and boisterous, and therefore not favourable
to bees, nevertheless every female flower which I examined had been
effectually fertilised by the bees, accidentally dusted with pollen,
having flown from tree to tree in search of nectar. But to return to
our imaginary case: as soon as the plant had been rendered so highly
attractive to insects that pollen was regularly carried from flower
to flower, another process might commence. No naturalist doubts the
advantage of what has been called the "physiological division of
labour;" hence we may believe that it would be advantageous to a plant
to produce stamens alone in one flower or on one whole plant, and
pistils alone in another flower or on another plant. In plants under
culture and placed under new conditions of life, sometimes the male
organs and sometimes the female organs become more or less impotent;
now if we suppose this to occur in ever so slight a degree under nature,
then as pollen is already carried regularly from flower to flower,
and as a more complete separation of the sexes of our plant would be
advantageous on the principle of the division of labour, individuals
with this tendency more and more increased, would be continually
favoured or selected, until at last a complete separation of the sexes
would be effected.

Let us now turn to the nectar-feeding insects in our imaginary case: we
may suppose the plant of which we have been slowly increasing the nectar
by continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many facts,
showing how anxious bees are to save time; for instance, their habit of
cutting holes and sucking the nectar at the bases of certain flowers,
which they can, with a very little more trouble, enter by the mouth.
Bearing such facts in mind, I can see no reason to doubt that an
accidental deviation in the size and form of the body, or in the
curvature and length of the proboscis, etc., far too slight to be
appreciated by us, might profit a bee or other insect, so that an
individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving descendants.
Its descendants would probably inherit a tendency to a similar slight
deviation of structure. The tubes of the corollas of the common red and
incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty
glance appear to differ in length; yet the hive-bee can easily suck
the nectar out of the incarnate clover, but not out of the common red
clover, which is visited by humble-bees alone; so that whole fields of
the red clover offer in vain an abundant supply of precious nectar to
the hive-bee. Thus it might be a great advantage to the hive-bee to have
a slightly longer or differently constructed proboscis. On the other
hand, I have found by experiment that the fertility of clover greatly
depends on bees visiting and moving parts of the corolla, so as to push
the pollen on to the stigmatic surface. Hence, again, if humble-bees
were to become rare in any country, it might be a great advantage to the
red clover to have a shorter or more deeply divided tube to its corolla,
so that the hive-bee could visit its flowers. Thus I can understand how
a flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted in the most perfect manner to each
other, by the continued preservation of individuals presenting mutual
and slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which were
at first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now very
seldom hear the action, for instance, of the coast-waves, called a
trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic beings, or of
any great and sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two
individuals must always unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaphrodites two individuals, either
occasionally or habitually, concur for the reproduction of their kind.
This view, I may add, was first suggested by Andrew Knight. We shall
presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample
discussion. All vertebrate animals, all insects, and some other large
groups of animals, pair for each birth. Modern research has much
diminished the number of supposed hermaphrodites, and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may be
asked, is there for supposing in these cases that two individuals ever
concur in reproduction? As it is impossible here to enter on details, I
must trust to some general considerations alone.

In the first place, I have collected so large a body of facts, showing,
in accordance with the almost universal belief of breeders, that with
animals and plants a cross between different varieties, or between
individuals of the same variety but of another strain, gives vigour
and fertility to the offspring; and on the other hand, that CLOSE
interbreeding diminishes vigour and fertility; that these facts alone
incline me to believe that it is a general law of nature (utterly
ignorant though we be of the meaning of the law) that no organic being
self-fertilises itself for an eternity of generations; but that a
cross with another individual is occasionally--perhaps at very long
intervals--indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any
other view are inexplicable. Every hybridizer knows how unfavourable
exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to
the weather! but if an occasional cross be indispensable, the fullest
freedom for the entrance of pollen from another individual will explain
this state of exposure, more especially as the plant's own anthers and
pistil generally stand so close together that self-fertilisation seems
almost inevitable. Many flowers, on the other hand, have their organs
of fructification closely enclosed, as in the great papilionaceous or
pea-family; but in several, perhaps in all, such flowers, there is a
very curious adaptation between the structure of the flower and the
manner in which bees suck the nectar; for, in doing this, they either
push the flower's own pollen on the stigma, or bring pollen from another
flower. So necessary are the visits of bees to papilionaceous flowers,
that I have found, by experiments published elsewhere, that their
fertility is greatly diminished if these visits be prevented. Now, it
is scarcely possible that bees should fly from flower to flower, and not
carry pollen from one to the other, to the great good, as I believe,
of the plant. Bees will act like a camel-hair pencil, and it is quite
sufficient just to touch the anthers of one flower and then the stigma
of another with the same brush to ensure fertilisation; but it must not
be supposed that bees would thus produce a multitude of hybrids between
distinct species; for if you bring on the same brush a plant's own
pollen and pollen from another species, the former will have such a
prepotent effect, that it will invariably and completely destroy, as has
been shown by Gartner, any influence from the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but, the agency of insects is often required to cause the
stamens to spring forward, as Kolreuter has shown to be the case with
the barberry; and curiously in this very genus, which seems to have a
special contrivance for self-fertilisation, it is well known that if
very closely-allied forms or varieties are planted near each other, it
is hardly possible to raise pure seedlings, so largely do they
naturally cross. In many other cases, far from there being any aids for
self-fertilisation, there are special contrivances, as I could show
from the writings of C. C. Sprengel and from my own observations, which
effectually prevent the stigma receiving pollen from its own flower: for
instance, in Lobelia fulgens, there is a really beautiful and
elaborate contrivance by which every one of the infinitely numerous
pollen-granules are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready to receive them;
and as this flower is never visited, at least in my garden, by insects,
it never sets a seed, though by placing pollen from one flower on the
stigma of another, I raised plenty of seedlings; and whilst another
species of Lobelia growing close by, which is visited by bees, seeds
freely. In very many other cases, though there be no special mechanical
contrivance to prevent the stigma of a flower receiving its own pollen,
yet, as C. C. Sprengel has shown, and as I can confirm, either the
anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that flower is ready, so that these
plants have in fact separated sexes, and must habitually be crossed.
How strange are these facts! How strange that the pollen and stigmatic
surface of the same flower, though placed so close together, as if
for the very purpose of self-fertilisation, should in so many cases be
mutually useless to each other! How simply are these facts explained
on the view of an occasional cross with a distinct individual being
advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I
have found, of the seedlings thus raised will turn out mongrels: for
instance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only 78 were true to
their kind, and some even of these were not perfectly true. Yet the
pistil of each cabbage-flower is surrounded not only by its own six
stamens, but by those of the many other flowers on the same plant. How,
then, comes it that such a vast number of the seedlings are mongrelized?
I suspect that it must arise from the pollen of a distinct VARIETY
having a prepotent effect over a flower's own pollen; and that this is
part of the general law of good being derived from the intercrossing
of distinct individuals of the same species. When distinct SPECIES are
crossed the case is directly the reverse, for a plant's own pollen
is always prepotent over foreign pollen; but to this subject we shall
return in a future chapter.

In the case of a gigantic tree covered with innumerable flowers, it may
be objected that pollen could seldom be carried from tree to tree, and
at most only from flower to flower on the same tree, and that flowers
on the same tree can be considered as distinct individuals only in a
limited sense. I believe this objection to be valid, but that nature has
largely provided against it by giving to trees a strong tendency to bear
flowers with separated sexes. When the sexes are separated, although
the male and female flowers may be produced on the same tree, we can see
that pollen must be regularly carried from flower to flower; and this
will give a better chance of pollen being occasionally carried from tree
to tree. That trees belonging to all Orders have their sexes more often
separated than other plants, I find to be the case in this country; and
at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa
Gray those of the United States, and the result was as I anticipated. On
the other hand, Dr. Hooker has recently informed me that he finds that
the rule does not hold in Australia; and I have made these few remarks
on the sexes of trees simply to call attention to the subject.

Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with terrestrial plants, on the view of an
occasional cross being indispensable, by considering the medium in which
terrestrial animals live, and the nature of the fertilising element; for
we know of no means, analogous to the action of insects and of the wind
in the case of plants, by which an occasional cross could be effected
with terrestrial animals without the concurrence of two individuals.
Of aquatic animals, there are many self-fertilising hermaphrodites;
but here currents in the water offer an obvious means for an occasional
cross. And, as in the case of flowers, I have as yet failed, after
consultation with one of the highest authorities, namely, Professor
Huxley, to discover a single case of an hermaphrodite animal with the
organs of reproduction so perfectly enclosed within the body, that
access from without and the occasional influence of a distinct
individual can be shown to be physically impossible. Cirripedes long
appeared to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance, elsewhere
to prove that two individuals, though both are self-fertilising
hermaphrodites, do sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the
case of both animals and plants, species of the same family and even of
the same genus, though agreeing closely with each other in almost their
whole organisation, yet are not rarely, some of them hermaphrodites,
and some of them unisexual. But if, in fact, all hermaphrodites do
occasionally intercross with other individuals, the difference between
hermaphrodites and unisexual species, as far as function is concerned,
becomes very small.

From these several considerations and from the many special facts which
I have collected, but which I am not here able to give, I am strongly
inclined to suspect that, both in the vegetable and animal kingdoms, an
occasional intercross with a distinct individual is a law of nature. I
am well aware that there are, on this view, many cases of difficulty,
some of which I am trying to investigate. Finally then, we may conclude
that in many organic beings, a cross between two individuals is an
obvious necessity for each birth; in many others it occurs perhaps only
at long intervals; but in none, as I suspect, can self-fertilisation go
on for perpetuity.

CIRCUMSTANCES FAVOURABLE TO NATURAL SELECTION.

This is an extremely intricate subject. A large amount of inheritable
and diversified variability is favourable, but I believe mere individual
differences suffice for the work. A large number of individuals, by
giving a better chance for the appearance within any given period
of profitable variations, will compensate for a lesser amount of
variability in each individual, and is, I believe, an extremely
important element of success. Though nature grants vast periods of time
for the work of natural selection, she does not grant an indefinite
period; for as all organic beings are striving, it may be said, to seize
on each place in the economy of nature, if any one species does
not become modified and improved in a corresponding degree with its
competitors, it will soon be exterminated.

In man's methodical selection, a breeder selects for some definite
object, and free intercrossing will wholly stop his work. But when many
men, without intending to alter the breed, have a nearly common standard
of perfection, and all try to get and breed from the best animals,
much improvement and modification surely but slowly follow from this
unconscious process of selection, notwithstanding a large amount of
crossing with inferior animals. Thus it will be in nature; for within a
confined area, with some place in its polity not so perfectly occupied
as might be, natural selection will always tend to preserve all the
individuals varying in the right direction, though in different degrees,
so as better to fill up the unoccupied place. But if the area be large,
its several districts will almost certainly present different conditions
of life; and then if natural selection be modifying and improving a
species in the several districts, there will be intercrossing with the
other individuals of the same species on the confines of each. And in
this case the effects of intercrossing can hardly be counterbalanced by
natural selection always tending to modify all the individuals in each
district in exactly the same manner to the conditions of each; for in a
continuous area, the conditions will generally graduate away insensibly
from one district to another. The intercrossing will most affect those
animals which unite for each birth, which wander much, and which do
not breed at a very quick rate. Hence in animals of this nature, for
instance in birds, varieties will generally be confined to separated
countries; and this I believe to be the case. In hermaphrodite organisms
which cross only occasionally, and likewise in animals which unite for
each birth, but which wander little and which can increase at a very
rapid rate, a new and improved variety might be quickly formed on any
one spot, and might there maintain itself in a body, so that whatever
intercrossing took place would be chiefly between the individuals of
the same new variety. A local variety when once thus formed might
subsequently slowly spread to other districts. On the above principle,
nurserymen always prefer getting seed from a large body of plants of
the same variety, as the chance of intercrossing with other varieties is
thus lessened.

Even in the case of slow-breeding animals, which unite for each birth,
we must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing
that within the same area, varieties of the same animal can long remain
distinct, from haunting different stations, from breeding at slightly
different seasons, or from varieties of the same kind preferring to pair
together.

Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but I have already
attempted to show that we have reason to believe that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the
young thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have a
better chance of surviving and propagating their kind; and thus, in the
long run, the influence of intercrosses, even at rare intervals, will be
great. If there exist organic beings which never intercross, uniformity
of character can be retained amongst them, as long as their conditions
of life remain the same, only through the principle of inheritance, and
through natural selection destroying any which depart from the
proper type; but if their conditions of life change and they undergo
modification, uniformity of character can be given to their modified
offspring, solely by natural selection preserving the same favourable
variations.

Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the
organic and inorganic conditions of life will generally be in a great
degree uniform; so that natural selection will tend to modify all the
individuals of a varying species throughout the area in the same
manner in relation to the same conditions. Intercrosses, also, with the
individuals of the same species, which otherwise would have inhabited
the surrounding and differently circumstanced districts, will be
prevented. But isolation probably acts more efficiently in checking the
immigration of better adapted organisms, after any physical change, such
as of climate or elevation of the land, etc.; and thus new places in the
natural economy of the country are left open for the old inhabitants
to struggle for, and become adapted to, through modifications in their
structure and constitution. Lastly, isolation, by checking immigration
and consequently competition, will give time for any new variety to
be slowly improved; and this may sometimes be of importance in the
production of new species. If, however, an isolated area be very small,
either from being surrounded by barriers, or from having very peculiar
physical conditions, the total number of the individuals supported on it
will necessarily be very small; and fewness of individuals will greatly
retard the production of new species through natural selection, by
decreasing the chance of the appearance of favourable variations.

If we turn to nature to test the truth of these remarks, and look at
any small isolated area, such as an oceanic island, although the total
number of the species inhabiting it, will be found to be small, as we
shall see in our chapter on geographical distribution; yet of these
species a very large proportion are endemic,--that is, have been
produced there, and nowhere else. Hence an oceanic island at first sight
seems to have been highly favourable for the production of new species.
But we may thus greatly deceive ourselves, for to ascertain whether a
small isolated area, or a large open area like a continent, has been
most favourable for the production of new organic forms, we ought to
make the comparison within equal times; and this we are incapable of
doing.

Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to believe
that largeness of area is of more importance, more especially in the
production of species, which will prove capable of enduring for a long
period, and of spreading widely. Throughout a great and open area, not
only will there be a better chance of favourable variations arising from
the large number of individuals of the same species there supported, but
the conditions of life are infinitely complex from the large number
of already existing species; and if some of these many species
become modified and improved, others will have to be improved in a
corresponding degree or they will be exterminated. Each new form, also,
as soon as it has been much improved, will be able to spread over the
open and continuous area, and will thus come into competition with many
others. Hence more new places will be formed, and the competition to
fill them will be more severe, on a large than on a small and
isolated area. Moreover, great areas, though now continuous, owing to
oscillations of level, will often have recently existed in a broken
condition, so that the good effects of isolation will generally, to a
certain extent, have concurred. Finally, I conclude that, although small
isolated areas probably have been in some respects highly favourable for
the production of new species, yet that the course of modification
will generally have been more rapid on large areas; and what is more
important, that the new forms produced on large areas, which already
have been victorious over many competitors, will be those that will
spread most widely, will give rise to most new varieties and species,
and will thus play an important part in the changing history of the
organic world.

We can, perhaps, on these views, understand some facts which will
be again alluded to in our chapter on geographical distribution; for
instance, that the productions of the smaller continent of Australia
have formerly yielded, and apparently are now yielding, before those
of the larger Europaeo-Asiatic area. Thus, also, it is that continental
productions have everywhere become so largely naturalised on islands. On
a small island, the race for life will have been less severe, and there
will have been less modification and less extermination. Hence, perhaps,
it comes that the flora of Madeira, according to Oswald Heer, resembles
the extinct tertiary flora of Europe. All fresh-water basins, taken
together, make a small area compared with that of the sea or of the
land; and, consequently, the competition between fresh-water productions
will have been less severe than elsewhere; new forms will have been
more slowly formed, and old forms more slowly exterminated. And it is
in fresh water that we find seven genera of Ganoid fishes, remnants of
a once preponderant order: and in fresh water we find some of the most
anomalous forms now known in the world, as the Ornithorhynchus and
Lepidosiren, which, like fossils, connect to a certain extent orders now
widely separated in the natural scale. These anomalous forms may almost
be called living fossils; they have endured to the present day, from
having inhabited a confined area, and from having thus been exposed to
less severe competition.

To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a
large continental area, which will probably undergo many oscillations
of level, and which consequently will exist for long periods in a broken
condition, will be the most favourable for the production of many new
forms of life, likely to endure long and to spread widely. For the area
will first have existed as a continent, and the inhabitants, at this
period numerous in individuals and kinds, will have been subjected
to very severe competition. When converted by subsidence into large
separate islands, there will still exist many individuals of the same
species on each island: intercrossing on the confines of the range of
each species will thus be checked: after physical changes of any kind,
immigration will be prevented, so that new places in the polity of
each island will have to be filled up by modifications of the old
inhabitants; and time will be allowed for the varieties in each to
become well modified and perfected. When, by renewed elevation, the
islands shall be re-converted into a continental area, there will again
be severe competition: the most favoured or improved varieties will be
enabled to spread: there will be much extinction of the less improved
forms, and the relative proportional numbers of the various inhabitants
of the renewed continent will again be changed; and again there will
be a fair field for natural selection to improve still further the
inhabitants, and thus produce new species.

That natural selection will always act with extreme slowness, I fully
admit. Its action depends on there being places in the polity of nature,
which can be better occupied by some of the inhabitants of the country
undergoing modification of some kind. The existence of such places will
often depend on physical changes, which are generally very slow, and
on the immigration of better adapted forms having been checked. But the
action of natural selection will probably still oftener depend on some
of the inhabitants becoming slowly modified; the mutual relations of
many of the other inhabitants being thus disturbed. Nothing can be
effected, unless favourable variations occur, and variation itself is
apparently always a very slow process. The process will often be greatly
retarded by free intercrossing. Many will exclaim that these several
causes are amply sufficient wholly to stop the action of natural
selection. I do not believe so. On the other hand, I do believe that
natural selection will always act very slowly, often only at long
intervals of time, and generally on only a very few of the inhabitants
of the same region at the same time. I further believe, that this very
slow, intermittent action of natural selection accords perfectly
well with what geology tells us of the rate and manner at which the
inhabitants of this world have changed.

Slow though the process of selection may be, if feeble man can do much
by his powers of artificial selection, I can see no limit to the amount
of change, to the beauty and infinite complexity of the coadaptations
between all organic beings, one with another and with their physical
conditions of life, which may be effected in the long course of time by
nature's power of selection.

EXTINCTION.

This subject will be more fully discussed in our chapter on Geology; but
it must be here alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. But as
from the high geometrical powers of increase of all organic beings, each
area is already fully stocked with inhabitants, it follows that as
each selected and favoured form increases in number, so will the less
favoured forms decrease and become rare. Rarity, as geology tells us, is
the precursor to extinction. We can, also, see that any form represented
by few individuals will, during fluctuations in the seasons or in the
number of its enemies, run a good chance of utter extinction. But we may
go further than this; for as new forms are continually and slowly being
produced, unless we believe that the number of specific forms goes on
perpetually and almost indefinitely increasing, numbers inevitably must
become extinct. That the number of specific forms has not indefinitely
increased, geology shows us plainly; and indeed we can see reason why
they should not have thus increased, for the number of places in the
polity of nature is not indefinitely great,--not that we have any means
of knowing that any one region has as yet got its maximum of species.
Probably no region is as yet fully stocked, for at the Cape of Good
Hope, where more species of plants are crowded together than in any
other quarter of the world, some foreign plants have become naturalised,
without causing, as far as we know, the extinction of any natives.

Furthermore, the species which are most numerous in individuals will
have the best chance of producing within any given period favourable
variations. We have evidence of this, in the facts given in the second
chapter, showing that it is the common species which afford the greatest
number of recorded varieties, or incipient species. Hence, rare species
will be less quickly modified or improved within any given period, and
they will consequently be beaten in the race for life by the modified
descendants of the commoner species.

From these several considerations I think it inevitably follows, that as
new species in the course of time are formed through natural selection,
others will become rarer and rarer, and finally extinct. The forms which
stand in closest competition with those undergoing modification and
improvement, will naturally suffer most. And we have seen in the
chapter on the Struggle for Existence that it is the most closely-allied
forms,--varieties of the same species, and species of the same genus
or of related genera,--which, from having nearly the same structure,
constitution, and habits, generally come into the severest competition
with each other. Consequently, each new variety or species, during the
progress of its formation, will generally press hardest on its nearest
kindred, and tend to exterminate them. We see the same process of
extermination amongst our domesticated productions, through the
selection of improved forms by man. Many curious instances could
be given showing how quickly new breeds of cattle, sheep, and other
animals, and varieties of flowers, take the place of older and inferior
kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these "were swept away
by the short-horns" (I quote the words of an agricultural writer) "as if
by some murderous pestilence."

DIVERGENCE OF CHARACTER.

The principle, which I have designated by this term, is of high
importance on my theory, and explains, as I believe, several important
facts. In the first place, varieties, even strongly-marked ones, though
having somewhat of the character of species--as is shown by the hopeless
doubts in many cases how to rank them--yet certainly differ from
each other far less than do good and distinct species. Nevertheless,
according to my view, varieties are species in the process of formation,
or are, as I have called them, incipient species. How, then, does the
lesser difference between varieties become augmented into the greater
difference between species? That this does habitually happen, we must
infer from most of the innumerable species throughout nature presenting
well-marked differences; whereas varieties, the supposed prototypes and
parents of future well-marked species, present slight and ill-defined
differences. Mere chance, as we may call it, might cause one variety
to differ in some character from its parents, and the offspring of this
variety again to differ from its parent in the very same character and
in a greater degree; but this alone would never account for so habitual
and large an amount of difference as that between varieties of the same
species and species of the same genus.

As has always been my practice, let us seek light on this head from our
domestic productions. We shall here find something analogous. A fancier
is struck by a pigeon having a slightly shorter beak; another fancier is
struck by a pigeon having a rather longer beak; and on the acknowledged
principle that "fanciers do not and will not admire a medium standard,
but like extremes," they both go on (as has actually occurred with
tumbler-pigeons) choosing and breeding from birds with longer and longer
beaks, or with shorter and shorter beaks. Again, we may suppose that at
an early period one man preferred swifter horses; another stronger and
more bulky horses. The early differences would be very slight; in the
course of time, from the continued selection of swifter horses by some
breeders, and of stronger ones by others, the differences would become
greater, and would be noted as forming two sub-breeds; finally, after
the lapse of centuries, the sub-breeds would become converted into two
well-established and distinct breeds. As the differences slowly become
greater, the inferior animals with intermediate characters, being
neither very swift nor very strong, will have been neglected, and will
have tended to disappear. Here, then, we see in man's productions
the action of what may be called the principle of divergence, causing
differences, at first barely appreciable, steadily to increase, and
the breeds to diverge in character both from each other and from their
common parent.

But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently, from the simple
circumstance that the more diversified the descendants from any one
species become in structure, constitution, and habits, by so much will
they be better enabled to seize on many and widely diversified places in
the polity of nature, and so be enabled to increase in numbers.

We can clearly see this in the case of animals with simple habits. Take
the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If
its natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions)
only by its varying descendants seizing on places at present occupied by
other animals: some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animal became, the more places they would
be enabled to occupy. What applies to one animal will apply throughout
all time to all animals--that is, if they vary--for otherwise natural
selection can do nothing. So it will be with plants. It has been
experimentally proved, that if a plot of ground be sown with one species
of grass, and a similar plot be sown with several distinct genera of
grasses, a greater number of plants and a greater weight of dry herbage
can thus be raised. The same has been found to hold good when first
one variety and then several mixed varieties of wheat have been sown on
equal spaces of ground. Hence, if any one species of grass were to go
on varying, and those varieties were continually selected which differed
from each other in at all the same manner as distinct species and genera
of grasses differ from each other, a greater number of individual plants
of this species of grass, including its modified descendants, would
succeed in living on the same piece of ground. And we well know that
each species and each variety of grass is annually sowing almost
countless seeds; and thus, as it may be said, is striving its utmost to
increase its numbers. Consequently, I cannot doubt that in the course
of many thousands of generations, the most distinct varieties of any one
species of grass would always have the best chance of succeeding and
of increasing in numbers, and thus of supplanting the less distinct
varieties; and varieties, when rendered very distinct from each other,
take the rank of species.

The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets; and so in small ponds of fresh water. Farmers find
that they can raise most food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any
small piece of ground, could live on it (supposing it not to be in
any way peculiar in its nature), and may be said to be striving to the
utmost to live there; but, it is seen, that where they come into the
closest competition with each other, the advantages of diversification
of structure, with the accompanying differences of habit and
constitution, determine that the inhabitants, which thus jostle each
other most closely, shall, as a general rule, belong to what we call
different genera and orders.

The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants
which have succeeded in becoming naturalised in any land would generally
have been closely allied to the indigenes; for these are commonly looked
at as specially created and adapted for their own country. It might,
also, perhaps have been expected that naturalised plants would have
belonged to a few groups more especially adapted to certain stations in
their new homes. But the case is very different; and Alph. De Candolle
has well remarked in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's 'Manual of the Flora of
the Northern United States,' 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants
are of a highly diversified nature. They differ, moreover, to a large
extent from the indigenes, for out of the 162 genera, no less than 100
genera are not there indigenous, and thus a large proportional addition
is made to the genera of these States.

By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we can gain some crude idea in what manner some of the
natives would have had to be modified, in order to have gained an
advantage over the other natives; and we may, I think, at least safely
infer that diversification of structure, amounting to new generic
differences, would have been profitable to them.

The advantage of diversification in the inhabitants of the same region
is, in fact, the same as that of the physiological division of labour in
the organs of the same individual body--a subject so well elucidated by
Milne Edwards. No physiologist doubts that a stomach by being adapted to
digest vegetable matter alone, or flesh alone, draws most nutriment from
these substances. So in the general economy of any land, the more widely
and perfectly the animals and plants are diversified for different
habits of life, so will a greater number of individuals be capable of
there supporting themselves. A set of animals, with their organisation
but little diversified, could hardly compete with a set more perfectly
diversified in structure. It may be doubted, for instance, whether
the Australian marsupials, which are divided into groups differing but
little from each other, and feebly representing, as Mr. Waterhouse and
others have remarked, our carnivorous, ruminant, and rodent mammals,
could successfully compete with these well-pronounced orders. In the
Australian mammals, we see the process of diversification in an early
and incomplete stage of development. After the foregoing discussion,
which ought to have been much amplified, we may, I think, assume that
the modified descendants of any one species will succeed by so much
the better as they become more diversified in structure, and are thus
enabled to encroach on places occupied by other beings. Now let us see
how this principle of great benefit being derived from divergence of
character, combined with the principles of natural selection and of
extinction, will tend to act.

The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances.
I have said a large genus, because we have seen in the second chapter,
that on an average more of the species of large genera vary than of
small genera; and the varying species of the large genera present a
greater number of varieties. We have, also, seen that the species, which
are the commonest and the most widely-diffused, vary more than rare
species with restricted ranges. Let (A) be a common, widely-diffused,
and varying species, belonging to a genus large in its own country. The
little fan of diverging dotted lines of unequal lengths proceeding from
(A), may represent its varying offspring. The variations are supposed
to be extremely slight, but of the most diversified nature; they are not
supposed all to appear simultaneously, but often after long intervals of
time; nor are they all supposed to endure for equal periods. Only
those variations which are in some way profitable will be preserved or
naturally selected. And here the importance of the principle of benefit
being derived from divergence of character comes in; for this
will generally lead to the most different or divergent variations
(represented by the outer dotted lines) being preserved and accumulated
by natural selection. When a dotted line reaches one of the horizontal
lines, and is there marked by a small numbered letter, a sufficient
amount of variation is supposed to have been accumulated to have formed
a fairly well-marked variety, such as would be thought worthy of record
in a systematic work.

The intervals between the horizontal lines in the diagram, may represent
each a thousand generations; but it would have been better if each had
represented ten thousand generations. After a thousand generations,
species (A) is supposed to have produced two fairly well-marked
varieties, namely a1 and m1. These two varieties will generally continue
to be exposed to the same conditions which made their parents variable,
and the tendency to variability is in itself hereditary, consequently
they will tend to vary, and generally to vary in nearly the same manner
as their parents varied. Moreover, these two varieties, being only
slightly modified forms, will tend to inherit those advantages which
made their common parent (A) more numerous than most of the other
inhabitants of the same country; they will likewise partake of those
more general advantages which made the genus to which the parent-species
belonged, a large genus in its own country. And these circumstances we
know to be favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of
their variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle
of divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and s2, differing
from each other, and more considerably from their common parent (A). We
may continue the process by similar steps for any length of time; some
of the varieties, after each thousand generations, producing only
a single variety, but in a more and more modified condition, some
producing two or three varieties, and some failing to produce any. Thus
the varieties or modified descendants, proceeding from the common
parent (A), will generally go on increasing in number and diverging
in character. In the diagram the process is represented up to the
ten-thousandth generation, and under a condensed and simplified form up
to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes
on so regularly as is represented in the diagram, though in itself
made somewhat irregular. I am far from thinking that the most divergent
varieties will invariably prevail and multiply: a medium form may
often long endure, and may or may not produce more than one modified
descendant; for natural selection will always act according to the
nature of the places which are either unoccupied or not perfectly
occupied by other beings; and this will depend on infinitely complex
relations. But as a general rule, the more diversified in structure the
descendants from any one species can be rendered, the more places they
will be enabled to seize on, and the more their modified progeny will
be increased. In our diagram the line of succession is broken at regular
intervals by small numbered letters marking the successive forms which
have become sufficiently distinct to be recorded as varieties. But
these breaks are imaginary, and might have been inserted anywhere, after
intervals long enough to have allowed the accumulation of a considerable
amount of divergent variation.

As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the
same advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in character:
this is represented in the diagram by the several divergent branches
proceeding from (A). The modified offspring from the later and more
highly improved branches in the lines of descent, will, it is probable,
often take the place of, and so destroy, the earlier and less improved
branches: this is represented in the diagram by some of the lower
branches not reaching to the upper horizontal lines. In some cases I do
not doubt that the process of modification will be confined to a
single line of descent, and the number of the descendants will not be
increased; although the amount of divergent modification may have been
increased in the successive generations. This case would be represented
in the diagram, if all the lines proceeding from (A) were removed,
excepting that from a1 to a10. In the same way, for instance, the
English race-horse and English pointer have apparently both gone on
slowly diverging in character from their original stocks, without either
having given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful category of
sub-species; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into well-defined species: thus the diagram illustrates
the steps by which the small differences distinguishing varieties
are increased into the larger differences distinguishing species. By
continuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we get eight
species, marked by the letters between a14 and m14, all descended from
(A). Thus, as I believe, species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of
change supposed to be represented between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n14 to z14, are supposed to have been produced. In each genus, the
species, which are already extremely different in character, will
generally tend to produce the greatest number of modified descendants;
for these will have the best chance of filling new and widely different
places in the polity of nature: hence in the diagram I have chosen the
extreme species (A), and the nearly extreme species (I), as those which
have largely varied, and have given rise to new varieties and species.
The other nine species (marked by capital letters) of our original
genus, may for a long period continue transmitting unaltered
descendants; and this is shown in the diagram by the dotted lines not
prolonged far upwards from want of space.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in
the improved descendants of any one species to supplant and exterminate
in each stage of descent their predecessors and their original parent.
For it should be remembered that the competition will generally be most
severe between those forms which are most nearly related to each other
in habits, constitution, and structure. Hence all the intermediate forms
between the earlier and later states, that is between the less and more
improved state of a species, as well as the original parent-species
itself, will generally tend to become extinct. So it probably will be
with many whole collateral lines of descent, which will be conquered by
later and improved lines of descent. If, however, the modified offspring
of a species get into some distinct country, or become quickly adapted
to some quite new station, in which child and parent do not come into
competition, both may continue to exist.

If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a14 to m14); and (I)
will have been replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally
the case in nature; species (A) being more nearly related to B, C, and
D, than to the other species; and species (I) more to G, H, K, L, than
to the others. These two species (A) and (I), were also supposed to be
very common and widely diffused species, so that they must originally
have had some advantage over most of the other species of the
genus. Their modified descendants, fourteen in number at the
fourteen-thousandth generation, will probably have inherited some of
the same advantages: they have also been modified and improved in
a diversified manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of their country.
It seems, therefore, to me extremely probable that they will have taken
the places of, and thus exterminated, not only their parents (A) and
(I), but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will
have transmitted offspring to the fourteen-thousandth generation. We may
suppose that only one (F), of the two species which were least closely
related to the other nine original species, has transmitted descendants
to this late stage of descent.

The new species in our diagram descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency
of natural selection, the extreme amount of difference in character
between species a14 and z14 will be much greater than that between
the most different of the original eleven species. The new species,
moreover, will be allied to each other in a widely different manner. Of
the eight descendants from (A) the three marked a14, q14, p14, will be
nearly related from having recently branched off from a10; b14 and
f14, from having diverged at an earlier period from a5, will be in some
degree distinct from the three first-named species; and lastly, o14,
e14, and m14, will be nearly related one to the other, but from having
diverged at the first commencement of the process of modification, will
be widely different from the other five species, and may constitute a
sub-genus or even a distinct genus.

The six descendants from (I) will form two sub-genera or even genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme points of the original genus, the six descendants
from (I) will, owing to inheritance, differ considerably from the eight
descendants from (A); the two groups, moreover, are supposed to have
gone on diverging in different directions. The intermediate species,
also (and this is a very important consideration), which connected the
original species (A) and (I), have all become, excepting (F), extinct,
and have left no descendants. Hence the six new species descended from
(I), and the eight descended from (A), will have to be ranked as very
distinct genera, or even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by
descent, with modification, from two or more species of the same genus.
And the two or more parent-species are supposed to have descended from
some one species of an earlier genus. In our diagram, this is indicated
by the broken lines, beneath the capital letters, converging in
sub-branches downwards towards a single point; this point representing a
single species, the supposed single parent of our several new sub-genera
and genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character,
but to have retained the form of (F), either unaltered or altered only
in a slight degree. In this case, its affinities to the other fourteen
new species will be of a curious and circuitous nature. Having descended
from a form which stood between the two parent-species (A) and (I),
now supposed to be extinct and unknown, it will be in some degree
intermediate in character between the two groups descended from these
species. But as these two groups have gone on diverging in character
from the type of their parents, the new species (F14) will not be
directly intermediate between them, but rather between types of the two
groups; and every naturalist will be able to bring some such case before
his mind.

In the diagram, each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or
hundred million generations, and likewise a section of the successive
strata of the earth's crust including extinct remains. We shall, when we
come to our chapter on Geology, have to refer again to this subject,
and I think we shall then see that the diagram throws light on the
affinities of extinct beings, which, though generally belonging to the
same orders, or families, or genera, with those now living, yet are
often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for the extinct species lived
at very ancient epochs when the branching lines of descent had diverged
less.

I see no reason to limit the process of modification, as now explained,
to the formation of genera alone. If, in our diagram, we suppose the
amount of change represented by each successive group of diverging
dotted lines to be very great, the forms marked a14 to p14, those marked
b14 and f14, and those marked o14 to m14, will form three very distinct
genera. We shall also have two very distinct genera descended from
(I) and as these latter two genera, both from continued divergence of
character and from inheritance from a different parent, will differ
widely from the three genera descended from (A), the two little groups
of genera will form two distinct families, or even orders, according to
the amount of divergent modification supposed to be represented in the
diagram. And the two new families, or orders, will have descended from
two species of the original genus; and these two species are supposed
to have descended from one species of a still more ancient and unknown
genus.

We have seen that in each country it is the species of the larger genera
which oftenest present varieties or incipient species. This, indeed,
might have been expected; for as natural selection acts through one form
having some advantage over other forms in the struggle for existence,
it will chiefly act on those which already have some advantage; and
the largeness of any group shows that its species have inherited from
a common ancestor some advantage in common. Hence, the struggle for the
production of new and modified descendants, will mainly lie between the
larger groups, which are all trying to increase in number. One large
group will slowly conquer another large group, reduce its numbers, and
thus lessen its chance of further variation and improvement. Within the
same large group, the later and more highly perfected sub-groups, from
branching out and seizing on many new places in the polity of Nature,
will constantly tend to supplant and destroy the earlier and less
improved sub-groups. Small and broken groups and sub-groups will finally
tend to disappear. Looking to the future, we can predict that the groups
of organic beings which are now large and triumphant, and which are
least broken up, that is, which as yet have suffered least extinction,
will for a long period continue to increase. But which groups will
ultimately prevail, no man can predict; for we well know that many
groups, formerly most extensively developed, have now become extinct.
Looking still more remotely to the future, we may predict that, owing to
the continued and steady increase of the larger groups, a multitude
of smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one
period, extremely few will transmit descendants to a remote futurity. I
shall have to return to this subject in the chapter on Classification,
but I may add that on this view of extremely few of the more ancient
species having transmitted descendants, and on the view of all the
descendants of the same species making a class, we can understand how
it is that there exist but very few classes in each main division of
the animal and vegetable kingdoms. Although extremely few of the most
ancient species may now have living and modified descendants, yet at the
most remote geological period, the earth may have been as well peopled
with many species of many genera, families, orders, and classes, as at
the present day.

SUMMARY OF CHAPTER.

If during the long course of ages and under varying conditions of life,
organic beings vary at all in the several parts of their organisation,
and I think this cannot be disputed; if there be, owing to the high
geometrical powers of increase of each species, at some age, season, or
year, a severe struggle for life, and this certainly cannot be disputed;
then, considering the infinite complexity of the relations of all
organic beings to each other and to their conditions of existence,
causing an infinite diversity in structure, constitution, and habits, to
be advantageous to them, I think it would be a most extraordinary fact
if no variation ever had occurred useful to each being's own welfare, in
the same way as so many variations have occurred useful to man. But if
variations useful to any organic being do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance they
will tend to produce offspring similarly characterised. This principle
of preservation, I have called, for the sake of brevity, Natural
Selection. Natural selection, on the principle of qualities being
inherited at corresponding ages, can modify the egg, seed, or young, as
easily as the adult. Amongst many animals, sexual selection will give
its aid to ordinary selection, by assuring to the most vigorous and best
adapted males the greatest number of offspring. Sexual selection will
also give characters useful to the males alone, in their struggles with
other males.

Whether natural selection has really thus acted in nature, in modifying
and adapting the various forms of life to their several conditions
and stations, must be judged of by the general tenour and balance of
evidence given in the following chapters. But we already see how it
entails extinction; and how largely extinction has acted in the world's
history, geology plainly declares. Natural selection, also, leads to
divergence of character; for more living beings can be supported on the
same area the more they diverge in structure, habits, and constitution,
of which we see proof by looking at the inhabitants of any small spot
or at naturalised productions. Therefore during the modification of the
descendants of any one species, and during the incessant struggle of all
species to increase in numbers, the more diversified these descendants
become, the better will be their chance of succeeding in the battle of
life. Thus the small differences distinguishing varieties of the same
species, will steadily tend to increase till they come to equal the
greater differences between species of the same genus, or even of
distinct genera.

We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary
most; and these will tend to transmit to their modified offspring
that superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly
wonderful fact--the wonder of which we are apt to overlook from
familiarity--that all animals and all plants throughout all time and
space should be related to each other in group subordinate to group,
in the manner which we everywhere behold--namely, varieties of the same
species most closely related together, species of the same genus less
closely and unequally related together, forming sections and sub-genera,
species of distinct genera much less closely related, and genera
related in different degrees, forming sub-families, families, orders,
sub-classes, and classes. The several subordinate groups in any class
cannot be ranked in a single file, but seem rather to be clustered
round points, and these round other points, and so on in almost endless
cycles. On the view that each species has been independently created, I
can see no explanation of this great fact in the classification of all
organic beings; but, to the best of my judgment, it is explained through
inheritance and the complex action of natural selection, entailing
extinction and divergence of character, as we have seen illustrated in
the diagram.

The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during each former year may represent the long succession
of extinct species. At each period of growth all the growing twigs
have tried to branch out on all sides, and to overtop and kill the
surrounding twigs and branches, in the same manner as species and groups
of species have tried to overmaster other species in the great battle
for life. The limbs divided into great branches, and these into lesser
and lesser branches, were themselves once, when the tree was small,
budding twigs; and this connexion of the former and present buds by
ramifying branches may well represent the classification of all extinct
and living species in groups subordinate to groups. Of the many twigs
which flourished when the tree was a mere bush, only two or three, now
grown into great branches, yet survive and bear all the other branches;
so with the species which lived during long-past geological periods,
very few now have living and modified descendants. From the first growth
of the tree, many a limb and branch has decayed and dropped off; and
these lost branches of various sizes may represent those whole orders,
families, and genera which have now no living representatives, and which
are known to us only from having been found in a fossil state. As we
here and there see a thin straggling branch springing from a fork low
down in a tree, and which by some chance has been favoured and is
still alive on its summit, so we occasionally see an animal like the
Ornithorhynchus or Lepidosiren, which in some small degree connects by
its affinities two large branches of life, and which has apparently been
saved from fatal competition by having inhabited a protected station. As
buds give rise by growth to fresh buds, and these, if vigorous, branch
out and overtop on all sides many a feebler branch, so by generation I
believe it has been with the great Tree of Life, which fills with its
dead and broken branches the crust of the earth, and covers the surface
with its ever branching and beautiful ramifications.




5. LAWS OF VARIATION.

Effects of external conditions. Use and disuse, combined with natural
selection; organs of flight and of vision. Acclimatisation. Correlation
of growth. Compensation and economy of growth. False correlations.
Multiple, rudimentary, and lowly organised structures variable. Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable.
Species of the same genus vary in an analogous manner. Reversions to
long lost characters. Summary.

I have hitherto sometimes spoken as if the variations--so common and
multiform in organic beings under domestication, and in a lesser degree
in those in a state of nature--had been due to chance. This, of course,
is a wholly incorrect expression, but it serves to acknowledge plainly
our ignorance of the cause of each particular variation. Some authors
believe it to be as much the function of the reproductive system to
produce individual differences, or very slight deviations of structure,
as to make the child like its parents. But the much greater variability,
as well as the greater frequency of monstrosities, under domestication
or cultivation, than under nature, leads me to believe that deviations
of structure are in some way due to the nature of the conditions of
life, to which the parents and their more remote ancestors have been
exposed during several generations. I have remarked in the first
chapter--but a long catalogue of facts which cannot be here given would
be necessary to show the truth of the remark--that the reproductive
system is eminently susceptible to changes in the conditions of life;
and to this system being functionally disturbed in the parents, I
chiefly attribute the varying or plastic condition of the offspring. The
male and female sexual elements seem to be affected before that union
takes place which is to form a new being. In the case of "sporting"
plants, the bud, which in its earliest condition does not apparently
differ essentially from an ovule, is alone affected. But why, because
the reproductive system is disturbed, this or that part should vary more
or less, we are profoundly ignorant. Nevertheless, we can here and there
dimly catch a faint ray of light, and we may feel sure that there must
be some cause for each deviation of structure, however slight.

How much direct effect difference of climate, food, etc., produces on
any being is extremely doubtful. My impression is, that the effect is
extremely small in the case of animals, but perhaps rather more in that
of plants. We may, at least, safely conclude that such influences cannot
have produced the many striking and complex co-adaptations of structure
between one organic being and another, which we see everywhere
throughout nature. Some little influence may be attributed to climate,
food, etc.: thus, E. Forbes speaks confidently that shells at their
southern limit, and when living in shallow water, are more brightly
coloured than those of the same species further north or from greater
depths. Gould believes that birds of the same species are more brightly
coloured under a clear atmosphere, than when living on islands or near
the coast. So with insects, Wollaston is convinced that residence near
the sea affects their colours. Moquin-Tandon gives a list of plants
which when growing near the sea-shore have their leaves in some degree
fleshy, though not elsewhere fleshy. Several other such cases could be
given.

The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree
some of the characters of such species, accords with our view that
species of all kinds are only well-marked and permanent varieties. Thus
the species of shells which are confined to tropical and shallow seas
are generally brighter-coloured than those confined to cold and deeper
seas. The birds which are confined to continents are, according to
Mr. Gould, brighter-coloured than those of islands. The insect-species
confined to sea-coasts, as every collector knows, are often brassy or
lurid. Plants which live exclusively on the sea-side are very apt to
have fleshy leaves. He who believes in the creation of each species,
will have to say that this shell, for instance, was created with bright
colours for a warm sea; but that this other shell became bright-coloured
by variation when it ranged into warmer or shallower waters.

When a variation is of the slightest use to a being, we cannot tell how
much of it to attribute to the accumulative action of natural selection,
and how much to the conditions of life. Thus, it is well known to
furriers that animals of the same species have thicker and better fur
the more severe the climate is under which they have lived; but who
can tell how much of this difference may be due to the warmest-clad
individuals having been favoured and preserved during many generations,
and how much to the direct action of the severe climate? for it would
appear that climate has some direct action on the hair of our domestic
quadrupeds.

Instances could be given of the same variety being produced under
conditions of life as different as can well be conceived; and, on the
other hand, of different varieties being produced from the same species
under the same conditions. Such facts show how indirectly the conditions
of life must act. Again, innumerable instances are known to every
naturalist of species keeping true, or not varying at all, although
living under the most opposite climates. Such considerations as these
incline me to lay very little weight on the direct action of the
conditions of life. Indirectly, as already remarked, they seem to play
an important part in affecting the reproductive system, and in thus
inducing variability; and natural selection will then accumulate
all profitable variations, however slight, until they become plainly
developed and appreciable by us.

EFFECTS OF USE AND DISUSE.

From the facts alluded to in the first chapter, I think there can be
little doubt that use in our domestic animals strengthens and enlarges
certain parts, and disuse diminishes them; and that such modifications
are inherited. Under free nature, we can have no standard of comparison,
by which to judge of the effects of long-continued use or disuse, for we
know not the parent-forms; but many animals have structures which can
be explained by the effects of disuse. As Professor Owen has remarked,
there is no greater anomaly in nature than a bird that cannot fly; yet
there are several in this state. The logger-headed duck of South America
can only flap along the surface of the water, and has its wings in
nearly the same condition as the domestic Aylesbury duck. As the larger
ground-feeding birds seldom take flight except to escape danger, I
believe that the nearly wingless condition of several birds, which now
inhabit or have lately inhabited several oceanic islands, tenanted by
no beast of prey, has been caused by disuse. The ostrich indeed inhabits
continents and is exposed to danger from which it cannot escape by
flight, but by kicking it can defend itself from enemies, as well as any
of the smaller quadrupeds. We may imagine that the early progenitor
of the ostrich had habits like those of a bustard, and that as natural
selection increased in successive generations the size and weight of
its body, its legs were used more, and its wings less, until they became
incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are very often broken
off; he examined seventeen specimens in his own collection, and not one
had even a relic left. In the Onites apelles the tarsi are so habitually
lost, that the insect has been described as not having them. In some
other genera they are present, but in a rudimentary condition. In the
Ateuchus or sacred beetle of the Egyptians, they are totally deficient.
There is not sufficient evidence to induce us to believe that
mutilations are ever inherited; and I should prefer explaining the
entire absence of the anterior tarsi in Ateuchus, and their rudimentary
condition in some other genera, by the long-continued effects of disuse
in their progenitors; for as the tarsi are almost always lost in many
dung-feeding beetles, they must be lost early in life, and therefore
cannot be much used by these insects.

In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly, due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of
the 550 species inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that of the twenty-nine endemic genera, no less
than twenty-three genera have all their species in this condition!
Several facts, namely, that beetles in many parts of the world are very
frequently blown to sea and perish; that the beetles in Madeira, as
observed by Mr. Wollaston, lie much concealed, until the wind lulls and
the sun shines; that the proportion of wingless beetles is larger on
the exposed Dezertas than in Madeira itself; and especially the
extraordinary fact, so strongly insisted on by Mr. Wollaston, of the
almost entire absence of certain large groups of beetles, elsewhere
excessively numerous, and which groups have habits of life almost
necessitating frequent flight;--these several considerations have made
me believe that the wingless condition of so many Madeira beetles is
mainly due to the action of natural selection, but combined probably
with disuse. For during thousands of successive generations each
individual beetle which flew least, either from its wings having been
ever so little less perfectly developed or from indolent habit, will
have had the best chance of surviving from not being blown out to sea;
and, on the other hand, those beetles which most readily took to flight
will oftenest have been blown to sea and thus have been destroyed.

The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their
wings to gain their subsistence, have, as Mr. Wollaston suspects, their
wings not at all reduced, but even enlarged. This is quite compatible
with the action of natural selection. For when a new insect first
arrived on the island, the tendency of natural selection to enlarge
or to reduce the wings, would depend on whether a greater number of
individuals were saved by successfully battling with the winds, or
by giving up the attempt and rarely or never flying. As with mariners
shipwrecked near a coast, it would have been better for the good
swimmers if they had been able to swim still further, whereas it would
have been better for the bad swimmers if they had not been able to swim
at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered up by skin and fur. This state of
the eyes is probably due to gradual reduction from disuse, but aided
perhaps by natural selection. In South America, a burrowing rodent, the
tuco-tuco, or Ctenomys, is even more subterranean in its habits than the
mole; and I was assured by a Spaniard, who had often caught them, that
they were frequently blind; one which I kept alive was certainly in
this condition, the cause, as appeared on dissection, having been
inflammation of the nictitating membrane. As frequent inflammation of
the eyes must be injurious to any animal, and as eyes are certainly not
indispensable to animals with subterranean habits, a reduction in their
size with the adhesion of the eyelids and growth of fur over them,
might in such case be an advantage; and if so, natural selection would
constantly aid the effects of disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind.
In some of the crabs the foot-stalk for the eye remains, though the eye
is gone; the stand for the telescope is there, though the telescope
with its glasses has been lost. As it is difficult to imagine that
eyes, though useless, could be in any way injurious to animals living in
darkness, I attribute their loss wholly to disuse. In one of the
blind animals, namely, the cave-rat, the eyes are of immense size; and
Professor Silliman thought that it regained, after living some days in
the light, some slight power of vision. In the same manner as in Madeira
the wings of some of the insects have been enlarged, and the wings of
others have been reduced by natural selection aided by use and disuse,
so in the case of the cave-rat natural selection seems to have struggled
with the loss of light and to have increased the size of the eyes;
whereas with all the other inhabitants of the caves, disuse by itself
seems to have done its work.

It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the
common view of the blind animals having been separately created for the
American and European caverns, close similarity in their organisation
and affinities might have been expected; but, as Schiodte and others
have remarked, this is not the case, and the cave-insects of the two
continents are not more closely allied than might have been anticipated
from the general resemblance of the other inhabitants of North America
and Europe. On my view we must suppose that American animals, having
ordinary powers of vision, slowly migrated by successive generations
from the outer world into the deeper and deeper recesses of the Kentucky
caves, as did European animals into the caves of Europe. We have some
evidence of this gradation of habit; for, as Schiodte remarks, "animals
not far remote from ordinary forms, prepare the transition from light to
darkness. Next follow those that are constructed for twilight; and, last
of all, those destined for total darkness." By the time that an animal
had reached, after numberless generations, the deepest recesses, disuse
will on this view have more or less perfectly obliterated its eyes, and
natural selection will often have effected other changes, such as an
increase in the length of the antennae or palpi, as a compensation for
blindness. Notwithstanding such modifications, we might expect still to
see in the cave-animals of America, affinities to the other inhabitants
of that continent, and in those of Europe, to the inhabitants of the
European continent. And this is the case with some of the American
cave-animals, as I hear from Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding
country. It would be most difficult to give any rational explanation of
the affinities of the blind cave-animals to the other inhabitants of the
two continents on the ordinary view of their independent creation. That
several of the inhabitants of the caves of the Old and New Worlds should
be closely related, we might expect from the well-known relationship of
most of their other productions. Far from feeling any surprise that some
of the cave-animals should be very anomalous, as Agassiz has remarked
in regard to the blind fish, the Amblyopsis, and as is the case with
the blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the inhabitants of these
dark abodes will probably have been exposed.

ACCLIMATISATION.

Habit is hereditary with plants, as in the period of flowering, in the
amount of rain requisite for seeds to germinate, in the time of sleep,
etc., and this leads me to say a few words on acclimatisation. As it is
extremely common for species of the same genus to inhabit very hot and
very cold countries, and as I believe that all the species of the same
genus have descended from a single parent, if this view be correct,
acclimatisation must be readily effected during long-continued descent.
It is notorious that each species is adapted to the climate of its own
home: species from an arctic or even from a temperate region cannot
endure a tropical climate, or conversely. So again, many succulent
plants cannot endure a damp climate. But the degree of adaptation of
species to the climates under which they live is often overrated. We
may infer this from our frequent inability to predict whether or not an
imported plant will endure our climate, and from the number of plants
and animals brought from warmer countries which here enjoy good health.
We have reason to believe that species in a state of nature are limited
in their ranges by the competition of other organic beings quite as much
as, or more than, by adaptation to particular climates. But whether or
not the adaptation be generally very close, we have evidence, in
the case of some few plants, of their becoming, to a certain
extent, naturally habituated to different temperatures, or becoming
acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from trees growing at different heights on the
Himalaya, were found in this country to possess different constitutional
powers of resisting cold. Mr. Thwaites informs me that he has observed
similar facts in Ceylon, and analogous observations have been made by
Mr. H. C. Watson on European species of plants brought from the Azores
to England. In regard to animals, several authentic cases could be given
of species within historical times having largely extended their
range from warmer to cooler latitudes, and conversely; but we do not
positively know that these animals were strictly adapted to their native
climate, but in all ordinary cases we assume such to be the case; nor
do we know that they have subsequently become acclimatised to their new
homes.

As I believe that our domestic animals were originally chosen by
uncivilised man because they were useful and bred readily under
confinement, and not because they were subsequently found capable
of far-extended transportation, I think the common and extraordinary
capacity in our domestic animals of not only withstanding the most
different climates but of being perfectly fertile (a far severer test)
under them, may be used as an argument that a large proportion of other
animals, now in a state of nature, could easily be brought to bear
widely different climates. We must not, however, push the foregoing
argument too far, on account of the probable origin of some of our
domestic animals from several wild stocks: the blood, for instance, of
a tropical and arctic wolf or wild dog may perhaps be mingled in our
domestic breeds. The rat and mouse cannot be considered as domestic
animals, but they have been transported by man to many parts of the
world, and now have a far wider range than any other rodent, living free
under the cold climate of Faroe in the north and of the Falklands in the
south, and on many islands in the torrid zones. Hence I am inclined to
look at adaptation to any special climate as a quality readily grafted
on an innate wide flexibility of constitution, which is common to most
animals. On this view, the capacity of enduring the most different
climates by man himself and by his domestic animals, and such facts
as that former species of the elephant and rhinoceros were capable
of enduring a glacial climate, whereas the living species are now all
tropical or sub-tropical in their habits, ought not to be looked at
as anomalies, but merely as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into play.

How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopaedias
of China, to be very cautious in transposing animals from one district
to another; for it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts: the result must, I think, be due to
habit. On the other hand, I can see no reason to doubt that natural
selection will continually tend to preserve those individuals which
are born with constitutions best adapted to their native countries. In
treatises on many kinds of cultivated plants, certain varieties are
said to withstand certain climates better than others: this is very
strikingly shown in works on fruit trees published in the United States,
in which certain varieties are habitually recommended for the northern,
and others for the southern States; and as most of these varieties are
of recent origin, they cannot owe their constitutional differences to
habit. The case of the Jerusalem artichoke, which is never propagated
by seed, and of which consequently new varieties have not been produced,
has even been advanced--for it is now as tender as ever it was--as
proving that acclimatisation cannot be effected! The case, also, of the
kidney-bean has been often cited for a similar purpose, and with
much greater weight; but until some one will sow, during a score of
generations, his kidney-beans so early that a very large proportion are
destroyed by frost, and then collect seed from the few survivors, with
care to prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to
have been even tried. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has
been published how much more hardy some seedlings appeared to be than
others.

On the whole, I think we may conclude that habit, use, and disuse, have,
in some cases, played a considerable part in the modification of the
constitution, and of the structure of various organs; but that the
effects of use and disuse have often been largely combined with, and
sometimes overmastered by, the natural selection of innate differences.

CORRELATION OF GROWTH.

I mean by this expression that the whole organisation is so tied
together during its growth and development, that when slight variations
in any one part occur, and are accumulated through natural selection,
other parts become modified. This is a very important subject, most
imperfectly understood. The most obvious case is, that modifications
accumulated solely for the good of the young or larva, will, it may
safely be concluded, affect the structure of the adult; in the same
manner as any malconformation affecting the early embryo, seriously
affects the whole organisation of the adult. The several parts of the
body which are homologous, and which, at an early embryonic period, are
alike, seem liable to vary in an allied manner: we see this in the right
and left sides of the body varying in the same manner; in the front and
hind legs, and even in the jaws and limbs, varying together, for the
lower jaw is believed to be homologous with the limbs. These tendencies,
I do not doubt, may be mastered more or less completely by natural
selection: thus a family of stags once existed with an antler only on
one side; and if this had been of any great use to the breed it might
probably have been rendered permanent by natural selection.

Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of the
petals of the corolla into a tube. Hard parts seem to affect the form of
adjoining soft parts; it is believed by some authors that the diversity
in the shape of the pelvis in birds causes the remarkable diversity in
the shape of their kidneys. Others believe that the shape of the pelvis
in the human mother influences by pressure the shape of the head of the
child. In snakes, according to Schlegel, the shape of the body and
the manner of swallowing determine the position of several of the most
important viscera.

The nature of the bond of correlation is very frequently quite obscure.
M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist,
without our being able to assign any reason. What can be more singular
than the relation between blue eyes and deafness in cats, and the
tortoise-shell colour with the female sex; the feathered feet and skin
between the outer toes in pigeons, and the presence of more or less down
on the young birds when first hatched, with the future colour of their
plumage; or, again, the relation between the hair and teeth in the naked
Turkish dog, though here probably homology comes into play? With respect
to this latter case of correlation, I think it can hardly be accidental,
that if we pick out the two orders of mammalia which are most
abnormal in their dermal coverings, viz. Cetacea (whales) and Edentata
(armadilloes, scaly ant-eaters, etc.), that these are likewise the most
abnormal in their teeth.

I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of utility
and, therefore, of natural selection, than that of the difference
between the outer and inner flowers in some Compositous and
Umbelliferous plants. Every one knows the difference in the ray and
central florets of, for instance, the daisy, and this difference is
often accompanied with the abortion of parts of the flower. But, in some
Compositous plants, the seeds also differ in shape and sculpture; and
even the ovary itself, with its accessory parts, differs, as has been
described by Cassini. These differences have been attributed by some
authors to pressure, and the shape of the seeds in the ray-florets in
some Compositae countenances this idea; but, in the case of the corolla
of the Umbelliferae, it is by no means, as Dr. Hooker informs me, in
species with the densest heads that the inner and outer flowers most
frequently differ. It might have been thought that the development of
the ray-petals by drawing nourishment from certain other parts of the
flower had caused their abortion; but in some Compositae there is a
difference in the seeds of the outer and inner florets without any
difference in the corolla. Possibly, these several differences may be
connected with some difference in the flow of nutriment towards the
central and external flowers: we know, at least, that in irregular
flowers, those nearest to the axis are oftenest subject to peloria, and
become regular. I may add, as an instance of this, and of a striking
case of correlation, that I have recently observed in some garden
pelargoniums, that the central flower of the truss often loses the
patches of darker colour in the two upper petals; and that when this
occurs, the adherent nectary is quite aborted; when the colour is
absent from only one of the two upper petals, the nectary is only much
shortened.

With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C.
C. Sprengel's idea that the ray-florets serve to attract insects, whose
agency is highly advantageous in the fertilisation of plants of these
two orders, is so far-fetched, as it may at first appear: and if it be
advantageous, natural selection may have come into play. But in regard
to the differences both in the internal and external structure of the
seeds, which are not always correlated with any differences in the
flowers, it seems impossible that they can be in any way advantageous
to the plant: yet in the Umbelliferae these differences are of such
apparent importance--the seeds being in some cases, according to Tausch,
orthospermous in the exterior flowers and coelospermous in the central
flowers,--that the elder De Candolle founded his main divisions of
the order on analogous differences. Hence we see that modifications of
structure, viewed by systematists as of high value, may be wholly due to
unknown laws of correlated growth, and without being, as far as we can
see, of the slightest service to the species.

We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and, after
thousands of generations, some other and independent modification; and
these two modifications, having been transmitted to a whole group
of descendants with diverse habits, would naturally be thought to be
correlated in some necessary manner. So, again, I do not doubt that some
apparent correlations, occurring throughout whole orders, are entirely
due to the manner alone in which natural selection can act. For
instance, Alph. De Candolle has remarked that winged seeds are never
found in fruits which do not open: I should explain the rule by the fact
that seeds could not gradually become winged through natural selection,
except in fruits which opened; so that the individual plants producing
seeds which were a little better fitted to be wafted further, might get
an advantage over those producing seed less fitted for dispersal; and
this process could not possibly go on in fruit which did not open.

The elder Geoffroy and Goethe propounded, at about the same period,
their law of compensation or balancement of growth; or, as Goethe
expressed it, "in order to spend on one side, nature is forced to
economise on the other side." I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one part
or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to fatten
readily. The same varieties of the cabbage do not yield abundant and
nutritious foliage and a copious supply of oil-bearing seeds. When the
seeds in our fruits become atrophied, the fruit itself gains largely in
size and quality. In our poultry, a large tuft of feathers on the head
is generally accompanied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature it can hardly
be maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will
not, however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoining
part being reduced by this same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the
excess of growth in another and adjoining part.

I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually
trying to economise in every part of the organisation. If under changed
conditions of life a structure before useful becomes less useful, any
diminution, however slight, in its development, will be seized on by
natural selection, for it will profit the individual not to have its
nutriment wasted in building up an useless structure. I can thus
only understand a fact with which I was much struck when examining
cirripedes, and of which many other instances could be given: namely,
that when a cirripede is parasitic within another and is thus protected,
it loses more or less completely its own shell or carapace. This is the
case with the male Ibla, and in a truly extraordinary manner with the
Proteolepas: for the carapace in all other cirripedes consists of
the three highly-important anterior segments of the head enormously
developed, and furnished with great nerves and muscles; but in the
parasitic and protected Proteolepas, the whole anterior part of the
head is reduced to the merest rudiment attached to the bases of the
prehensile antennae. Now the saving of a large and complex structure,
when rendered superfluous by the parasitic habits of the Proteolepas,
though effected by slow steps, would be a decided advantage to each
successive individual of the species; for in the struggle for life to
which every animal is exposed, each individual Proteolepas would have
a better chance of supporting itself, by less nutriment being wasted in
developing a structure now become useless.

Thus, as I believe, natural selection will always succeed in the long
run in reducing and saving every part of the organisation, as soon as it
is rendered superfluous, without by any means causing some other part
to be largely developed in a corresponding degree. And, conversely, that
natural selection may perfectly well succeed in largely developing any
organ, without requiring as a necessary compensation the reduction of
some adjoining part.

It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in
varieties and in species, that when any part or organ is repeated many
times in the structure of the same individual (as the vertebrae in
snakes, and the stamens in polyandrous flowers) the number is variable;
whereas the number of the same part or organ, when it occurs in lesser
numbers, is constant. The same author and some botanists have further
remarked that multiple parts are also very liable to variation in
structure. Inasmuch as this "vegetative repetition," to use Professor
Owen's expression, seems to be a sign of low organisation; the foregoing
remark seems connected with the very general opinion of naturalists,
that beings low in the scale of nature are more variable than those
which are higher. I presume that lowness in this case means that the
several parts of the organisation have been but little specialised
for particular functions; and as long as the same part has to perform
diversified work, we can perhaps see why it should remain variable, that
is, why natural selection should have preserved or rejected each little
deviation of form less carefully than when the part has to serve for one
special purpose alone. In the same way that a knife which has to cut
all sorts of things may be of almost any shape; whilst a tool for
some particular object had better be of some particular shape. Natural
selection, it should never be forgotten, can act on each part of each
being, solely through and for its advantage.

Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to the
general subject of rudimentary and aborted organs; and I will here only
add that their variability seems to be owing to their uselessness, and
therefore to natural selection having no power to check deviations in
their structure. Thus rudimentary parts are left to the free play of the
various laws of growth, to the effects of long-continued disuse, and to
the tendency to reversion.

A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER,
IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.

Several years ago I was much struck with a remark, nearly to the above
effect, published by Mr. Waterhouse. I infer also from an observation
made by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is
hopeless to attempt to convince any one of the truth of this proposition
without giving the long array of facts which I have collected, and which
cannot possibly be here introduced. I can only state my conviction that
it is a rule of high generality. I am aware of several causes of
error, but I hope that I have made due allowance for them. It should
be understood that the rule by no means applies to any part, however
unusually developed, unless it be unusually developed in comparison with
the same part in closely allied species. Thus, the bat's wing is a most
abnormal structure in the class mammalia; but the rule would not here
apply, because there is a whole group of bats having wings; it would
apply only if some one species of bat had its wings developed in some
remarkable manner in comparison with the other species of the same
genus. The rule applies very strongly in the case of secondary sexual
characters, when displayed in any unusual manner. The term, secondary
sexual characters, used by Hunter, applies to characters which are
attached to one sex, but are not directly connected with the act of
reproduction. The rule applies to males and females; but as females more
rarely offer remarkable secondary sexual characters, it applies more
rarely to them. The rule being so plainly applicable in the case of
secondary sexual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual manner--of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case
of hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse's remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in different
genera; but in the several species of one genus, Pyrgoma, these valves
present a marvellous amount of diversification: the homologous valves
in the different species being sometimes wholly unlike in shape; and the
amount of variation in the individuals of several of the species is so
great, that it is no exaggeration to state that the varieties differ
more from each other in the characters of these important valves than do
other species of distinct genera.

As birds within the same country vary in a remarkably small degree, I
have particularly attended to them, and the rule seems to me certainly
to hold good in this class. I cannot make out that it applies to plants,
and this would seriously have shaken my belief in its truth, had not the
great variability in plants made it particularly difficult to compare
their relative degrees of variability.

When we see any part or organ developed in a remarkable degree or manner
in any species, the fair presumption is that it is of high importance to
that species; nevertheless the part in this case is eminently liable to
variation. Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see
no explanation. But on the view that groups of species have descended
from other species, and have been modified through natural selection, I
think we can obtain some light. In our domestic animals, if any part,
or the whole animal, be neglected and no selection be applied, that part
(for instance, the comb in the Dorking fowl) or the whole breed will
cease to have a nearly uniform character. The breed will then be said
to have degenerated. In rudimentary organs, and in those which have
been but little specialised for any particular purpose, and perhaps in
polymorphic groups, we see a nearly parallel natural case; for in such
cases natural selection either has not or cannot come into full play,
and thus the organisation is left in a fluctuating condition. But what
here more especially concerns us is, that in our domestic animals
those points, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at the
breeds of the pigeon; see what a prodigious amount of difference there
is in the beak of the different tumblers, in the beak and wattle of
the different carriers, in the carriage and tail of our fantails, etc.,
these being the points now mainly attended to by English fanciers. Even
in the sub-breeds, as in the short-faced tumbler, it is notoriously
difficult to breed them nearly to perfection, and frequently individuals
are born which depart widely from the standard. There may be truly
said to be a constant struggle going on between, on the one hand, the
tendency to reversion to a less modified state, as well as an innate
tendency to further variability of all kinds, and, on the other hand,
the power of steady selection to keep the breed true. In the long run
selection gains the day, and we do not expect to fail so far as to breed
a bird as coarse as a common tumbler from a good short-faced strain. But
as long as selection is rapidly going on, there may always be expected
to be much variability in the structure undergoing modification. It
further deserves notice that these variable characters, produced by
man's selection, sometimes become attached, from causes quite unknown
to us, more to one sex than to the other, generally to the male sex, as
with the wattle of carriers and the enlarged crop of pouters.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species
of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification, since the period when the species
branched off from the common progenitor of the genus. This period will
seldom be remote in any extreme degree, as species very rarely endure
for more than one geological period. An extraordinary amount of
modification implies an unusually large and long-continued amount of
variability, which has continually been accumulated by natural
selection for the benefit of the species. But as the variability of
the extraordinarily-developed part or organ has been so great and
long-continued within a period not excessively remote, we might, as a
general rule, expect still to find more variability in such parts than
in other parts of the organisation, which have remained for a much
longer period nearly constant. And this, I am convinced, is the case.
That the struggle between natural selection on the one hand, and the
tendency to reversion and variability on the other hand, will in the
course of time cease; and that the most abnormally developed organs may
be made constant, I can see no reason to doubt. Hence when an organ,
however abnormal it may be, has been transmitted in approximately the
same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to my theory, for an
immense period in nearly the same state; and thus it comes to be no more
variable than any other structure. It is only in those cases in which
the modification has been comparatively recent and extraordinarily great
that we ought to find the GENERATIVE VARIABILITY, as it may be called,
still present in a high degree. For in this case the variability
will seldom as yet have been fixed by the continued selection of the
individuals varying in the required manner and degree, and by the
continued rejection of those tending to revert to a former and less
modified condition.

The principle included in these remarks may be extended. It is notorious
that specific characters are more variable than generic. To explain by a
simple example what is meant. If some species in a large genus of plants
had blue flowers and some had red, the colour would be only a specific
character, and no one would be surprised at one of the blue species
varying into red, or conversely; but if all the species had blue
flowers, the colour would become a generic character, and its variation
would be a more unusual circumstance. I have chosen this example because
an explanation is not in this case applicable, which most naturalists
would advance, namely, that specific characters are more variable
than generic, because they are taken from parts of less physiological
importance than those commonly used for classing genera. I believe this
explanation is partly, yet only indirectly, true; I shall, however, have
to return to this subject in our chapter on Classification. It would be
almost superfluous to adduce evidence in support of the above statement,
that specific characters are more variable than generic; but I have
repeatedly noticed in works on natural history, that when an author
has remarked with surprise that some IMPORTANT organ or part, which is
generally very constant throughout large groups of species, has DIFFERED
considerably in closely-allied species, that it has, also, been VARIABLE
in the individuals of some of the species. And this fact shows that a
character, which is generally of generic value, when it sinks in value
and becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same kind
applies to monstrosities: at least Is. Geoffroy St. Hilaire seems to
entertain no doubt, that the more an organ normally differs in
the different species of the same group, the more subject it is to
individual anomalies.

On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same
part in other independently-created species of the same genus, be
more variable than those parts which are closely alike in the several
species? I do not see that any explanation can be given. But on the
view of species being only strongly marked and fixed varieties, we might
surely expect to find them still often continuing to vary in those parts
of their structure which have varied within a moderately recent period,
and which have thus come to differ. Or to state the case in another
manner:--the points in which all the species of a genus resemble each
other, and in which they differ from the species of some other genus,
are called generic characters; and these characters in common I
attribute to inheritance from a common progenitor, for it can rarely
have happened that natural selection will have modified several species,
fitted to more or less widely-different habits, in exactly the same
manner: and as these so-called generic characters have been inherited
from a remote period, since that period when the species first branched
off from their common progenitor, and subsequently have not varied or
come to differ in any degree, or only in a slight degree, it is not
probable that they should vary at the present day. On the other hand,
the points in which species differ from other species of the same genus,
are called specific characters; and as these specific characters have
varied and come to differ within the period of the branching off of the
species from a common progenitor, it is probable that they should still
often be in some degree variable,--at least more variable than those
parts of the organisation which have for a very long period remained
constant.

In connexion with the present subject, I will make only two other
remarks. I think it will be admitted, without my entering on details,
that secondary sexual characters are very variable; I think it also will
be admitted that species of the same group differ from each other more
widely in their secondary sexual characters, than in other parts of
their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference between
their females; and the truth of this proposition will be granted. The
cause of the original variability of secondary sexual characters is
not manifest; but we can see why these characters should not have been
rendered as constant and uniform as other parts of the organisation; for
secondary sexual characters have been accumulated by sexual selection,
which is less rigid in its action than ordinary selection, as it does
not entail death, but only gives fewer offspring to the less favoured
males. Whatever the cause may be of the variability of secondary sexual
characters, as they are highly variable, sexual selection will have had
a wide scope for action, and may thus readily have succeeded in giving
to the species of the same group a greater amount of difference in their
sexual characters, than in other parts of their structure.

It is a remarkable fact, that the secondary sexual differences between
the two sexes of the same species are generally displayed in the very
same parts of the organisation in which the different species of
the same genus differ from each other. Of this fact I will give in
illustration two instances, the first which happen to stand on my list;
and as the differences in these cases are of a very unusual nature,
the relation can hardly be accidental. The same number of joints in the
tarsi is a character generally common to very large groups of beetles,
but in the Engidae, as Westwood has remarked, the number varies greatly;
and the number likewise differs in the two sexes of the same species:
again in fossorial hymenoptera, the manner of neuration of the wings is
a character of the highest importance, because common to large groups;
but in certain genera the neuration differs in the different species,
and likewise in the two sexes of the same species. This relation has a
clear meaning on my view of the subject: I look at all the species
of the same genus as having as certainly descended from the same
progenitor, as have the two sexes of any one of the species.
Consequently, whatever part of the structure of the common progenitor,
or of its early descendants, became variable; variations of this part
would it is highly probable, be taken advantage of by natural and sexual
selection, in order to fit the several species to their several places
in the economy of nature, and likewise to fit the two sexes of the same
species to each other, or to fit the males and females to different
habits of life, or the males to struggle with other males for the
possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;--that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part
in its congeners; and the not great degree of variability in a part,
however extraordinarily it may be developed, if it be common to a
whole group of species;--that the great variability of secondary sexual
characters, and the great amount of difference in these same characters
between closely allied species;--that secondary sexual and ordinary
specific differences are generally displayed in the same parts of the
organisation,--are all principles closely connected together. All being
mainly due to the species of the same group having descended from a
common progenitor, from whom they have inherited much in common,--to
parts which have recently and largely varied being more likely still
to go on varying than parts which have long been inherited and have not
varied,--to natural selection having more or less completely, according
to the lapse of time, overmastered the tendency to reversion and to
further variability,--to sexual selection being less rigid than ordinary
selection,--and to variations in the same parts having been accumulated
by natural and sexual selection, and thus adapted for secondary sexual,
and for ordinary specific purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS; AND A VARIETY OF ONE
SPECIES OFTEN ASSUMES SOME OF THE CHARACTERS OF AN ALLIED SPECIES, OR
REVERTS TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.

These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of pigeons, in countries most
widely apart, present sub-varieties with reversed feathers on the head
and feathers on the feet,--characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers
in the pouter, may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences.
In the vegetable kingdom we have a case of analogous variation, in the
enlarged stems, or roots as commonly called, of the Swedish turnip and
Ruta baga, plants which several botanists rank as varieties produced by
cultivation from a common parent: if this be not so, the case will then
be one of analogous variation in two so-called distinct species; and to
these a third may be added, namely, the common turnip. According to
the ordinary view of each species having been independently created, we
should have to attribute this similarity in the enlarged stems of these
three plants, not to the vera causa of community of descent, and a
consequent tendency to vary in a like manner, but to three separate yet
closely related acts of creation.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, a white rump, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these
marks are characteristic of the parent rock-pigeon, I presume that no
one will doubt that this is a case of reversion, and not of a new yet
analogous variation appearing in the several breeds. We may I think
confidently come to this conclusion, because, as we have seen, these
coloured marks are eminently liable to appear in the crossed offspring
of two distinct and differently coloured breeds; and in this case there
is nothing in the external conditions of life to cause the reappearance
of the slaty-blue, with the several marks, beyond the influence of the
mere act of crossing on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations--some say, for a dozen or even a
score of generations. After twelve generations, the proportion of blood,
to use a common expression, of any one ancestor, is only 1 in 2048; and
yet, as we see, it is generally believed that a tendency to reversion
is retained by this very small proportion of foreign blood. In a breed
which has not been crossed, but in which BOTH parents have lost some
character which their progenitor possessed, the tendency, whether strong
or weak, to reproduce the lost character might be, as was formerly
remarked, for all that we can see to the contrary, transmitted for
almost any number of generations. When a character which has been lost
in a breed, reappears after a great number of generations, the most
probable hypothesis is, not that the offspring suddenly takes after an
ancestor some hundred generations distant, but that in each successive
generation there has been a tendency to reproduce the character in
question, which at last, under unknown favourable conditions, gains an
ascendancy. For instance, it is probable that in each generation of the
barb-pigeon, which produces most rarely a blue and black-barred bird,
there has been a tendency in each generation in the plumage to assume
this colour. This view is hypothetical, but could be supported by some
facts; and I can see no more abstract improbability in a tendency
to produce any character being inherited for an endless number of
generations, than in quite useless or rudimentary organs being, as we
all know them to be, thus inherited. Indeed, we may sometimes observe
a mere tendency to produce a rudiment inherited: for instance, in the
common snapdragon (Antirrhinum) a rudiment of a fifth stamen so often
appears, that this plant must have an inherited tendency to produce it.

As all the species of the same genus are supposed, on my theory, to have
descended from a common parent, it might be expected that they would
occasionally vary in an analogous manner; so that a variety of one
species would resemble in some of its characters another species; this
other species being on my view only a well-marked and permanent variety.
But characters thus gained would probably be of an unimportant nature,
for the presence of all important characters will be governed by natural
selection, in accordance with the diverse habits of the species, and
will not be left to the mutual action of the conditions of life and of
a similar inherited constitution. It might further be expected that the
species of the same genus would occasionally exhibit reversions to lost
ancestral characters. As, however, we never know the exact character
of the common ancestor of a group, we could not distinguish these two
cases: if, for instance, we did not know that the rock-pigeon was not
feather-footed or turn-crowned, we could not have told, whether these
characters in our domestic breeds were reversions or only analogous
variations; but we might have inferred that the blueness was a case of
reversion, from the number of the markings, which are correlated with
the blue tint, and which it does not appear probable would all appear
together from simple variation. More especially we might have inferred
this, from the blue colour and marks so often appearing when distinct
breeds of diverse colours are crossed. Hence, though under nature
it must generally be left doubtful, what cases are reversions to an
anciently existing character, and what are new but analogous variations,
yet we ought, on my theory, sometimes to find the varying offspring of
a species assuming characters (either from reversion or from analogous
variation) which already occur in some other members of the same group.
And this undoubtedly is the case in nature.

A considerable part of the difficulty in recognising a variable species
in our systematic works, is due to its varieties mocking, as it were,
some of the other species of the same genus. A considerable catalogue,
also, could be given of forms intermediate between two other forms,
which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as
independently created species, that the one in varying has assumed some
of the characters of the other, so as to produce the intermediate form.
But the best evidence is afforded by parts or organs of an important and
uniform nature occasionally varying so as to acquire, in some degree,
the character of the same part or organ in an allied species. I have
collected a long list of such cases; but here, as before, I lie under
a great disadvantage in not being able to give them. I can only repeat
that such cases certainly do occur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several species
of the same genus, partly under domestication and partly under nature.
It is a case apparently of reversion. The ass not rarely has very
distinct transverse bars on its legs, like those on the legs of a zebra:
it has been asserted that these are plainest in the foal, and from
inquiries which I have made, I believe this to be true. It has also
been asserted that the stripe on each shoulder is sometimes double.
The shoulder stripe is certainly very variable in length and outline. A
white ass, but NOT an albino, has been described without either spinal
or shoulder-stripe; and these stripes are sometimes very obscure, or
actually quite lost, in dark-coloured asses. The koulan of Pallas is
said to have been seen with a double shoulder-stripe. The hemionus has
no shoulder-stripe; but traces of it, as stated by Mr. Blyth and others,
occasionally appear: and I have been informed by Colonel Poole that the
foals of this species are generally striped on the legs, and faintly on
the shoulder. The quagga, though so plainly barred like a zebra over the
body, is without bars on the legs; but Dr. Gray has figured one specimen
with very distinct zebra-like bars on the hocks.

With respect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of ALL colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut: a faint shoulder-stripe may sometimes be seen
in duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes; and a man, whom I can
implicitly trust, has examined for me a small dun Welch pony with THREE
short parallel stripes on each shoulder.

In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
the breed for the Indian Government, a horse without stripes is not
considered as purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The stripes are plainest in the foal; and sometimes
quite disappear in old horses. Colonel Poole has seen both gray and
bay Kattywar horses striped when first foaled. I have, also, reason to
suspect, from information given me by Mr. W. W. Edwards, that with the
English race-horse the spinal stripe is much commoner in the foal than
in the full-grown animal. Without here entering on further details, I
may state that I have collected cases of leg and shoulder stripes in
horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago in
the south. In all parts of the world these stripes occur far oftenest
in duns and mouse-duns; by the term dun a large range of colour is
included, from one between brown and black to a close approach to
cream-colour.

I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse have descended from
several aboriginal species--one of which, the dun, was striped; and that
the above-described appearances are all due to ancient crosses with the
dun stock. But I am not at all satisfied with this theory, and should be
loth to apply it to breeds so distinct as the heavy Belgian cart-horse,
Welch ponies, cobs, the lanky Kattywar race, etc., inhabiting the most
distant parts of the world.

Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and horse
is particularly apt to have bars on its legs. I once saw a mule with its
legs so much striped that any one at first would have thought that it
must have been the product of a zebra; and Mr. W. C. Martin, in his
excellent treatise on the horse, has given a figure of a similar mule.
In four coloured drawings, which I have seen, of hybrids between the ass
and zebra, the legs were much more plainly barred than the rest of the
body; and in one of them there was a double shoulder-stripe. In Lord
Moreton's famous hybrid from a chestnut mare and male quagga, the
hybrid, and even the pure offspring subsequently produced from the mare
by a black Arabian sire, were much more plainly barred across the
legs than is even the pure quagga. Lastly, and this is another most
remarkable case, a hybrid has been figured by Dr. Gray (and he informs
me that he knows of a second case) from the ass and the hemionus; and
this hybrid, though the ass seldom has stripes on its legs and the
hemionus has none and has not even a shoulder-stripe, nevertheless had
all four legs barred, and had three short shoulder-stripes, like those
on the dun Welch pony, and even had some zebra-like stripes on the sides
of its face. With respect to this last fact, I was so convinced that not
even a stripe of colour appears from what would commonly be called an
accident, that I was led solely from the occurrence of the face-stripes
on this hybrid from the ass and hemionus, to ask Colonel Poole whether
such face-stripes ever occur in the eminently striped Kattywar breed of
horses, and was, as we have seen, answered in the affirmative.

What now are we to say to these several facts? We see several very
distinct species of the horse-genus becoming, by simple variation,
striped on the legs like a zebra, or striped on the shoulders like
an ass. In the horse we see this tendency strong whenever a dun tint
appears--a tint which approaches to that of the general colouring of
the other species of the genus. The appearance of the stripes is not
accompanied by any change of form or by any other new character. We see
this tendency to become striped most strongly displayed in hybrids from
between several of the most distinct species. Now observe the case
of the several breeds of pigeons: they are descended from a pigeon
(including two or three sub-species or geographical races) of a bluish
colour, with certain bars and other marks; and when any breed assumes
by simple variation a bluish tint, these bars and other marks invariably
reappear; but without any other change of form or character. When the
oldest and truest breeds of various colours are crossed, we see a
strong tendency for the blue tint and bars and marks to reappear in the
mongrels. I have stated that the most probable hypothesis to account
for the reappearance of very ancient characters, is--that there is
a TENDENCY in the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown causes,
sometimes prevails. And we have just seen that in several species of the
horse-genus the stripes are either plainer or appear more commonly in
the young than in the old. Call the breeds of pigeons, some of which
have bred true for centuries, species; and how exactly parallel is the
case with that of the species of the horse-genus! For myself, I venture
confidently to look back thousands on thousands of generations, and
I see an animal striped like a zebra, but perhaps otherwise very
differently constructed, the common parent of our domestic horse,
whether or not it be descended from one or more wild stocks, of the ass,
the hemionus, quagga, and zebra.

He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like other species of
the genus; and that each has been created with a strong tendency,
when crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents, but
other species of the genus. To admit this view is, as it seems to me, to
reject a real for an unreal, or at least for an unknown, cause. It makes
the works of God a mere mockery and deception; I would almost as soon
believe with the old and ignorant cosmogonists, that fossil shells had
never lived, but had been created in stone so as to mock the shells now
living on the sea-shore.

SUMMARY.

Our ignorance of the laws of variation is profound. Not in one case out
of a hundred can we pretend to assign any reason why this or that part
differs, more or less, from the same part in the parents. But whenever
we have the means of instituting a comparison, the same laws appear to
have acted in producing the lesser differences between varieties of the
same species, and the greater differences between species of the same
genus. The external conditions of life, as climate and food, etc.,
seem to have induced some slight modifications. Habit in producing
constitutional differences, and use in strengthening, and disuse in
weakening and diminishing organs, seem to have been more potent in their
effects. Homologous parts tend to vary in the same way, and homologous
parts tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining
parts; and every part of the structure which can be saved without
detriment to the individual, will be saved. Changes of structure at an
early age will generally affect parts subsequently developed; and there
are very many other correlations of growth, the nature of which we are
utterly unable to understand. Multiple parts are variable in number and
in structure, perhaps arising from such parts not having been closely
specialised to any particular function, so that their modifications have
not been closely checked by natural selection. It is probably from
this same cause that organic beings low in the scale of nature are
more variable than those which have their whole organisation more
specialised, and are higher in the scale. Rudimentary organs, from being
useless, will be disregarded by natural selection, and hence probably
are variable. Specific characters--that is, the characters which have
come to differ since the several species of the same genus branched
off from a common parent--are more variable than generic characters, or
those which have long been inherited, and have not differed within
this same period. In these remarks we have referred to special parts or
organs being still variable, because they have recently varied and thus
come to differ; but we have also seen in the second Chapter that the
same principle applies to the whole individual; for in a district where
many species of any genus are found--that is, where there has been much
former variation and differentiation, or where the manufactory of new
specific forms has been actively at work--there, on an average, we now
find most varieties or incipient species. Secondary sexual characters
are highly variable, and such characters differ much in the species of
the same group. Variability in the same parts of the organisation has
generally been taken advantage of in giving secondary sexual differences
to the sexes of the same species, and specific differences to the
several species of the same genus. Any part or organ developed to an
extraordinary size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have gone through an
extraordinary amount of modification since the genus arose; and thus we
can understand why it should often still be variable in a much higher
degree than other parts; for variation is a long-continued and slow
process, and natural selection will in such cases not as yet have had
time to overcome the tendency to further variability and to
reversion to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants--which on my view must be a very slow process, requiring
a long lapse of time--in this case, natural selection may readily
have succeeded in giving a fixed character to the organ, in however
extraordinary a manner it may be developed. Species inheriting nearly
the same constitution from a common parent and exposed to similar
influences will naturally tend to present analogous variations, and
these same species may occasionally revert to some of the characters of
their ancient progenitors. Although new and important modifications may
not arise from reversion and analogous variation, such modifications
will add to the beautiful and harmonious diversity of nature.

Whatever the cause may be of each slight difference in the offspring
from their parents--and a cause for each must exist--it is the steady
accumulation, through natural selection, of such differences, when
beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face
of this earth are enabled to struggle with each other, and the best
adapted to survive.




6. DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification. Transitions.
Absence or rarity of transitional varieties. Transitions in habits of
life. Diversified habits in the same species. Species with habits widely
different from those of their allies. Organs of extreme perfection.
Means of transition. Cases of difficulty. Natura non facit saltum.
Organs of small importance. Organs not in all cases absolutely perfect.
The law of Unity of Type and of the Conditions of Existence embraced by
the theory of Natural Selection.

Long before having arrived at this part of my work, a crowd of
difficulties will have occurred to the reader. Some of them are so grave
that to this day I can never reflect on them without being staggered;
but, to the best of my judgment, the greater number are only apparent,
and those that are real are not, I think, fatal to my theory.

These difficulties and objections may be classed under the following
heads:--

Firstly, why, if species have descended from other species by insensibly
fine gradations, do we not everywhere see innumerable transitional
forms? Why is not all nature in confusion instead of the species being,
as we see them, well defined?

Secondly, is it possible that an animal having, for instance,
the structure and habits of a bat, could have been formed by the
modification of some animal with wholly different habits? Can we
believe that natural selection could produce, on the one hand, organs
of trifling importance, such as the tail of a giraffe, which serves as a
fly-flapper, and, on the other hand, organs of such wonderful structure,
as the eye, of which we hardly as yet fully understand the inimitable
perfection?

Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to so marvellous an instinct as that which
leads the bee to make cells, which have practically anticipated the
discoveries of profound mathematicians?

Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?

The two first heads shall be here discussed--Instinct and Hybridism in
separate chapters.

ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.

As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to
take the place of, and finally to exterminate, its own less improved
parent or other less-favoured forms with which it comes into
competition. Thus extinction and natural selection will, as we have
seen, go hand in hand. Hence, if we look at each species as descended
from some other unknown form, both the parent and all the transitional
varieties will generally have been exterminated by the very process of
formation and perfection of the new form.

But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of
the earth? It will be much more convenient to discuss this question in
the chapter on the Imperfection of the geological record; and I will
here only state that I believe the answer mainly lies in the record
being incomparably less perfect than is generally supposed; the
imperfection of the record being chiefly due to organic beings not
inhabiting profound depths of the sea, and to their remains being
embedded and preserved to a future age only in masses of sediment
sufficiently thick and extensive to withstand an enormous amount of
future degradation; and such fossiliferous masses can be accumulated
only where much sediment is deposited on the shallow bed of the sea,
whilst it slowly subsides. These contingencies will concur only rarely,
and after enormously long intervals. Whilst the bed of the sea
is stationary or is rising, or when very little sediment is being
deposited, there will be blanks in our geological history. The crust of
the earth is a vast museum; but the natural collections have been made
only at intervals of time immensely remote.

But it may be urged that when several closely-allied species inhabit
the same territory we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals
with closely allied or representative species, evidently filling nearly
the same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and
rarer, the other becomes more and more frequent, till the one replaces
the other. But if we compare these species where they intermingle, they
are generally as absolutely distinct from each other in every detail of
structure as are specimens taken from the metropolis inhabited by each.
By my theory these allied species have descended from a common parent;
and during the process of modification, each has become adapted to
the conditions of life of its own region, and has supplanted and
exterminated its original parent and all the transitional varieties
between its past and present states. Hence we ought not to expect at
the present time to meet with numerous transitional varieties in each
region, though they must have existed there, and may be embedded
there in a fossil condition. But in the intermediate region, having
intermediate conditions of life, why do we not now find closely-linking
intermediate varieties? This difficulty for a long time quite confounded
me. But I think it can be in large part explained.

In the first place we should be extremely cautious in inferring, because
an area is now continuous, that it has been continuous during a long
period. Geology would lead us to believe that almost every continent has
been broken up into islands even during the later tertiary periods;
and in such islands distinct species might have been separately formed
without the possibility of intermediate varieties existing in the
intermediate zones. By changes in the form of the land and of climate,
marine areas now continuous must often have existed within recent times
in a far less continuous and uniform condition than at present. But I
will pass over this way of escaping from the difficulty; for I believe
that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken
condition of areas now continuous has played an important part in the
formation of new species, more especially with freely-crossing and
wandering animals.

In looking at species as they are now distributed over a wide area,
we generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it
is quite remarkable how abruptly, as Alph. De Candolle has observed,
a common alpine species disappears. The same fact has been noticed by
Forbes in sounding the depths of the sea with the dredge. To those who
look at climate and the physical conditions of life as the all-important
elements of distribution, these facts ought to cause surprise, as
climate and height or depth graduate away insensibly. But when we
bear in mind that almost every species, even in its metropolis, would
increase immensely in numbers, were it not for other competing species;
that nearly all either prey on or serve as prey for others; in short,
that each organic being is either directly or indirectly related in
the most important manner to other organic beings, we must see that the
range of the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in large part on the
presence of other species, on which it depends, or by which it is
destroyed, or with which it comes into competition; and as these species
are already defined objects (however they may have become so), not
blending one into another by insensible gradations, the range of any one
species, depending as it does on the range of others, will tend to be
sharply defined. Moreover, each species on the confines of its range,
where it exists in lessened numbers, will, during fluctuations in the
number of its enemies or of its prey, or in the seasons, be extremely
liable to utter extermination; and thus its geographical range will come
to be still more sharply defined.

If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each
has a wide range, with a comparatively narrow neutral territory between
them, in which they become rather suddenly rarer and rarer; then, as
varieties do not essentially differ from species, the same rule will
probably apply to both; and if we in imagination adapt a varying species
to a very large area, we shall have to adapt two varieties to two
large areas, and a third variety to a narrow intermediate zone. The
intermediate variety, consequently, will exist in lesser numbers from
inhabiting a narrow and lesser area; and practically, as far as I can
make out, this rule holds good with varieties in a state of nature. I
have met with striking instances of the rule in the case of varieties
intermediate between well-marked varieties in the genus Balanus. And it
would appear from information given me by Mr. Watson, Dr. Asa Gray, and
Mr. Wollaston, that generally when varieties intermediate between two
other forms occur, they are much rarer numerically than the forms which
they connect. Now, if we may trust these facts and inferences, and
therefore conclude that varieties linking two other varieties together
have generally existed in lesser numbers than the forms which they
connect, then, I think, we can understand why intermediate varieties
should not endure for very long periods;--why as a general rule they
should be exterminated and disappear, sooner than the forms which they
originally linked together.

For any form existing in lesser numbers would, as already remarked,
run a greater chance of being exterminated than one existing in large
numbers; and in this particular case the intermediate form would be
eminently liable to the inroads of closely allied forms existing on both
sides of it. But a far more important consideration, as I believe, is
that, during the process of further modification, by which two varieties
are supposed on my theory to be converted and perfected into two
distinct species, the two which exist in larger numbers from inhabiting
larger areas, will have a great advantage over the intermediate variety,
which exists in smaller numbers in a narrow and intermediate zone.
For forms existing in larger numbers will always have a better chance,
within any given period, of presenting further favourable variations for
natural selection to seize on, than will the rarer forms which exist in
lesser numbers. Hence, the more common forms, in the race for life, will
tend to beat and supplant the less common forms, for these will be
more slowly modified and improved. It is the same principle which, as
I believe, accounts for the common species in each country, as shown
in the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what I
mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly
tract; and a third to wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks
by selection; the chances in this case will be strongly in favour of the
great holders on the mountains or on the plains improving their breeds
more quickly than the small holders on the intermediate narrow, hilly
tract; and consequently the improved mountain or plain breed will soon
take the place of the less improved hill breed; and thus the two breeds,
which originally existed in greater numbers, will come into close
contact with each other, without the interposition of the supplanted,
intermediate hill-variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are
very slowly formed, for variation is a very slow process, and natural
selection can do nothing until favourable variations chance to occur,
and until a place in the natural polity of the country can be better
filled by some modification of some one or more of its inhabitants.
And such new places will depend on slow changes of climate, or on the
occasional immigration of new inhabitants, and, probably, in a still
more important degree, on some of the old inhabitants becoming slowly
modified, with the new forms thus produced and the old ones acting and
reacting on each other. So that, in any one region and at any one time,
we ought only to see a few species presenting slight modifications of
structure in some degree permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more especially
amongst the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the several
representative species and their common parent, must formerly have
existed in each broken portion of the land, but these links will
have been supplanted and exterminated during the process of natural
selection, so that they will no longer exist in a living state.

Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will, it
is probable, at first have been formed in the intermediate zones, but
they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely from what we know
of the actual distribution of closely allied or representative species,
and likewise of acknowledged varieties), exist in the intermediate zones
in lesser numbers than the varieties which they tend to connect. From
this cause alone the intermediate varieties will be liable to accidental
extermination; and during the process of further modification through
natural selection, they will almost certainly be beaten and supplanted
by the forms which they connect; for these from existing in greater
numbers will, in the aggregate, present more variation, and thus be
further improved through natural selection and gain further advantages.

Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but the
very process of natural selection constantly tends, as has been so often
remarked, to exterminate the parent forms and the intermediate links.
Consequently evidence of their former existence could be found only
amongst fossil remains, which are preserved, as we shall in a future
chapter attempt to show, in an extremely imperfect and intermittent
record.

ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.

It has been asked by the opponents of such views as I hold, how, for
instance, a land carnivorous animal could have been converted into one
with aquatic habits; for how could the animal in its transitional state
have subsisted? It would be easy to show that within the same group
carnivorous animals exist having every intermediate grade between
truly aquatic and strictly terrestrial habits; and as each exists by a
struggle for life, it is clear that each is well adapted in its habits
to its place in nature. Look at the Mustela vison of North America,
which has webbed feet and which resembles an otter in its fur, short
legs, and form of tail; during summer this animal dives for and preys on
fish, but during the long winter it leaves the frozen waters, and preys
like other polecats on mice and land animals. If a different case had
been taken, and it had been asked how an insectivorous quadruped could
possibly have been converted into a flying bat, the question would have
been far more difficult, and I could have given no answer. Yet I think
such difficulties have very little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for out
of the many striking cases which I have collected, I can give only one
or two instances of transitional habits and structures in closely allied
species of the same genus; and of diversified habits, either constant
or occasional, in the same species. And it seems to me that nothing less
than a long list of such cases is sufficient to lessen the difficulty in
any particular case like that of the bat.

Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as
Sir J. Richardson has remarked, with the posterior part of their bodies
rather wide and with the skin on their flanks rather full, to the
so-called flying squirrels; and flying squirrels have their limbs and
even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to
an astonishing distance from tree to tree. We cannot doubt that each
structure is of use to each kind of squirrel in its own country, by
enabling it to escape birds or beasts of prey, or to collect food more
quickly, or, as there is reason to believe, by lessening the danger
from occasional falls. But it does not follow from this fact that the
structure of each squirrel is the best that it is possible to conceive
under all natural conditions. Let the climate and vegetation change,
let other competing rodents or new beasts of prey immigrate, or old ones
become modified, and all analogy would lead us to believe that some at
least of the squirrels would decrease in numbers or become exterminated,
unless they also became modified and improved in structure in a
corresponding manner. Therefore, I can see no difficulty, more
especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes,
each modification being useful, each being propagated, until by the
accumulated effects of this process of natural selection, a perfect
so-called flying squirrel was produced.

Now look at the Galeopithecus or flying lemur, which formerly was
falsely ranked amongst bats. It has an extremely wide flank-membrane,
stretching from the corners of the jaw to the tail, and including the
limbs and the elongated fingers: the flank membrane is, also, furnished
with an extensor muscle. Although no graduated links of structure,
fitted for gliding through the air, now connect the Galeopithecus with
the other Lemuridae, yet I can see no difficulty in supposing that such
links formerly existed, and that each had been formed by the same steps
as in the case of the less perfectly gliding squirrels; and that each
grade of structure had been useful to its possessor. Nor can I see
any insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might be
greatly lengthened by natural selection; and this, as far as the organs
of flight are concerned, would convert it into a bat. In bats which have
the wing-membrane extended from the top of the shoulder to the
tail, including the hind-legs, we perhaps see traces of an apparatus
originally constructed for gliding through the air rather than for
flight.

If about a dozen genera of birds had become extinct or were unknown, who
would have ventured to have surmised that birds might have existed
which used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land,
like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the Apteryx. Yet the structure of each of these birds is
good for it, under the conditions of life to which it is exposed, for
each has to live by a struggle; but it is not necessarily the best
possible under all possible conditions. It must not be inferred from
these remarks that any of the grades of wing-structure here alluded to,
which perhaps may all have resulted from disuse, indicate the natural
steps by which birds have acquired their perfect power of flight; but
they serve, at least, to show what diversified means of transition are
possible.

Seeing that a few members of such water-breathing classes as the
Crustacea and Mollusca are adapted to live on the land, and seeing that
we have flying birds and mammals, flying insects of the most diversified
types, and formerly had flying reptiles, it is conceivable that
flying-fish, which now glide far through the air, slightly rising and
turning by the aid of their fluttering fins, might have been modified
into perfectly winged animals. If this had been effected, who would
have ever imagined that in an early transitional state they had been
inhabitants of the open ocean, and had used their incipient organs of
flight exclusively, as far as we know, to escape being devoured by other
fish?

When we see any structure highly perfected for any particular habit,
as the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom
continue to exist to the present day, for they will have been
supplanted by the very process of perfection through natural selection.
Furthermore, we may conclude that transitional grades between structures
fitted for very different habits of life will rarely have been developed
at an early period in great numbers and under many subordinate forms.
Thus, to return to our imaginary illustration of the flying-fish, it
does not seem probable that fishes capable of true flight would have
been developed under many subordinate forms, for taking prey of many
kinds in many ways, on the land and in the water, until their organs of
flight had come to a high stage of perfection, so as to have given them
a decided advantage over other animals in the battle for life. Hence the
chance of discovering species with transitional grades of structure in
a fossil condition will always be less, from their having existed
in lesser numbers, than in the case of species with fully developed
structures.

I will now give two or three instances of diversified and of changed
habits in the individuals of the same species. When either case occurs,
it would be easy for natural selection to fit the animal, by some
modification of its structure, for its changed habits, or exclusively
for one of its several different habits. But it is difficult to tell,
and immaterial for us, whether habits generally change first and
structure afterwards; or whether slight modifications of structure lead
to changed habits; both probably often change almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of the
many British insects which now feed on exotic plants, or exclusively on
artificial substances. Of diversified habits innumerable instances
could be given: I have often watched a tyrant flycatcher (Saurophagus
sulphuratus) in South America, hovering over one spot and then
proceeding to another, like a kestrel, and at other times standing
stationary on the margin of water, and then dashing like a kingfisher at
a fish. In our own country the larger titmouse (Parus major) may be seen
climbing branches, almost like a creeper; it often, like a shrike, kills
small birds by blows on the head; and I have many times seen and heard
it hammering the seeds of the yew on a branch, and thus breaking them
like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, like a whale,
insects in the water. Even in so extreme a case as this, if the supply
of insects were constant, and if better adapted competitors did not
already exist in the country, I can see no difficulty in a race of bears
being rendered, by natural selection, more and more aquatic in their
structure and habits, with larger and larger mouths, till a creature was
produced as monstrous as a whale.

As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other species
of the same genus, we might expect, on my theory, that such individuals
would occasionally have given rise to new species, having anomalous
habits, and with their structure either slightly or considerably
modified from that of their proper type. And such instances do occur in
nature. Can a more striking instance of adaptation be given than that of
a woodpecker for climbing trees and for seizing insects in the chinks of
the bark? Yet in North America there are woodpeckers which feed largely
on fruit, and others with elongated wings which chase insects on the
wing; and on the plains of La Plata, where not a tree grows, there is a
woodpecker, which in every essential part of its organisation, even in
its colouring, in the harsh tone of its voice, and undulatory flight,
told me plainly of its close blood-relationship to our common species;
yet it is a woodpecker which never climbs a tree!

Petrels are the most aerial and oceanic of birds, yet in the quiet
Sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, its manner of swimming, and
of flying when unwillingly it takes flight, would be mistaken by any one
for an auk or grebe; nevertheless, it is essentially a petrel, but with
many parts of its organisation profoundly modified. On the other hand,
the acutest observer by examining the dead body of the water-ouzel would
never have suspected its sub-aquatic habits; yet this anomalous
member of the strictly terrestrial thrush family wholly subsists by
diving,--grasping the stones with its feet and using its wings under
water.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not at all in agreement. What can be plainer than
that the webbed feet of ducks and geese are formed for swimming? yet
there are upland geese with webbed feet which rarely or never go near
the water; and no one except Audubon has seen the frigate-bird, which
has all its four toes webbed, alight on the surface of the sea. On the
other hand, grebes and coots are eminently aquatic, although their toes
are only bordered by membrane. What seems plainer than that the long
toes of grallatores are formed for walking over swamps and floating
plants, yet the water-hen is nearly as aquatic as the coot; and the
landrail nearly as terrestrial as the quail or partridge. In such
cases, and many others could be given, habits have changed without a
corresponding change of structure. The webbed feet of the upland goose
may be said to have become rudimentary in function, though not in
structure. In the frigate-bird, the deeply-scooped membrane between the
toes shows that structure has begun to change.

He who believes in separate and innumerable acts of creation will say,
that in these cases it has pleased the Creator to cause a being of one
type to take the place of one of another type; but this seems to me
only restating the fact in dignified language. He who believes in the
struggle for existence and in the principle of natural selection, will
acknowledge that every organic being is constantly endeavouring to
increase in numbers; and that if any one being vary ever so little,
either in habits or structure, and thus gain an advantage over some
other inhabitant of the country, it will seize on the place of that
inhabitant, however different it may be from its own place. Hence it
will cause him no surprise that there should be geese and frigate-birds
with webbed feet, either living on the dry land or most rarely alighting
on the water; that there should be long-toed corncrakes living in
meadows instead of in swamps; that there should be woodpeckers where not
a tree grows; that there should be diving thrushes, and petrels with the
habits of auks.

ORGANS OF EXTREME PERFECTION AND COMPLICATION.

To suppose that the eye, with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I freely
confess, absurd in the highest possible degree. Yet reason tells me,
that if numerous gradations from a perfect and complex eye to one very
imperfect and simple, each grade being useful to its possessor, can be
shown to exist; if further, the eye does vary ever so slightly, and
the variations be inherited, which is certainly the case; and if any
variation or modification in the organ be ever useful to an animal under
changing conditions of life, then the difficulty of believing that a
perfect and complex eye could be formed by natural selection, though
insuperable by our imagination, can hardly be considered real. How a
nerve comes to be sensitive to light, hardly concerns us more than how
life itself first originated; but I may remark that several facts make
me suspect that any sensitive nerve may be rendered sensitive to light,
and likewise to those coarser vibrations of the air which produce sound.

In looking for the gradations by which an organ in any species has been
perfected, we ought to look exclusively to its lineal ancestors; but
this is scarcely ever possible, and we are forced in each case to look
to species of the same group, that is to the collateral descendants
from the same original parent-form, in order to see what gradations are
possible, and for the chance of some gradations having been transmitted
from the earlier stages of descent, in an unaltered or little altered
condition. Amongst existing Vertebrata, we find but a small amount of
gradation in the structure of the eye, and from fossil species we can
learn nothing on this head. In this great class we should probably
have to descend far beneath the lowest known fossiliferous stratum to
discover the earlier stages, by which the eye has been perfected.

In the Articulata we can commence a series with an optic nerve merely
coated with pigment, and without any other mechanism; and from this
low stage, numerous gradations of structure, branching off in two
fundamentally different lines, can be shown to exist, until we reach
a moderately high stage of perfection. In certain crustaceans, for
instance, there is a double cornea, the inner one divided into
facets, within each of which there is a lens-shaped swelling. In other
crustaceans the transparent cones which are coated by pigment, and which
properly act only by excluding lateral pencils of light, are convex at
their upper ends and must act by convergence; and at their lower ends
there seems to be an imperfect vitreous substance. With these facts,
here far too briefly and imperfectly given, which show that there is
much graduated diversity in the eyes of living crustaceans, and bearing
in mind how small the number of living animals is in proportion to those
which have become extinct, I can see no very great difficulty (not more
than in the case of many other structures) in believing that natural
selection has converted the simple apparatus of an optic nerve merely
coated with pigment and invested by transparent membrane, into an
optical instrument as perfect as is possessed by any member of the great
Articulate class.

He who will go thus far, if he find on finishing this treatise that
large bodies of facts, otherwise inexplicable, can be explained by the
theory of descent, ought not to hesitate to go further, and to admit
that a structure even as perfect as the eye of an eagle might be formed
by natural selection, although in this case he does not know any of the
transitional grades. His reason ought to conquer his imagination; though
I have felt the difficulty far too keenly to be surprised at any degree
of hesitation in extending the principle of natural selection to such
startling lengths.

It is scarcely possible to avoid comparing the eye to a telescope.
We know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that the
eye has been formed by a somewhat analogous process. But may not this
inference be presumptuous? Have we any right to assume that the Creator
works by intellectual powers like those of man? If we must compare the
eye to an optical instrument, we ought in imagination to take a thick
layer of transparent tissue, with a nerve sensitive to light beneath,
and then suppose every part of this layer to be continually changing
slowly in density, so as to separate into layers of different densities
and thicknesses, placed at different distances from each other, and
with the surfaces of each layer slowly changing in form. Further we
must suppose that there is a power always intently watching each slight
accidental alteration in the transparent layers; and carefully selecting
each alteration which, under varied circumstances, may in any way, or in
any degree, tend to produce a distincter image. We must suppose each new
state of the instrument to be multiplied by the million; and each to
be preserved till a better be produced, and then the old ones to
be destroyed. In living bodies, variation will cause the slight
alterations, generation will multiply them almost infinitely, and
natural selection will pick out with unerring skill each improvement.
Let this process go on for millions on millions of years; and during
each year on millions of individuals of many kinds; and may we not
believe that a living optical instrument might thus be formed as
superior to one of glass, as the works of the Creator are to those of
man?

If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to my theory, there has been much
extinction. Or again, if we look to an organ common to all the members
of a large class, for in this latter case the organ must have been first
formed at an extremely remote period, since which all the many members
of the class have been developed; and in order to discover the early
transitional grades through which the organ has passed, we should have
to look to very ancient ancestral forms, long since become extinct.

We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous cases
could be given amongst the lower animals of the same organ performing
at the same time wholly distinct functions; thus the alimentary canal
respires, digests, and excretes in the larva of the dragon-fly and in
the fish Cobites. In the Hydra, the animal may be turned inside out, and
the exterior surface will then digest and the stomach respire. In such
cases natural selection might easily specialise, if any advantage were
thus gained, a part or organ, which had performed two functions, for one
function alone, and thus wholly change its nature by insensible steps.
Two distinct organs sometimes perform simultaneously the same function
in the same individual; to give one instance, there are fish with gills
or branchiae that breathe the air dissolved in the water, at the same
time that they breathe free air in their swimbladders, this latter organ
having a ductus pneumaticus for its supply, and being divided by highly
vascular partitions. In these cases, one of the two organs might with
ease be modified and perfected so as to perform all the work by itself,
being aided during the process of modification by the other organ;
and then this other organ might be modified for some other and quite
distinct purpose, or be quite obliterated.

The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a wholly different purpose, namely respiration. The swimbladder has,
also, been worked in as an accessory to the auditory organs of certain
fish, or, for I do not know which view is now generally held, a part
of the auditory apparatus has been worked in as a complement to the
swimbladder. All physiologists admit that the swimbladder is homologous,
or "ideally similar," in position and structure with the lungs of
the higher vertebrate animals: hence there seems to me to be no great
difficulty in believing that natural selection has actually converted a
swimbladder into a lung, or organ used exclusively for respiration.

I can, indeed, hardly doubt that all vertebrate animals having true
lungs have descended by ordinary generation from an ancient prototype,
of which we know nothing, furnished with a floating apparatus or
swimbladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis
is closed. In the higher Vertebrata the branchiae have wholly
disappeared--the slits on the sides of the neck and the loop-like course
of the arteries still marking in the embryo their former position. But
it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some quite distinct
purpose: in the same manner as, on the view entertained by some
naturalists that the branchiae and dorsal scales of Annelids are
homologous with the wings and wing-covers of insects, it is probable
that organs which at a very ancient period served for respiration have
been actually converted into organs of flight.

In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give one more instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means
of a sticky secretion, to retain the eggs until they are hatched within
the sack. These cirripedes have no branchiae, the whole surface of the
body and sack, including the small frena, serving for respiration. The
Balanidae or sessile cirripedes, on the other hand, have no ovigerous
frena, the eggs lying loose at the bottom of the sack, in the
well-enclosed shell; but they have large folded branchiae. Now I think
no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchiae of the other family; indeed, they
graduate into each other. Therefore I do not doubt that little folds of
skin, which originally served as ovigerous frena, but which, likewise,
very slightly aided the act of respiration, have been gradually
converted by natural selection into branchiae, simply through an
increase in their size and the obliteration of their adhesive glands.
If all pedunculated cirripedes had become extinct, and they have already
suffered far more extinction than have sessile cirripedes, who would
ever have imagined that the branchiae in this latter family had
originally existed as organs for preventing the ova from being washed
out of the sack?

Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, grave cases of difficulty occur, some of
which will be discussed in my future work.

One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but
this case will be treated of in the next chapter. The electric organs
of fishes offer another case of special difficulty; it is impossible to
conceive by what steps these wondrous organs have been produced; but,
as Owen and others have remarked, their intimate structure closely
resembles that of common muscle; and as it has lately been shown that
Rays have an organ closely analogous to the electric apparatus, and yet
do not, as Matteuchi asserts, discharge any electricity, we must own
that we are far too ignorant to argue that no transition of any kind is
possible.

The electric organs offer another and even more serious difficulty; for
they occur in only about a dozen fishes, of which several are widely
remote in their affinities. Generally when the same organ appears in
several members of the same class, especially if in members having very
different habits of life, we may attribute its presence to inheritance
from a common ancestor; and its absence in some of the members to its
loss through disuse or natural selection. But if the electric organs had
been inherited from one ancient progenitor thus provided, we might have
expected that all electric fishes would have been specially related to
each other. Nor does geology at all lead to the belief that formerly
most fishes had electric organs, which most of their modified
descendants have lost. The presence of luminous organs in a few insects,
belonging to different families and orders, offers a parallel case of
difficulty. Other cases could be given; for instance in plants, the very
curious contrivance of a mass of pollen-grains, borne on a
foot-stalk with a sticky gland at the end, is the same in Orchis and
Asclepias,--genera almost as remote as possible amongst flowering
plants. In all these cases of two very distinct species furnished
with apparently the same anomalous organ, it should be observed that,
although the general appearance and function of the organ may be the
same, yet some fundamental difference can generally be detected. I
am inclined to believe that in nearly the same way as two men have
sometimes independently hit on the very same invention, so natural
selection, working for the good of each being and taking advantage of
analogous variations, has sometimes modified in very nearly the same
manner two parts in two organic beings, which owe but little of their
structure in common to inheritance from the same ancestor.

Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering that the proportion of living and known forms to the extinct
and unknown is very small, I have been astonished how rarely an organ
can be named, towards which no transitional grade is known to lead.
The truth of this remark is indeed shown by that old canon in natural
history of "Natura non facit saltum." We meet with this admission in the
writings of almost every experienced naturalist; or, as Milne Edwards
has well expressed it, nature is prodigal in variety, but niggard in
innovation. Why, on the theory of Creation, should this be so? Why
should all the parts and organs of many independent beings, each
supposed to have been separately created for its proper place in nature,
be so invariably linked together by graduated steps? Why should not
Nature have taken a leap from structure to structure? On the theory of
natural selection, we can clearly understand why she should not; for
natural selection can act only by taking advantage of slight successive
variations; she can never take a leap, but must advance by the shortest
and slowest steps.

ORGANS OF LITTLE APPARENT IMPORTANCE.

As natural selection acts by life and death,--by the preservation of
individuals with any favourable variation, and by the destruction of
those with any unfavourable deviation of structure,--I have sometimes
felt much difficulty in understanding the origin of simple parts, of
which the importance does not seem sufficient to cause the preservation
of successively varying individuals. I have sometimes felt as much
difficulty, though of a very different kind, on this head, as in the
case of an organ as perfect and complex as the eye.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications would
be of importance or not. In a former chapter I have given instances of
most trifling characters, such as the down on fruit and the colour of
the flesh, which, from determining the attacks of insects or from being
correlated with constitutional differences, might assuredly be acted on
by natural selection. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible that this
could have been adapted for its present purpose by successive slight
modifications, each better and better, for so trifling an object as
driving away flies; yet we should pause before being too positive even
in this case, for we know that the distribution and existence of cattle
and other animals in South America absolutely depends on their power of
resisting the attacks of insects: so that individuals which could by any
means defend themselves from these small enemies, would be able to range
into new pastures and thus gain a great advantage. It is not that the
larger quadrupeds are actually destroyed (except in some rare cases) by
the flies, but they are incessantly harassed and their strength reduced,
so that they are more subject to disease, or not so well enabled in a
coming dearth to search for food, or to escape from beasts of prey.

Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly
perfected at a former period, have been transmitted in nearly the
same state, although now become of very slight use; and any actually
injurious deviations in their structure will always have been checked by
natural selection. Seeing how important an organ of locomotion the
tail is in most aquatic animals, its general presence and use for many
purposes in so many land animals, which in their lungs or modified
swim-bladders betray their aquatic origin, may perhaps be thus accounted
for. A well-developed tail having been formed in an aquatic animal, it
might subsequently come to be worked in for all sorts of purposes, as
a fly-flapper, an organ of prehension, or as an aid in turning, as with
the dog, though the aid must be slight, for the hare, with hardly any
tail, can double quickly enough.

In the second place, we may sometimes attribute importance to characters
which are really of very little importance, and which have originated
from quite secondary causes, independently of natural selection. We
should remember that climate, food, etc., probably have some little
direct influence on the organisation; that characters reappear from
the law of reversion; that correlation of growth will have had a most
important influence in modifying various structures; and finally,
that sexual selection will often have largely modified the external
characters of animals having a will, to give one male an advantage
in fighting with another or in charming the females. Moreover when a
modification of structure has primarily arisen from the above or
other unknown causes, it may at first have been of no advantage to
the species, but may subsequently have been taken advantage of by the
descendants of the species under new conditions of life and with newly
acquired habits.

To give a few instances to illustrate these latter remarks. If green
woodpeckers alone had existed, and we did not know that there were many
black and pied kinds, I dare say that we should have thought that the
green colour was a beautiful adaptation to hide this tree-frequenting
bird from its enemies; and consequently that it was a character of
importance and might have been acquired through natural selection; as it
is, I have no doubt that the colour is due to some quite distinct cause,
probably to sexual selection. A trailing bamboo in the Malay Archipelago
climbs the loftiest trees by the aid of exquisitely constructed hooks
clustered around the ends of the branches, and this contrivance, no
doubt, is of the highest service to the plant; but as we see nearly
similar hooks on many trees which are not climbers, the hooks on the
bamboo may have arisen from unknown laws of growth, and have been
subsequently taken advantage of by the plant undergoing further
modification and becoming a climber. The naked skin on the head of a
vulture is generally looked at as a direct adaptation for wallowing in
putridity; and so it may be, or it may possibly be due to the direct
action of putrid matter; but we should be very cautious in drawing
any such inference, when we see that the skin on the head of the
clean-feeding male turkey is likewise naked. The sutures in the skulls
of young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable
for this act; but as sutures occur in the skulls of young birds and
reptiles, which have only to escape from a broken egg, we may infer that
this structure has arisen from the laws of growth, and has been taken
advantage of in the parturition of the higher animals.

We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are immediately made conscious of this by
reflecting on the differences in the breeds of our domesticated animals
in different countries,--more especially in the less civilized countries
where there has been but little artificial selection. Careful observers
are convinced that a damp climate affects the growth of the hair, and
that with the hair the horns are correlated. Mountain breeds always
differ from lowland breeds; and a mountainous country would probably
affect the hind limbs from exercising them more, and possibly even the
form of the pelvis; and then by the law of homologous variation, the
front limbs and even the head would probably be affected. The shape,
also, of the pelvis might affect by pressure the shape of the head of
the young in the womb. The laborious breathing necessary in high regions
would, we have some reason to believe, increase the size of the chest;
and again correlation would come into play. Animals kept by savages in
different countries often have to struggle for their own subsistence,
and would be exposed to a certain extent to natural selection, and
individuals with slightly different constitutions would succeed
best under different climates; and there is reason to believe that
constitution and colour are correlated. A good observer, also, states
that in cattle susceptibility to the attacks of flies is correlated with
colour, as is the liability to be poisoned by certain plants; so that
colour would be thus subjected to the action of natural selection. But
we are far too ignorant to speculate on the relative importance of the
several known and unknown laws of variation; and I have here alluded
to them only to show that, if we are unable to account for the
characteristic differences of our domestic breeds, which nevertheless we
generally admit to have arisen through ordinary generation, we ought
not to lay too much stress on our ignorance of the precise cause of the
slight analogous differences between species. I might have adduced for
this same purpose the differences between the races of man, which are
so strongly marked; I may add that some little light can apparently
be thrown on the origin of these differences, chiefly through sexual
selection of a particular kind, but without here entering on copious
details my reasoning would appear frivolous.

The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists, against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor.
They believe that very many structures have been created for beauty in
the eyes of man, or for mere variety. This doctrine, if true, would be
absolutely fatal to my theory. Yet I fully admit that many structures
are of no direct use to their possessors. Physical conditions probably
have had some little effect on structure, quite independently of any
good thus gained. Correlation of growth has no doubt played a most
important part, and a useful modification of one part will often have
entailed on other parts diversified changes of no direct use. So again
characters which formerly were useful, or which formerly had arisen from
correlation of growth, or from other unknown cause, may reappear from
the law of reversion, though now of no direct use. The effects of sexual
selection, when displayed in beauty to charm the females, can be called
useful only in rather a forced sense. But by far the most important
consideration is that the chief part of the organisation of every
being is simply due to inheritance; and consequently, though each being
assuredly is well fitted for its place in nature, many structures now
have no direct relation to the habits of life of each species. Thus, we
can hardly believe that the webbed feet of the upland goose or of the
frigate-bird are of special use to these birds; we cannot believe that
the same bones in the arm of the monkey, in the fore leg of the horse,
in the wing of the bat, and in the flipper of the seal, are of special
use to these animals. We may safely attribute these structures to
inheritance. But to the progenitor of the upland goose and of the
frigate-bird, webbed feet no doubt were as useful as they now are to the
most aquatic of existing birds. So we may believe that the progenitor of
the seal had not a flipper, but a foot with five toes fitted for walking
or grasping; and we may further venture to believe that the several
bones in the limbs of the monkey, horse, and bat, which have been
inherited from a common progenitor, were formerly of more special use to
that progenitor, or its progenitors, than they now are to these animals
having such widely diversified habits. Therefore we may infer that
these several bones might have been acquired through natural selection,
subjected formerly, as now, to the several laws of inheritance,
reversion, correlation of growth, etc. Hence every detail of structure
in every living creature (making some little allowance for the direct
action of physical conditions) may be viewed, either as having been of
special use to some ancestral form, or as being now of special use to
the descendants of this form--either directly, or indirectly through the
complex laws of growth.

Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang
of the adder, and in the ovipositor of the ichneumon, by which its eggs
are deposited in the living bodies of other insects. If it could be
proved that any part of the structure of any one species had been
formed for the exclusive good of another species, it would annihilate my
theory, for such could not have been produced through natural selection.
Although many statements may be found in works on natural history to
this effect, I cannot find even one which seems to me of any weight. It
is admitted that the rattlesnake has a poison-fang for its own defence
and for the destruction of its prey; but some authors suppose that at
the same time this snake is furnished with a rattle for its own injury,
namely, to warn its prey to escape. I would almost as soon believe that
the cat curls the end of its tail when preparing to spring, in order to
warn the doomed mouse. But I have not space here to enter on this and
other such cases.

Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each.
No organ will be formed, as Paley has remarked, for the purpose of
causing pain or for doing an injury to its possessor. If a fair balance
be struck between the good and evil caused by each part, each will be
found on the whole advantageous. After the lapse of time, under changing
conditions of life, if any part comes to be injurious, it will be
modified; or if it be not so, the being will become extinct, as myriads
have become extinct.

Natural selection tends only to make each organic being as perfect as,
or slightly more perfect than, the other inhabitants of the same country
with which it has to struggle for existence. And we see that this is the
degree of perfection attained under nature. The endemic productions of
New Zealand, for instance, are perfect one compared with another; but
they are now rapidly yielding before the advancing legions of plants
and animals introduced from Europe. Natural selection will not produce
absolute perfection, nor do we always meet, as far as we can judge, with
this high standard under nature. The correction for the aberration of
light is said, on high authority, not to be perfect even in that most
perfect organ, the eye. If our reason leads us to admire with enthusiasm
a multitude of inimitable contrivances in nature, this same reason tells
us, though we may easily err on both sides, that some other contrivances
are less perfect. Can we consider the sting of the wasp or of the bee
as perfect, which, when used against many attacking animals, cannot be
withdrawn, owing to the backward serratures, and so inevitably causes
the death of the insect by tearing out its viscera?

If we look at the sting of the bee, as having originally existed in a
remote progenitor as a boring and serrated instrument, like that in so
many members of the same great order, and which has been modified
but not perfected for its present purpose, with the poison originally
adapted to cause galls subsequently intensified, we can perhaps
understand how it is that the use of the sting should so often cause the
insect's own death: for if on the whole the power of stinging be
useful to the community, it will fulfil all the requirements of natural
selection, though it may cause the death of some few members. If we
admire the truly wonderful power of scent by which the males of many
insects find their females, can we admire the production for this
single purpose of thousands of drones, which are utterly useless to the
community for any other end, and which are ultimately slaughtered by
their industrious and sterile sisters? It may be difficult, but we ought
to admire the savage instinctive hatred of the queen-bee, which urges
her instantly to destroy the young queens her daughters as soon as born,
or to perish herself in the combat; for undoubtedly this is for the
good of the community; and maternal love or maternal hatred, though
the latter fortunately is most rare, is all the same to the inexorable
principle of natural selection. If we admire the several ingenious
contrivances, by which the flowers of the orchis and of many other
plants are fertilised through insect agency, can we consider as equally
perfect the elaboration by our fir-trees of dense clouds of pollen, in
order that a few granules may be wafted by a chance breeze on to the
ovules?

SUMMARY OF CHAPTER.

We have in this chapter discussed some of the difficulties and
objections which may be urged against my theory. Many of them are very
grave; but I think that in the discussion light has been thrown on
several facts, which on the theory of independent acts of creation are
utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural selection
will always be very slow, and will act, at any one time, only on a very
few forms; and partly because the very process of natural selection
almost implies the continual supplanting and extinction of preceding
and intermediate gradations. Closely allied species, now living on
a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be
formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two
forms which it connects; consequently the two latter, during the course
of further modification, from existing in greater numbers, will have a
great advantage over the less numerous intermediate variety, and will
thus generally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each
other; that a bat, for instance, could not have been formed by natural
selection from an animal which at first could only glide through the
air.

We have seen that a species may under new conditions of life change its
habits, or have diversified habits, with some habits very unlike those
of its nearest congeners. Hence we can understand, bearing in mind that
each organic being is trying to live wherever it can live, how it has
arisen that there are upland geese with webbed feet, ground woodpeckers,
diving thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one; yet
in the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing conditions
of life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the cases
in which we know of no intermediate or transitional states, we should
be very cautious in concluding that none could have existed, for the
homologies of many organs and their intermediate states show that
wonderful metamorphoses in function are at least possible. For instance,
a swim-bladder has apparently been converted into an air-breathing lung.
The same organ having performed simultaneously very different functions,
and then having been specialised for one function; and two very distinct
organs having performed at the same time the same function, the one
having been perfected whilst aided by the other, must often have largely
facilitated transitions.

We are far too ignorant, in almost every case, to be enabled to assert
that any part or organ is so unimportant for the welfare of a species,
that modifications in its structure could not have been slowly
accumulated by means of natural selection. But we may confidently
believe that many modifications, wholly due to the laws of growth, and
at first in no way advantageous to a species, have been subsequently
taken advantage of by the still further modified descendants of this
species. We may, also, believe that a part formerly of high importance
has often been retained (as the tail of an aquatic animal by its
terrestrial descendants), though it has become of such small importance
that it could not, in its present state, have been acquired by natural
selection,--a power which acts solely by the preservation of profitable
variations in the struggle for life.

Natural selection will produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the owner.
Natural selection in each well-stocked country, must act chiefly through
the competition of the inhabitants one with another, and consequently
will produce perfection, or strength in the battle for life, only
according to the standard of that country. Hence the inhabitants of one
country, generally the smaller one, will often yield, as we see they do
yield, to the inhabitants of another and generally larger country. For
in the larger country there will have existed more individuals, and more
diversified forms, and the competition will have been severer, and
thus the standard of perfection will have been rendered higher. Natural
selection will not necessarily produce absolute perfection; nor, as far
as we can judge by our limited faculties, can absolute perfection be
everywhere found.

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look only to the present inhabitants of the world, is
not strictly correct, but if we include all those of past times, it must
by my theory be strictly true.

It is generally acknowledged that all organic beings have been formed on
two great laws--Unity of Type, and the Conditions of Existence. By unity
of type is meant that fundamental agreement in structure, which we see
in organic beings of the same class, and which is quite independent of
their habits of life. On my theory, unity of type is explained by unity
of descent. The expression of conditions of existence, so often insisted
on by the illustrious Cuvier, is fully embraced by the principle of
natural selection. For natural selection acts by either now adapting the
varying parts of each being to its organic and inorganic conditions of
life; or by having adapted them during long-past periods of time: the
adaptations being aided in some cases by use and disuse, being slightly
affected by the direct action of the external conditions of life, and
being in all cases subjected to the several laws of growth. Hence, in
fact, the law of the Conditions of Existence is the higher law; as it
includes, through the inheritance of former adaptations, that of Unity
of Type.




7. INSTINCT.

Instincts comparable with habits, but different in their origin.
Instincts graduated. Aphides and ants. Instincts variable. Domestic
instincts, their origin. Natural instincts of the cuckoo, ostrich, and
parasitic bees. Slave-making ants. Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts. Neuter
or sterile insects. Summary.

The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat
the subject separately, especially as so wonderful an instinct as that
of the hive-bee making its cells will probably have occurred to many
readers, as a difficulty sufficient to overthrow my whole theory. I must
premise, that I have nothing to do with the origin of the primary mental
powers, any more than I have with that of life itself. We are concerned
only with the diversities of instinct and of the other mental qualities
of animals within the same class.

I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests.
An action, which we ourselves should require experience to enable us to
perform, when performed by an animal, more especially by a very young
one, without any experience, and when performed by many individuals in
the same way, without their knowing for what purpose it is performed,
is usually said to be instinctive. But I could show that none of these
characters of instinct are universal. A little dose, as Pierre Huber
expresses it, of judgment or reason, often comes into play, even in
animals very low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably
accurate notion of the frame of mind under which an instinctive action
is performed, but not of its origin. How unconsciously many habitual
actions are performed, indeed not rarely in direct opposition to our
conscious will! yet they may be modified by the will or reason. Habits
easily become associated with other habits, and with certain periods
of time and states of the body. When once acquired, they often remain
constant throughout life. Several other points of resemblance between
instincts and habits could be pointed out. As in repeating a well-known
song, so in instincts, one action follows another by a sort of rhythm;
if a person be interrupted in a song, or in repeating anything by rote,
he is generally forced to go back to recover the habitual train of
thought: so P. Huber found it was with a caterpillar, which makes a very
complicated hammock; for if he took a caterpillar which had completed
its hammock up to, say, the sixth stage of construction, and put it into
a hammock completed up only to the third stage, the caterpillar simply
re-performed the fourth, fifth, and sixth stages of construction.
If, however, a caterpillar were taken out of a hammock made up, for
instance, to the third stage, and were put into one finished up to the
sixth stage, so that much of its work was already done for it, far from
feeling the benefit of this, it was much embarrassed, and, in order to
complete its hammock, seemed forced to start from the third stage, where
it had left off, and thus tried to complete the already finished work.

If we suppose any habitual action to become inherited--and I think
it can be shown that this does sometimes happen--then the resemblance
between what originally was a habit and an instinct becomes so close as
not to be distinguished. If Mozart, instead of playing the pianoforte at
three years old with wonderfully little practice, had played a tune
with no practice at all, he might truly be said to have done so
instinctively. But it would be the most serious error to suppose that
the greater number of instincts have been acquired by habit in
one generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been thus acquired.

It will be universally admitted that instincts are as important as
corporeal structure for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that may
be profitable. It is thus, as I believe, that all the most complex
and wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are of quite
subordinate importance to the effects of the natural selection of what
may be called accidental variations of instincts;--that is of variations
produced by the same unknown causes which produce slight deviations of
bodily structure.

No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures,
we ought to find in nature, not the actual transitional gradations by
which each complex instinct has been acquired--for these could be found
only in the lineal ancestors of each species--but we ought to find in
the collateral lines of descent some evidence of such gradations; or
we ought at least to be able to show that gradations of some kind are
possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little
observed except in Europe and North America, and for no instinct being
known amongst extinct species, how very generally gradations, leading to
the most complex instincts, can be discovered. The canon of "Natura non
facit saltum" applies with almost equal force to instincts as to bodily
organs. Changes of instinct may sometimes be facilitated by the same
species having different instincts at different periods of life, or
at different seasons of the year, or when placed under different
circumstances, etc.; in which case either one or the other instinct
might be preserved by natural selection. And such instances of diversity
of instinct in the same species can be shown to occur in nature.

Again as in the case of corporeal structure, and conformably with my
theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others.
One of the strongest instances of an animal apparently performing an
action for the sole good of another, with which I am acquainted, is that
of aphides voluntarily yielding their sweet excretion to ants: that they
do so voluntarily, the following facts show. I removed all the ants from
a group of about a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval, I felt sure that
the aphides would want to excrete. I watched them for some time through
a lens, but not one excreted; I then tickled and stroked them with a
hair in the same manner, as well as I could, as the ants do with their
antennae; but not one excreted. Afterwards I allowed an ant to visit
them, and it immediately seemed, by its eager way of running about, to
be well aware what a rich flock it had discovered; it then began to play
with its antennae on the abdomen first of one aphis and then of another;
and each aphis, as soon as it felt the antennae, immediately lifted up
its abdomen and excreted a limpid drop of sweet juice, which was eagerly
devoured by the ant. Even the quite young aphides behaved in this
manner, showing that the action was instinctive, and not the result of
experience. But as the excretion is extremely viscid, it is probably a
convenience to the aphides to have it removed; and therefore probably
the aphides do not instinctively excrete for the sole good of the ants.
Although I do not believe that any animal in the world performs an
action for the exclusive good of another of a distinct species, yet
each species tries to take advantage of the instincts of others, as each
takes advantage of the weaker bodily structure of others. So again, in
some few cases, certain instincts cannot be considered as absolutely
perfect; but as details on this and other such points are not
indispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to have been here
given; but want of space prevents me. I can only assert, that instincts
certainly do vary--for instance, the migratory instinct, both in extent
and direction, and in its total loss. So it is with the nests of birds,
which vary partly in dependence on the situations chosen, and on the
nature and temperature of the country inhabited, but often from causes
wholly unknown to us: Audubon has given several remarkable cases of
differences in nests of the same species in the northern and southern
United States. Fear of any particular enemy is certainly an instinctive
quality, as may be seen in nestling birds, though it is strengthened by
experience, and by the sight of fear of the same enemy in other animals.
But fear of man is slowly acquired, as I have elsewhere shown, by
various animals inhabiting desert islands; and we may see an instance
of this, even in England, in the greater wildness of all our large birds
than of our small birds; for the large birds have been most persecuted
by man. We may safely attribute the greater wildness of our large birds
to this cause; for in uninhabited islands large birds are not more
fearful than small; and the magpie, so wary in England, is tame in
Norway, as is the hooded crow in Egypt.

That the general disposition of individuals of the same species, born in
a state of nature, is extremely diversified, can be shown by a multitude
of facts. Several cases also, could be given, of occasional and strange
habits in certain species, which might, if advantageous to the species,
give rise, through natural selection, to quite new instincts. But I am
well aware that these general statements, without facts given in detail,
can produce but a feeble effect on the reader's mind. I can only repeat
my assurance, that I do not speak without good evidence.

The possibility, or even probability, of inherited variations
of instinct in a state of nature will be strengthened by briefly
considering a few cases under domestication. We shall thus also be
enabled to see the respective parts which habit and the selection of
so-called accidental variations have played in modifying the mental
qualities of our domestic animals. A number of curious and authentic
instances could be given of the inheritance of all shades of disposition
and tastes, and likewise of the oddest tricks, associated with certain
frames of mind or periods of time. But let us look to the familiar case
of the several breeds of dogs: it cannot be doubted that young pointers
(I have myself seen a striking instance) will sometimes point and even
back other dogs the very first time that they are taken out; retrieving
is certainly in some degree inherited by retrievers; and a tendency to
run round, instead of at, a flock of sheep, by shepherd-dogs. I cannot
see that these actions, performed without experience by the young,
and in nearly the same manner by each individual, performed with eager
delight by each breed, and without the end being known,--for the young
pointer can no more know that he points to aid his master, than
the white butterfly knows why she lays her eggs on the leaf of the
cabbage,--I cannot see that these actions differ essentially from true
instincts. If we were to see one kind of wolf, when young and without
any training, as soon as it scented its prey, stand motionless like a
statue, and then slowly crawl forward with a peculiar gait; and another
kind of wolf rushing round, instead of at, a herd of deer, and driving
them to a distant point, we should assuredly call these actions
instinctive. Domestic instincts, as they may be called, are certainly
far less fixed or invariable than natural instincts; but they have been
acted on by far less rigorous selection, and have been transmitted for
an incomparably shorter period, under less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with
a bull-dog has affected for many generations the courage and obstinacy
of greyhounds; and a cross with a greyhound has given to a whole family
of shepherd-dogs a tendency to hunt hares. These domestic instincts,
when thus tested by crossing, resemble natural instincts, which in a
like manner become curiously blended together, and for a long period
exhibit traces of the instincts of either parent: for example, Le Roy
describes a dog, whose great-grandfather was a wolf, and this dog
showed a trace of its wild parentage only in one way, by not coming in a
straight line to his master when called.

Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this,
I think, is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler-pigeon to tumble,--an action
which, as I have witnessed, is performed by young birds, that have
never seen a pigeon tumble. We may believe that some one pigeon showed
a slight tendency to this strange habit, and that the long-continued
selection of the best individuals in successive generations made
tumblers what they now are; and near Glasgow there are house-tumblers,
as I hear from Mr. Brent, which cannot fly eighteen inches high without
going head over heels. It may be doubted whether any one would have
thought of training a dog to point, had not some one dog naturally shown
a tendency in this line; and this is known occasionally to happen, as I
once saw in a pure terrier. When the first tendency was once displayed,
methodical selection and the inherited effects of compulsory training in
each successive generation would soon complete the work; and unconscious
selection is still at work, as each man tries to procure, without
intending to improve the breed, dogs which will stand and hunt best.
On the other hand, habit alone in some cases has sufficed; no animal is
more difficult to tame than the young of the wild rabbit; scarcely any
animal is tamer than the young of the tame rabbit; but I do not suppose
that domestic rabbits have ever been selected for tameness; and I
presume that we must attribute the whole of the inherited change from
extreme wildness to extreme tameness, simply to habit and long-continued
close confinement.

Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how universally and largely the minds of our
domestic animals have been modified by domestication. It is scarcely
possible to doubt that the love of man has become instinctive in the
dog. All wolves, foxes, jackals, and species of the cat genus, when
kept tame, are most eager to attack poultry, sheep, and pigs; and this
tendency has been found incurable in dogs which have been brought home
as puppies from countries, such as Tierra del Fuego and Australia, where
the savages do not keep these domestic animals. How rarely, on the other
hand, do our civilised dogs, even when quite young, require to be taught
not to attack poultry, sheep, and pigs! No doubt they occasionally
do make an attack, and are then beaten; and if not cured, they are
destroyed; so that habit, with some degree of selection, has probably
concurred in civilising by inheritance our dogs. On the other hand,
young chickens have lost, wholly by habit, that fear of the dog and cat
which no doubt was originally instinctive in them, in the same way as it
is so plainly instinctive in young pheasants, though reared under a hen.
It is not that chickens have lost all fear, but fear only of dogs and
cats, for if the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal themselves in
the surrounding grass or thickets; and this is evidently done for the
instinctive purpose of allowing, as we see in wild ground-birds, their
mother to fly away. But this instinct retained by our chickens has
become useless under domestication, for the mother-hen has almost lost
by disuse the power of flight.

Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar
mental habits and actions, which at first appeared from what we must in
our ignorance call an accident. In some cases compulsory habit alone
has sufficed to produce such inherited mental changes; in other cases
compulsory habit has done nothing, and all has been the result of
selection, pursued both methodically and unconsciously; but in most
cases, probably, habit and selection have acted together.

We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection, by considering a few cases. I will
select only three, out of the several which I shall have to discuss in
my future work,--namely, the instinct which leads the cuckoo to lay her
eggs in other birds' nests; the slave-making instinct of certain ants;
and the comb-making power of the hive-bee: these two latter instincts
have generally, and most justly, been ranked by naturalists as the most
wonderful of all known instincts.

It is now commonly admitted that the more immediate and final cause
of the cuckoo's instinct is, that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own
nest and sit on her own eggs, those first laid would have to be left
for some time unincubated, or there would be eggs and young birds of
different ages in the same nest. If this were the case, the process of
laying and hatching might be inconveniently long, more especially as she
has to migrate at a very early period; and the first hatched young would
probably have to be fed by the male alone. But the American cuckoo is
in this predicament; for she makes her own nest and has eggs and young
successively hatched, all at the same time. It has been asserted that
the American cuckoo occasionally lays her eggs in other birds' nests;
but I hear on the high authority of Dr. Brewer, that this is a mistake.
Nevertheless, I could give several instances of various birds which have
been known occasionally to lay their eggs in other birds' nests. Now let
us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo; but that occasionally she laid an egg in
another bird's nest. If the old bird profited by this occasional habit,
or if the young were made more vigorous by advantage having been taken
of the mistaken maternal instinct of another bird, than by their own
mother's care, encumbered as she can hardly fail to be by having eggs
and young of different ages at the same time; then the old birds or the
fostered young would gain an advantage. And analogy would lead me
to believe, that the young thus reared would be apt to follow by
inheritance the occasional and aberrant habit of their mother, and in
their turn would be apt to lay their eggs in other birds' nests, and
thus be successful in rearing their young. By a continued process of
this nature, I believe that the strange instinct of our cuckoo could be,
and has been, generated. I may add that, according to Dr. Gray and
to some other observers, the European cuckoo has not utterly lost all
maternal love and care for her own offspring.

The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with
the Gallinaceae; and this perhaps explains the origin of a singular
instinct in the allied group of ostriches. For several hen ostriches,
at least in the case of the American species, unite and lay first a
few eggs in one nest and then in another; and these are hatched by the
males. This instinct may probably be accounted for by the fact of the
hens laying a large number of eggs; but, as in the case of the cuckoo,
at intervals of two or three days. This instinct, however, of the
American ostrich has not as yet been perfected; for a surprising number
of eggs lie strewed over the plains, so that in one day's hunting I
picked up no less than twenty lost and wasted eggs.

Many bees are parasitic, and always lay their eggs in the nests of bees
of other kinds. This case is more remarkable than that of the cuckoo;
for these bees have not only their instincts but their structure
modified in accordance with their parasitic habits; for they do not
possess the pollen-collecting apparatus which would be necessary if
they had to store food for their own young. Some species, likewise, of
Sphegidae (wasp-like insects) are parasitic on other species; and M.
Fabre has lately shown good reason for believing that although the
Tachytes nigra generally makes its own burrow and stores it with
paralysed prey for its own larvae to feed on, yet that when this insect
finds a burrow already made and stored by another sphex, it takes
advantage of the prize, and becomes for the occasion parasitic. In this
case, as with the supposed case of the cuckoo, I can see no difficulty
in natural selection making an occasional habit permanent, if of
advantage to the species, and if the insect whose nest and stored food
are thus feloniously appropriated, be not thus exterminated.

SLAVE-MAKING INSTINCT.

This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves; without their
aid, the species would certainly become extinct in a single year. The
males and fertile females do no work. The workers or sterile females,
though most energetic and courageous in capturing slaves, do no other
work. They are incapable of making their own nests, or of feeding their
own larvae. When the old nest is found inconvenient, and they have to
migrate, it is the slaves which determine the migration, and actually
carry their masters in their jaws. So utterly helpless are the masters,
that when Huber shut up thirty of them without a slave, but with plenty
of the food which they like best, and with their larvae and pupae to
stimulate them to work, they did nothing; they could not even feed
themselves, and many perished of hunger. Huber then introduced a single
slave (F. fusca), and she instantly set to work, fed and saved the
survivors; made some cells and tended the larvae, and put all to rights.
What can be more extraordinary than these well-ascertained facts? If we
had not known of any other slave-making ant, it would have been
hopeless to have speculated how so wonderful an instinct could have been
perfected.

Formica sanguinea was likewise first discovered by P. Huber to be
a slave-making ant. This species is found in the southern parts of
England, and its habits have been attended to by Mr. F. Smith, of the
British Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and
Mr. Smith, I tried to approach the subject in a sceptical frame of
mind, as any one may well be excused for doubting the truth of so
extraordinary and odious an instinct as that of making slaves. Hence
I will give the observations which I have myself made, in some little
detail. I opened fourteen nests of F. sanguinea, and found a few slaves
in all. Males and fertile females of the slave-species are found only in
their own proper communities, and have never been observed in the nests
of F. sanguinea. The slaves are black and not above half the size of
their red masters, so that the contrast in their appearance is very
great. When the nest is slightly disturbed, the slaves occasionally come
out, and like their masters are much agitated and defend the nest: when
the nest is much disturbed and the larvae and pupae are exposed, the
slaves work energetically with their masters in carrying them away to a
place of safety. Hence, it is clear, that the slaves feel quite at home.
During the months of June and July, on three successive years, I have
watched for many hours several nests in Surrey and Sussex, and never
saw a slave either leave or enter a nest. As, during these months,
the slaves are very few in number, I thought that they might behave
differently when more numerous; but Mr. Smith informs me that he has
watched the nests at various hours during May, June and August, both in
Surrey and Hampshire, and has never seen the slaves, though present
in large numbers in August, either leave or enter the nest. Hence he
considers them as strictly household slaves. The masters, on the other
hand, may be constantly seen bringing in materials for the nest, and
food of all kinds. During the present year, however, in the month of
July, I came across a community with an unusually large stock of slaves,
and I observed a few slaves mingled with their masters leaving the nest,
and marching along the same road to a tall Scotch-fir-tree, twenty-five
yards distant, which they ascended together, probably in search of
aphides or cocci. According to Huber, who had ample opportunities
for observation, in Switzerland the slaves habitually work with their
masters in making the nest, and they alone open and close the doors in
the morning and evening; and, as Huber expressly states, their principal
office is to search for aphides. This difference in the usual habits of
the masters and slaves in the two countries, probably depends merely
on the slaves being captured in greater numbers in Switzerland than in
England.

One day I fortunately chanced to witness a migration from one nest to
another, and it was a most interesting spectacle to behold the masters
carefully carrying, as Huber has described, their slaves in their jaws.
Another day my attention was struck by about a score of the slave-makers
haunting the same spot, and evidently not in search of food; they
approached and were vigorously repulsed by an independent community of
the slave species (F. fusca); sometimes as many as three of these
ants clinging to the legs of the slave-making F. sanguinea. The latter
ruthlessly killed their small opponents, and carried their dead
bodies as food to their nest, twenty-nine yards distant; but they were
prevented from getting any pupae to rear as slaves. I then dug up a
small parcel of the pupae of F. fusca from another nest, and put them
down on a bare spot near the place of combat; they were eagerly seized,
and carried off by the tyrants, who perhaps fancied that, after all,
they had been victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of the nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr. Smith.
Although so small a species, it is very courageous, and I have seen it
ferociously attack other ants. In one instance I found to my surprise
an independent community of F. flava under a stone beneath a nest of the
slave-making F. sanguinea; and when I had accidentally disturbed both
nests, the little ants attacked their big neighbours with surprising
courage. Now I was curious to ascertain whether F. sanguinea could
distinguish the pupae of F. fusca, which they habitually make into
slaves, from those of the little and furious F. flava, which they rarely
capture, and it was evident that they did at once distinguish them:
for we have seen that they eagerly and instantly seized the pupae of F.
fusca, whereas they were much terrified when they came across the pupae,
or even the earth from the nest of F. flava, and quickly ran away; but
in about a quarter of an hour, shortly after all the little yellow ants
had crawled away, they took heart and carried off the pupae.

One evening I visited another community of F. sanguinea, and found a
number of these ants entering their nest, carrying the dead bodies of F.
fusca (showing that it was not a migration) and numerous pupae. I traced
the returning file burthened with booty, for about forty yards, to
a very thick clump of heath, whence I saw the last individual of F.
sanguinea emerge, carrying a pupa; but I was not able to find the
desolated nest in the thick heath. The nest, however, must have been
close at hand, for two or three individuals of F. fusca were rushing
about in the greatest agitation, and one was perched motionless with its
own pupa in its mouth on the top of a spray of heath over its ravaged
home.

Such are the facts, though they did not need confirmation by me, in
regard to the wonderful instinct of making slaves. Let it be observed
what a contrast the instinctive habits of F. sanguinea present with
those of the F. rufescens. The latter does not build its own nest, does
not determine its own migrations, does not collect food for itself or
its young, and cannot even feed itself: it is absolutely dependent on
its numerous slaves. Formica sanguinea, on the other hand, possesses
much fewer slaves, and in the early part of the summer extremely few.
The masters determine when and where a new nest shall be formed, and
when they migrate, the masters carry the slaves. Both in Switzerland and
England the slaves seem to have the exclusive care of the larvae, and
the masters alone go on slave-making expeditions. In Switzerland the
slaves and masters work together, making and bringing materials for the
nest: both, but chiefly the slaves, tend, and milk as it may be called,
their aphides; and thus both collect food for the community. In England
the masters alone usually leave the nest to collect building materials
and food for themselves, their slaves and larvae. So that the masters in
this country receive much less service from their slaves than they do in
Switzerland.

By what steps the instinct of F. sanguinea originated I will not pretend
to conjecture. But as ants, which are not slave-makers, will, as I have
seen, carry off pupae of other species, if scattered near their nests,
it is possible that pupae originally stored as food might become
developed; and the ants thus unintentionally reared would then follow
their proper instincts, and do what work they could. If their presence
proved useful to the species which had seized them--if it were more
advantageous to this species to capture workers than to procreate
them--the habit of collecting pupae originally for food might by natural
selection be strengthened and rendered permanent for the very different
purpose of raising slaves. When the instinct was once acquired, if
carried out to a much less extent even than in our British F. sanguinea,
which, as we have seen, is less aided by its slaves than the same
species in Switzerland, I can see no difficulty in natural selection
increasing and modifying the instinct--always supposing each
modification to be of use to the species--until an ant was formed as
abjectly dependent on its slaves as is the Formica rufescens.

CELL-MAKING INSTINCT OF THE HIVE-BEE.

I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be
a dull man who can examine the exquisite structure of a comb, so
beautifully adapted to its end, without enthusiastic admiration. We
hear from mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape to hold the
greatest possible amount of honey, with the least possible consumption
of precious wax in their construction. It has been remarked that a
skilful workman, with fitting tools and measures, would find it
very difficult to make cells of wax of the true form, though this is
perfectly effected by a crowd of bees working in a dark hive. Grant
whatever instincts you please, and it seems at first quite inconceivable
how they can make all the necessary angles and planes, or even perceive
when they are correctly made. But the difficulty is not nearly so great
as it at first appears: all this beautiful work can be shown, I think,
to follow from a few very simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered only
as a modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of
work. At one end of a short series we have humble-bees, which use their
old cocoons to hold honey, sometimes adding to them short tubes of wax,
and likewise making separate and very irregular rounded cells of wax. At
the other end of the series we have the cells of the hive-bee, placed in
a double layer: each cell, as is well known, is an hexagonal prism, with
the basal edges of its six sides bevelled so as to join on to a pyramid,
formed of three rhombs. These rhombs have certain angles, and the three
which form the pyramidal base of a single cell on one side of the comb,
enter into the composition of the bases of three adjoining cells on the
opposite side. In the series between the extreme perfection of the cells
of the hive-bee and the simplicity of those of the humble-bee, we have
the cells of the Mexican Melipona domestica, carefully described
and figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble bee, but more nearly related to
the latter: it forms a nearly regular waxen comb of cylindrical cells,
in which the young are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly spherical and of
nearly equal sizes, and are aggregated into an irregular mass. But the
important point to notice, is that these cells are always made at that
degree of nearness to each other, that they would have intersected or
broken into each other, if the spheres had been completed; but this is
never permitted, the bees building perfectly flat walls of wax between
the spheres which thus tend to intersect. Hence each cell consists of
an outer spherical portion and of two, three, or more perfectly flat
surfaces, according as the cell adjoins two, three or more other cells.
When one cell comes into contact with three other cells, which, from
the spheres being nearly of the same size, is very frequently and
necessarily the case, the three flat surfaces are united into a pyramid;
and this pyramid, as Huber has remarked, is manifestly a gross imitation
of the three-sided pyramidal basis of the cell of the hive-bee. As in
the cells of the hive-bee, so here, the three plane surfaces in any one
cell necessarily enter into the construction of three adjoining cells.
It is obvious that the Melipona saves wax by this manner of building;
for the flat walls between the adjoining cells are not double, but are
of the same thickness as the outer spherical portions, and yet each flat
portion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would probably have been as perfect as the comb of
the hive-bee. Accordingly I wrote to Professor Miller, of Cambridge,
and this geometer has kindly read over the following statement, drawn up
from his information, and tells me that it is strictly correct:--

If a number of equal spheres be described with their centres placed in
two parallel layers; with the centre of each sphere at the distance of
radius x the square root of 2 or radius x 1.41421 (or at some lesser
distance), from the centres of the six surrounding spheres in the
same layer; and at the same distance from the centres of the adjoining
spheres in the other and parallel layer; then, if planes of intersection
between the several spheres in both layers be formed, there will result
a double layer of hexagonal prisms united together by pyramidal bases
formed of three rhombs; and the rhombs and the sides of the hexagonal
prisms will have every angle identically the same with the best
measurements which have been made of the cells of the hive-bee.

Hence we may safely conclude that if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as
that of the hive-bee. We must suppose the Melipona to make her cells
truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows in wood many insects can
make, apparently by turning round on a fixed point. We must suppose the
Melipona to arrange her cells in level layers, as she already does her
cylindrical cells; and we must further suppose, and this is the greatest
difficulty, that she can somehow judge accurately at what distance to
stand from her fellow-labourers when several are making their spheres;
but she is already so far enabled to judge of distance, that she always
describes her spheres so as to intersect largely; and then she unites
the points of intersection by perfectly flat surfaces. We have further
to suppose, but this is no difficulty, that after hexagonal prisms have
been formed by the intersection of adjoining spheres in the same layer,
she can prolong the hexagon to any length requisite to hold the stock of
honey; in the same way as the rude humble-bee adds cylinders of wax
to the circular mouths of her old cocoons. By such modifications of
instincts in themselves not very wonderful,--hardly more wonderful than
those which guide a bird to make its nest,--I believe that the hive-bee
has acquired, through natural selection, her inimitable architectural
powers.

But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, square strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made
them wider and wider until they were converted into shallow basins,
appearing to the eye perfectly true or parts of a sphere, and of about
the diameter of a cell. It was most interesting to me to observe that
wherever several bees had begun to excavate these basins near together,
they had begun their work at such a distance from each other, that by
the time the basins had acquired the above stated width (i.e. about the
width of an ordinary cell), and were in depth about one sixth of the
diameter of the sphere of which they formed a part, the rims of the
basins intersected or broke into each other. As soon as this occurred,
the bees ceased to excavate, and began to build up flat walls of wax
on the lines of intersection between the basins, so that each hexagonal
prism was built upon the festooned edge of a smooth basin, instead of on
the straight edges of a three-sided pyramid as in the case of ordinary
cells.

I then put into the hive, instead of a thick, square piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin, that
the bottoms of the basins, if they had been excavated to the same depth
as in the former experiment, would have broken into each other from the
opposite sides. The bees, however, did not suffer this to happen, and
they stopped their excavations in due time; so that the basins, as soon
as they had been a little deepened, came to have flat bottoms; and these
flat bottoms, formed by thin little plates of the vermilion wax having
been left ungnawed, were situated, as far as the eye could judge,
exactly along the planes of imaginary intersection between the basins on
the opposite sides of the ridge of wax. In parts, only little bits, in
other parts, large portions of a rhombic plate had been left between the
opposed basins, but the work, from the unnatural state of things, had
not been neatly performed. The bees must have worked at very nearly the
same rate on the opposite sides of the ridge of vermilion wax, as they
circularly gnawed away and deepened the basins on both sides, in order
to have succeeded in thus leaving flat plates between the basins, by
stopping work along the intermediate planes or planes of intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip
of wax, perceiving when they have gnawed the wax away to the proper
thinness, and then stopping their work. In ordinary combs it has
appeared to me that the bees do not always succeed in working at exactly
the same rate from the opposite sides; for I have noticed half-completed
rhombs at the base of a just-commenced cell, which were slightly concave
on one side, where I suppose that the bees had excavated too quickly,
and convex on the opposed side, where the bees had worked less quickly.
In one well-marked instance, I put the comb back into the hive, and
allowed the bees to go on working for a short time, and again examined
the cell, and I found that the rhombic plate had been completed, and had
become PERFECTLY FLAT: it was absolutely impossible, from the extreme
thinness of the little rhombic plate, that they could have effected this
by gnawing away the convex side; and I suspect that the bees in such
cases stand in the opposed cells and push and bend the ductile and warm
wax (which as I have tried is easily done) into its proper intermediate
plane, and thus flatten it.

From the experiment of the ridge of vermilion wax, we can clearly see
that if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing the
spheres to break into each other. Now bees, as may be clearly seen by
examining the edge of a growing comb, do make a rough, circumferential
wall or rim all round the comb; and they gnaw into this from the
opposite sides, always working circularly as they deepen each cell. They
do not make the whole three-sided pyramidal base of any one cell at the
same time, but only the one rhombic plate which stands on the extreme
growing margin, or the two plates, as the case may be; and they never
complete the upper edges of the rhombic plates, until the hexagonal
walls are commenced. Some of these statements differ from those made by
the justly celebrated elder Huber, but I am convinced of their accuracy;
and if I had space, I could show that they are conformable with my
theory.

Huber's statement that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of wax;
but I will not here enter on these details. We see how important a part
excavation plays in the construction of the cells; but it would be a
great error to suppose that the bees cannot build up a rough wall of wax
in the proper position--that is, along the plane of intersection between
two adjoining spheres. I have several specimens showing clearly that
they can do this. Even in the rude circumferential rim or wall of wax
round a growing comb, flexures may sometimes be observed, corresponding
in position to the planes of the rhombic basal plates of future cells.
But the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build is
curious; they always make the first rough wall from ten to twenty times
thicker than the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by supposing
masons first to pile up a broad ridge of cement, and then to begin
cutting it away equally on both sides near the ground, till a smooth,
very thin wall is left in the middle; the masons always piling up the
cut-away cement, and adding fresh cement, on the summit of the ridge. We
shall thus have a thin wall steadily growing upward; but always crowned
by a gigantic coping. From all the cells, both those just commenced and
those completed, being thus crowned by a strong coping of wax, the
bees can cluster and crawl over the comb without injuring the delicate
hexagonal walls, which are only about one four-hundredth of an inch in
thickness; the plates of the pyramidal basis being about twice as thick.
By this singular manner of building, strength is continually given to
the comb, with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commencement
of the first cell. I was able practically to show this fact, by covering
the edges of the hexagonal walls of a single cell, or the extreme margin
of the circumferential rim of a growing comb, with an extremely thin
layer of melted vermilion wax; and I invariably found that the colour
was most delicately diffused by the bees--as delicately as a painter
could have done with his brush--by atoms of the coloured wax having been
taken from the spot on which it had been placed, and worked into the
growing edges of the cells all round. The work of construction seems
to be a sort of balance struck between many bees, all instinctively
standing at the same relative distance from each other, all trying to
sweep equal spheres, and then building up, or leaving ungnawed, the
planes of intersection between these spheres. It was really curious to
note in cases of difficulty, as when two pieces of comb met at an angle,
how often the bees would entirely pull down and rebuild in different
ways the same cell, sometimes recurring to a shape which they had at
first rejected.

When bees have a place on which they can stand in their proper positions
for working,--for instance, on a slip of wood, placed directly under the
middle of a comb growing downwards so that the comb has to be built over
one face of the slip--in this case the bees can lay the foundations
of one wall of a new hexagon, in its strictly proper place, projecting
beyond the other completed cells. It suffices that the bees should be
enabled to stand at their proper relative distances from each other
and from the walls of the last completed cells, and then, by striking
imaginary spheres, they can build up a wall intermediate between two
adjoining spheres; but, as far as I have seen, they never gnaw away and
finish off the angles of a cell till a large part both of that cell and
of the adjoining cells has been built. This capacity in bees of laying
down under certain circumstances a rough wall in its proper place
between two just-commenced cells, is important, as it bears on a fact,
which seems at first quite subversive of the foregoing theory; namely,
that the cells on the extreme margin of wasp-combs are sometimes
strictly hexagonal; but I have not space here to enter on this subject.
Nor does there seem to me any great difficulty in a single insect (as
in the case of a queen-wasp) making hexagonal cells, if she work
alternately on the inside and outside of two or three cells commenced at
the same time, always standing at the proper relative distance from
the parts of the cells just begun, sweeping spheres or cylinders, and
building up intermediate planes. It is even conceivable that an insect
might, by fixing on a point at which to commence a cell, and then moving
outside, first to one point, and then to five other points, at the
proper relative distances from the central point and from each other,
strike the planes of intersection, and so make an isolated hexagon: but
I am not aware that any such case has been observed; nor would any good
be derived from a single hexagon being built, as in its construction
more materials would be required than for a cylinder.

As natural selection acts only by the accumulation of slight
modifications of structure or instinct, each profitable to the
individual under its conditions of life, it may reasonably be asked, how
a long and graduated succession of modified architectural instincts,
all tending towards the present perfect plan of construction, could
have profited the progenitors of the hive-bee? I think the answer is
not difficult: it is known that bees are often hard pressed to get
sufficient nectar; and I am informed by Mr. Tegetmeier that it has been
experimentally found that no less than from twelve to fifteen pounds of
dry sugar are consumed by a hive of bees for the secretion of each pound
of wax; so that a prodigious quantity of fluid nectar must be collected
and consumed by the bees in a hive for the secretion of the wax
necessary for the construction of their combs. Moreover, many bees have
to remain idle for many days during the process of secretion. A large
store of honey is indispensable to support a large stock of bees during
the winter; and the security of the hive is known mainly to depend on a
large number of bees being supported. Hence the saving of wax by largely
saving honey must be a most important element of success in any family
of bees. Of course the success of any species of bee may be dependent
on the number of its parasites or other enemies, or on quite distinct
causes, and so be altogether independent of the quantity of honey which
the bees could collect. But let us suppose that this latter circumstance
determined, as it probably often does determine, the numbers of a
humble-bee which could exist in a country; and let us further suppose
that the community lived throughout the winter, and consequently
required a store of honey: there can in this case be no doubt that it
would be an advantage to our humble-bee, if a slight modification of
her instinct led her to make her waxen cells near together, so as to
intersect a little; for a wall in common even to two adjoining cells,
would save some little wax. Hence it would continually be more and more
advantageous to our humble-bee, if she were to make her cells more and
more regular, nearer together, and aggregated into a mass, like the
cells of the Melipona; for in this case a large part of the bounding
surface of each cell would serve to bound other cells, and much wax
would be saved. Again, from the same cause, it would be advantageous to
the Melipona, if she were to make her cells closer together, and more
regular in every way than at present; for then, as we have seen, the
spherical surfaces would wholly disappear, and would all be replaced by
plane surfaces; and the Melipona would make a comb as perfect as that of
the hive-bee. Beyond this stage of perfection in architecture, natural
selection could not lead; for the comb of the hive-bee, as far as we can
see, is absolutely perfect in economising wax.

Thus, as I believe, the most wonderful of all known instincts, that
of the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having by slow degrees, more and more
perfectly, led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax along
the planes of intersection. The bees, of course, no more knowing that
they swept their spheres at one particular distance from each other,
than they know what are the several angles of the hexagonal prisms and
of the basal rhombic plates. The motive power of the process of natural
selection having been economy of wax; that individual swarm which wasted
least honey in the secretion of wax, having succeeded best, and having
transmitted by inheritance its newly acquired economical instinct to new
swarms, which in their turn will have had the best chance of succeeding
in the struggle for existence.

No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection,--cases, in which we cannot see
how an instinct could possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently such trifling importance, that they could hardly have been
acted on by natural selection; cases of instincts almost identically the
same in animals so remote in the scale of nature, that we cannot account
for their similarity by inheritance from a common parent, and must
therefore believe that they have been acquired by independent acts of
natural selection. I will not here enter on these several cases, but
will confine myself to one special difficulty, which at first appeared
to me insuperable, and actually fatal to my whole theory. I allude to
the neuters or sterile females in insect-communities: for these neuters
often differ widely in instinct and in structure from both the males
and fertile females, and yet, from being sterile, they cannot propagate
their kind.

The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much greater
than that of any other striking modification of structure; for it can
be shown that some insects and other articulate animals in a state of
nature occasionally become sterile; and if such insects had been social,
and it had been profitable to the community that a number should have
been annually born capable of work, but incapable of procreation, I
can see no very great difficulty in this being effected by natural
selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the thorax and
in being destitute of wings and sometimes of eyes, and in instinct. As
far as instinct alone is concerned, the prodigious difference in this
respect between the workers and the perfect females, would have been
far better exemplified by the hive-bee. If a working ant or other
neuter insect had been an animal in the ordinary state, I should have
unhesitatingly assumed that all its characters had been slowly acquired
through natural selection; namely, by an individual having been born
with some slight profitable modification of structure, this being
inherited by its offspring, which again varied and were again selected,
and so onwards. But with the working ant we have an insect differing
greatly from its parents, yet absolutely sterile; so that it could never
have transmitted successively acquired modifications of structure or
instinct to its progeny. It may well be asked how is it possible to
reconcile this case with the theory of natural selection?

First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all sorts
of differences of structure which have become correlated to certain
ages, and to either sex. We have differences correlated not only to
one sex, but to that short period alone when the reproductive system is
active, as in the nuptial plumage of many birds, and in the hooked jaws
of the male salmon. We have even slight differences in the horns of
different breeds of cattle in relation to an artificially imperfect
state of the male sex; for oxen of certain breeds have longer horns than
in other breeds, in comparison with the horns of the bulls or cows of
these same breeds. Hence I can see no real difficulty in any character
having become correlated with the sterile condition of certain members
of insect-communities: the difficulty lies in understanding how such
correlated modifications of structure could have been slowly accumulated
by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied
to the family, as well as to the individual, and may thus gain the
desired end. Thus, a well-flavoured vegetable is cooked, and the
individual is destroyed; but the horticulturist sows seeds of the same
stock, and confidently expects to get nearly the same variety; breeders
of cattle wish the flesh and fat to be well marbled together; the animal
has been slaughtered, but the breeder goes with confidence to the same
family. I have such faith in the powers of selection, that I do not
doubt that a breed of cattle, always yielding oxen with extraordinarily
long horns, could be slowly formed by carefully watching which
individual bulls and cows, when matched, produced oxen with the longest
horns; and yet no one ox could ever have propagated its kind. Thus
I believe it has been with social insects: a slight modification of
structure, or instinct, correlated with the sterile condition of certain
members of the community, has been advantageous to the community:
consequently the fertile males and females of the same community
flourished, and transmitted to their fertile offspring a tendency to
produce sterile members having the same modification. And I believe
that this process has been repeated, until that prodigious amount of
difference between the fertile and sterile females of the same species
has been produced, which we see in many social insects.

But we have not as yet touched on the climax of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes.
The castes, moreover, do not generally graduate into each other, but are
perfectly well defined; being as distinct from each other, as are any
two species of the same genus, or rather as any two genera of the same
family. Thus in Eciton, there are working and soldier neuters, with jaws
and instincts extraordinarily different: in Cryptocerus, the workers of
one caste alone carry a wonderful sort of shield on their heads, the use
of which is quite unknown: in the Mexican Myrmecocystus, the workers of
one caste never leave the nest; they are fed by the workers of another
caste, and they have an enormously developed abdomen which secretes a
sort of honey, supplying the place of that excreted by the aphides, or
the domestic cattle as they may be called, which our European ants guard
or imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females,--in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few
alone; and that by the long-continued selection of the fertile parents
which produced most neuters with the profitable modification, all the
neuters ultimately came to have the desired character. On this view we
ought occasionally to find neuter-insects of the same species, in the
same nest, presenting gradations of structure; and this we do find,
even often, considering how few neuter-insects out of Europe have been
carefully examined. Mr. F. Smith has shown how surprisingly the neuters
of several British ants differ from each other in size and sometimes
in colour; and that the extreme forms can sometimes be perfectly linked
together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often happens that the
larger or the smaller sized workers are the most numerous; or that both
large and small are numerous, with those of an intermediate size scanty
in numbers. Formica flava has larger and smaller workers, with some of
intermediate size; and, in this species, as Mr. F. Smith has observed,
the larger workers have simple eyes (ocelli), which though small can
be plainly distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these
workers, I can affirm that the eyes are far more rudimentary in the
smaller workers than can be accounted for merely by their proportionally
lesser size; and I fully believe, though I dare not assert so
positively, that the workers of intermediate size have their ocelli in
an exactly intermediate condition. So that we here have two bodies of
sterile workers in the same nest, differing not only in size, but
in their organs of vision, yet connected by some few members in an
intermediate condition. I may digress by adding, that if the smaller
workers had been the most useful to the community, and those males and
females had been continually selected, which produced more and more
of the smaller workers, until all the workers had come to be in this
condition; we should then have had a species of ant with neuters very
nearly in the same condition with those of Myrmica. For the workers of
Myrmica have not even rudiments of ocelli, though the male and female
ants of this genus have well-developed ocelli.

I may give one other case: so confidently did I expect to find
gradations in important points of structure between the different castes
of neuters in the same species, that I gladly availed myself of Mr. F.
Smith's offer of numerous specimens from the same nest of the driver
ant (Anomma) of West Africa. The reader will perhaps best appreciate
the amount of difference in these workers, by my giving not the actual
measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house of whom
many were five feet four inches high, and many sixteen feet high; but
we must suppose that the larger workmen had heads four instead of three
times as big as those of the smaller men, and jaws nearly five times
as big. The jaws, moreover, of the working ants of the several sizes
differed wonderfully in shape, and in the form and number of the teeth.
But the important fact for us is, that though the workers can be grouped
into castes of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as Mr. Lubbock made drawings for
me with the camera lucida of the jaws which I had dissected from the
workers of the several sizes.

With these facts before me, I believe that natural selection, by acting
on the fertile parents, could form a species which should regularly
produce neuters, either all of large size with one form of jaw, or all
of small size with jaws having a widely different structure; or lastly,
and this is our climax of difficulty, one set of workers of one size and
structure, and simultaneously another set of workers of a different size
and structure;--a graduated series having been first formed, as in the
case of the driver ant, and then the extreme forms, from being the most
useful to the community, having been produced in greater and greater
numbers through the natural selection of the parents which generated
them; until none with an intermediate structure were produced.

Thus, as I believe, the wonderful fact of two distinctly defined castes
of sterile workers existing in the same nest, both widely different from
each other and from their parents, has originated. We can see how useful
their production may have been to a social community of insects, on the
same principle that the division of labour is useful to civilised man.
As ants work by inherited instincts and by inherited tools or weapons,
and not by acquired knowledge and manufactured instruments, a perfect
division of labour could be effected with them only by the workers being
sterile; for had they been fertile, they would have intercrossed, and
their instincts and structure would have become blended. And nature
has, as I believe, effected this admirable division of labour in the
communities of ants, by the means of natural selection. But I am bound
to confess, that, with all my faith in this principle, I should never
have anticipated that natural selection could have been efficient in so
high a degree, had not the case of these neuter insects convinced me
of the fact. I have, therefore, discussed this case, at some little
but wholly insufficient length, in order to show the power of natural
selection, and likewise because this is by far the most serious special
difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount
of modification in structure can be effected by the accumulation of
numerous, slight, and as we must call them accidental, variations, which
are in any manner profitable, without exercise or habit having come into
play. For no amount of exercise, or habit, or volition, in the utterly
sterile members of a community could possibly have affected the
structure or instincts of the fertile members, which alone leave
descendants. I am surprised that no one has advanced this demonstrative
case of neuter insects, against the well-known doctrine of Lamarck.

SUMMARY.

I have endeavoured briefly in this chapter to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore I can see no
difficulty, under changing conditions of life, in natural selection
accumulating slight modifications of instinct to any extent, in any
useful direction. In some cases habit or use and disuse have probably
come into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and
are liable to mistakes;--that no instinct has been produced for the
exclusive good of other animals, but that each animal takes advantage of
the instincts of others;--that the canon in natural history, of "natura
non facit saltum" is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views, but is
otherwise inexplicable,--all tend to corroborate the theory of natural
selection.

This theory is, also, strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and living
under considerably different conditions of life, yet often retaining
nearly the same instincts. For instance, we can understand on the
principle of inheritance, how it is that the thrush of South America
lines its nest with mud, in the same peculiar manner as does our British
thrush: how it is that the male wrens (Troglodytes) of North America,
build "cock-nests," to roost in, like the males of our distinct
Kitty-wrens,--a habit wholly unlike that of any other known bird.
Finally, it may not be a logical deduction, but to my imagination it
is far more satisfactory to look at such instincts as the young cuckoo
ejecting its foster-brothers,--ants making slaves,--the larvae of
ichneumonidae feeding within the live bodies of caterpillars,--not as
specially endowed or created instincts, but as small consequences of one
general law, leading to the advancement of all organic beings, namely,
multiply, vary, let the strongest live and the weakest die.




8. HYBRIDISM.

Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication. Laws governing the sterility
of hybrids. Sterility not a special endowment, but incidental on other
differences. Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing. Fertility of varieties when crossed and of their mongrel
offspring not universal. Hybrids and mongrels compared independently of
their fertility. Summary.

The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility,
in order to prevent the confusion of all organic forms. This view
certainly seems at first probable, for species within the same country
could hardly have kept distinct had they been capable of crossing
freely. The importance of the fact that hybrids are very generally
sterile, has, I think, been much underrated by some late writers. On the
theory of natural selection the case is especially important, inasmuch
as the sterility of hybrids could not possibly be of any advantage
to them, and therefore could not have been acquired by the continued
preservation of successive profitable degrees of sterility. I hope,
however, to be able to show that sterility is not a specially acquired
or endowed quality, but is incidental on other acquired differences.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together;
namely, the sterility of two species when first crossed, and the
sterility of the hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no
offspring. Hybrids, on the other hand, have their reproductive organs
functionally impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the organs themselves are
perfect in structure, as far as the microscope reveals. In the first
case the two sexual elements which go to form the embryo are perfect; in
the second case they are either not at all developed, or are imperfectly
developed. This distinction is important, when the cause of the
sterility, which is common to the two cases, has to be considered. The
distinction has probably been slurred over, owing to the sterility in
both cases being looked on as a special endowment, beyond the province
of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to
have descended from common parents, when intercrossed, and likewise
the fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility.
Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he unhesitatingly ranks them
as varieties. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these and
in many other cases, Gartner is obliged carefully to count the seeds, in
order to show that there is any degree of sterility. He always compares
the maximum number of seeds produced by two species when crossed and by
their hybrid offspring, with the average number produced by both pure
parent-species in a state of nature. But a serious cause of error seems
to me to be here introduced: a plant to be hybridised must be castrated,
and, what is often more important, must be secluded in order to prevent
pollen being brought to it by insects from other plants. Nearly all the
plants experimentised on by Gartner were potted, and apparently were
kept in a chamber in his house. That these processes are often injurious
to the fertility of a plant cannot be doubted; for Gartner gives in
his table about a score of cases of plants which he castrated, and
artificially fertilised with their own pollen, and (excluding all cases
such as the Leguminosae, in which there is an acknowledged difficulty
in the manipulation) half of these twenty plants had their fertility
in some degree impaired. Moreover, as Gartner during several years
repeatedly crossed the primrose and cowslip, which we have such good
reason to believe to be varieties, and only once or twice succeeded in
getting fertile seed; as he found the common red and blue pimpernels
(Anagallis arvensis and coerulea), which the best botanists rank as
varieties, absolutely sterile together; and as he came to the same
conclusion in several other analogous cases; it seems to me that we
may well be permitted to doubt whether many other species are really so
sterile, when intercrossed, as Gartner believes.

It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so insensibly,
and, on the other hand, that the fertility of pure species is so easily
affected by various circumstances, that for all practical purposes it is
most difficult to say where perfect fertility ends and sterility begins.
I think no better evidence of this can be required than that the two
most experienced observers who have ever lived, namely, Kolreuter and
Gartner, should have arrived at diametrically opposite conclusions
in regard to the very same species. It is also most instructive to
compare--but I have not space here to enter on details--the evidence
advanced by our best botanists on the question whether certain doubtful
forms should be ranked as species or varieties, with the evidence from
fertility adduced by different hybridisers, or by the same author,
from experiments made during different years. It can thus be shown that
neither sterility nor fertility affords any clear distinction between
species and varieties; but that the evidence from this source graduates
away, and is doubtful in the same degree as is the evidence derived from
other constitutional and structural differences.

In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for
ten generations, yet he asserts positively that their fertility never
increased, but generally greatly decreased. I do not doubt that this is
usually the case, and that the fertility often suddenly decreases in
the first few generations. Nevertheless I believe that in all these
experiments the fertility has been diminished by an independent cause,
namely, from close interbreeding. I have collected so large a body of
facts, showing that close interbreeding lessens fertility, and, on
the other hand, that an occasional cross with a distinct individual or
variety increases fertility, that I cannot doubt the correctness of this
almost universal belief amongst breeders. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: hence hybrids
will generally be fertilised during each generation by their own
individual pollen; and I am convinced that this would be injurious
to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Gartner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in
artificial fertilisation pollen is as often taken by chance (as I know
from my own experience) from the anthers of another flower, as from the
anthers of the flower itself which is to be fertilised; so that a cross
between two flowers, though probably on the same plant, would be thus
effected. Moreover, whenever complicated experiments are in progress,
so careful an observer as Gartner would have castrated his hybrids, and
this would have insured in each generation a cross with the pollen from
a distinct flower, either from the same plant or from another plant of
the same hybrid nature. And thus, the strange fact of the increase
of fertility in the successive generations of ARTIFICIALLY FERTILISED
hybrids may, I believe, be accounted for by close interbreeding having
been avoided.

Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Honourable and Reverend W. Herbert. He is as
emphatic in his conclusion that some hybrids are perfectly fertile--as
fertile as the pure parent-species--as are Kolreuter and Gartner that
some degree of sterility between distinct species is a universal law
of nature. He experimentised on some of the very same species as did
Gartner. The difference in their results may, I think, be in part
accounted for by Herbert's great horticultural skill, and by his having
hothouses at his command. Of his many important statements I will here
give only a single one as an example, namely, that "every ovule in a pod
of Crinum capense fertilised by C. revolutum produced a plant, which
(he says) I never saw to occur in a case of its natural fecundation." So
that we here have perfect, or even more than commonly perfect, fertility
in a first cross between two distinct species.

This case of the Crinum leads me to refer to a most singular fact,
namely, that there are individual plants, as with certain species of
Lobelia, and with all the species of the genus Hippeastrum, which can
be far more easily fertilised by the pollen of another and distinct
species, than by their own pollen. For these plants have been found to
yield seed to the pollen of a distinct species, though quite sterile
with their own pollen, notwithstanding that their own pollen was found
to be perfectly good, for it fertilised distinct species. So that
certain individual plants and all the individuals of certain species
can actually be hybridised much more readily than they can be
self-fertilised! For instance, a bulb of Hippeastrum aulicum produced
four flowers; three were fertilised by Herbert with their own pollen,
and the fourth was subsequently fertilised by the pollen of a compound
hybrid descended from three other and distinct species: the result was
that "the ovaries of the three first flowers soon ceased to grow, and
after a few days perished entirely, whereas the pod impregnated by
the pollen of the hybrid made vigorous growth and rapid progress to
maturity, and bore good seed, which vegetated freely." In a letter to
me, in 1839, Mr. Herbert told me that he had then tried the experiment
during five years, and he continued to try it during several subsequent
years, and always with the same result. This result has, also, been
confirmed by other observers in the case of Hippeastrum with its
sub-genera, and in the case of some other genera, as Lobelia, Passiflora
and Verbascum. Although the plants in these experiments appeared
perfectly healthy, and although both the ovules and pollen of the same
flower were perfectly good with respect to other species, yet as they
were functionally imperfect in their mutual self-action, we must infer
that the plants were in an unnatural state. Nevertheless these facts
show on what slight and mysterious causes the lesser or greater
fertility of species when crossed, in comparison with the same species
when self-fertilised, sometimes depends.

The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these
hybrids seed freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely dissimilar
in general habit, "reproduced itself as perfectly as if it had been a
natural species from the mountains of Chile." I have taken some pains
to ascertain the degree of fertility of some of the complex crosses of
Rhododendrons, and I am assured that many of them are perfectly fertile.
Mr. C. Noble, for instance, informs me that he raises stocks for
grafting from a hybrid between Rhododendron Ponticum and Catawbiense,
and that this hybrid "seeds as freely as it is possible to imagine." Had
hybrids, when fairly treated, gone on decreasing in fertility in each
successive generation, as Gartner believes to be the case, the fact
would have been notorious to nurserymen. Horticulturists raise large
beds of the same hybrids, and such alone are fairly treated, for by
insect agency the several individuals of the same hybrid variety are
allowed to freely cross with each other, and the injurious influence
of close interbreeding is thus prevented. Any one may readily convince
himself of the efficiency of insect-agency by examining the flowers of
the more sterile kinds of hybrid rhododendrons, which produce no pollen,
for he will find on their stigmas plenty of pollen brought from other
flowers.

In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that
is if the genera of animals are as distinct from each other, as are the
genera of plants, then we may infer that animals more widely separated
in the scale of nature can be more easily crossed than in the case of
plants; but the hybrids themselves are, I think, more sterile. I doubt
whether any case of a perfectly fertile hybrid animal can be considered
as thoroughly well authenticated. It should, however, be borne in
mind that, owing to few animals breeding freely under confinement, few
experiments have been fairly tried: for instance, the canary-bird has
been crossed with nine other finches, but as not one of these nine
species breeds freely in confinement, we have no right to expect that
the first crosses between them and the canary, or that their hybrids,
should be perfectly fertile. Again, with respect to the fertility in
successive generations of the more fertile hybrid animals, I hardly know
of an instance in which two families of the same hybrid have been raised
at the same time from different parents, so as to avoid the ill effects
of close interbreeding. On the contrary, brothers and sisters have
usually been crossed in each successive generation, in opposition to the
constantly repeated admonition of every breeder. And in this case, it is
not at all surprising that the inherent sterility in the hybrids should
have gone on increasing. If we were to act thus, and pair brothers and
sisters in the case of any pure animal, which from any cause had the
least tendency to sterility, the breed would assuredly be lost in a very
few generations.

Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that
the hybrids from Cervulus vaginalis and Reevesii, and from Phasianus
colchicus with P. torquatus and with P. versicolor are perfectly
fertile. The hybrids from the common and Chinese geese (A. cygnoides),
species which are so different that they are generally ranked in
distinct genera, have often bred in this country with either pure
parent, and in one single instance they have bred inter se. This was
effected by Mr. Eyton, who raised two hybrids from the same parents but
from different hatches; and from these two birds he raised no less than
eight hybrids (grandchildren of the pure geese) from one nest. In India,
however, these cross-bred geese must be far more fertile; for I am
assured by two eminently capable judges, namely Mr. Blyth and Capt.
Hutton, that whole flocks of these crossed geese are kept in various
parts of the country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly fertile.

A doctrine which originated with Pallas, has been largely accepted
by modern naturalists; namely, that most of our domestic animals have
descended from two or more aboriginal species, since commingled by
intercrossing. On this view, the aboriginal species must either at first
have produced quite fertile hybrids, or the hybrids must have become in
subsequent generations quite fertile under domestication. This latter
alternative seems to me the most probable, and I am inclined to believe
in its truth, although it rests on no direct evidence. I believe, for
instance, that our dogs have descended from several wild stocks; yet,
with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; and analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again there
is reason to believe that our European and the humped Indian cattle are
quite fertile together; but from facts communicated to me by Mr. Blyth,
I think they must be considered as distinct species. On this view of
the origin of many of our domestic animals, we must either give up the
belief of the almost universal sterility of distinct species of
animals when crossed; or we must look at sterility, not as an indelible
characteristic, but as one capable of being removed by domestication.

Finally, looking to all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result;
but that it cannot, under our present state of knowledge, be considered
as absolutely universal.

LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the circumstances and
rules governing the sterility of first crosses and of hybrids. Our chief
object will be to see whether or not the rules indicate that species
have specially been endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following rules
and conclusions are chiefly drawn up from Gartner's admirable work on
the hybridisation of plants. I have taken much pains to ascertain how
far the rules apply to animals, and considering how scanty our knowledge
is in regard to hybrid animals, I have been surprised to find how
generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown to
exist; but only the barest outline of the facts can here be given. When
pollen from a plant of one family is placed on the stigma of a plant of
a distinct family, it exerts no more influence than so much inorganic
dust. From this absolute zero of fertility, the pollen of different
species of the same genus applied to the stigma of some one species,
yields a perfect gradation in the number of seeds produced, up to nearly
complete or even quite complete fertility; and, as we have seen, in
certain abnormal cases, even to an excess of fertility, beyond that
which the plant's own pollen will produce. So in hybrids themselves,
there are some which never have produced, and probably never would
produce, even with the pollen of either pure parent, a single fertile
seed: but in some of these cases a first trace of fertility may be
detected, by the pollen of one of the pure parent-species causing the
flower of the hybrid to wither earlier than it otherwise would have
done; and the early withering of the flower is well known to be a sign
of incipient fertilisation. From this extreme degree of sterility we
have self-fertilised hybrids producing a greater and greater number of
seeds up to perfect fertility.

Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are
generally confounded together--is by no means strict. There are many
cases, in which two pure species can be united with unusual facility,
and produce numerous hybrid-offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed
very rarely, or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the same genus,
for instance in Dianthus, these two opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure
species. But the degree of fertility is likewise innately variable; for
it is not always the same when the same two species are crossed under
the same circumstances, but depends in part upon the constitution of the
individuals which happen to have been chosen for the experiment. So it
is with hybrids, for their degree of fertility is often found to differ
greatly in the several individuals raised from seed out of the same
capsule and exposed to exactly the same conditions.

By the term systematic affinity is meant, the resemblance between
species in structure and in constitution, more especially in the
structure of parts which are of high physiological importance and which
differ little in the allied species. Now the fertility of first crosses
between species, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely allied
species generally uniting with facility. But the correspondence between
systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species which
will not unite, or only with extreme difficulty; and on the other hand
of very distinct species which unite with the utmost facility. In
the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene,
in which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the
same genus, we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than the species of
almost any other genus; but Gartner found that N. acuminata, which is
not a particularly distinct species, obstinately failed to fertilise, or
to be fertilised by, no less than eight other species of Nicotiana. Very
many analogous facts could be given.

No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences in
every part of the flower, even in the pollen, in the fruit, and in the
cotyledons, can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and fitted for
extremely different climates, can often be crossed with ease.

By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass, and
then a male-ass with a mare: these two species may then be said to have
been reciprocally crossed. There is often the widest possible difference
in the facility of making reciprocal crosses. Such cases are highly
important, for they prove that the capacity in any two species to cross
is often completely independent of their systematic affinity, or of any
recognisable difference in their whole organisation. On the other hand,
these cases clearly show that the capacity for crossing is connected
with constitutional differences imperceptible by us, and confined to the
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalappa can easily be fertilised by the pollen of
M. longiflora, and the hybrids thus produced are sufficiently fertile;
but Kolreuter tried more than two hundred times, during eight following
years, to fertilise reciprocally M. longiflora with the pollen of M.
jalappa, and utterly failed. Several other equally striking cases could
be given. Thuret has observed the same fact with certain sea-weeds
or Fuci. Gartner, moreover, found that this difference of facility in
making reciprocal crosses is extremely common in a lesser degree. He has
observed it even between forms so closely related (as Matthiola annua
and glabra) that many botanists rank them only as varieties. It is also
a remarkable fact, that hybrids raised from reciprocal crosses, though
of course compounded of the very same two species, the one species
having first been used as the father and then as the mother, generally
differ in fertility in a small, and occasionally in a high degree.

Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species;
other species of the same genus have a remarkable power of impressing
their likeness on their hybrid offspring; but these two powers do not at
all necessarily go together. There are certain hybrids which instead
of having, as is usual, an intermediate character between their two
parents, always closely resemble one of them; and such hybrids, though
externally so like one of their pure parent-species, are with rare
exceptions extremely sterile. So again amongst hybrids which are
usually intermediate in structure between their parents, exceptional and
abnormal individuals sometimes are born, which closely resemble one of
their pure parents; and these hybrids are almost always utterly sterile,
even when the other hybrids raised from seed from the same capsule have
a considerable degree of fertility. These facts show how completely
fertility in the hybrid is independent of its external resemblance to
either pure parent.

Considering the several rules now given, which govern the fertility
of first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable. That it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross. That the
fertility of hybrids is not related to the degree in which they resemble
in external appearance either parent. And lastly, that the facility of
making a first cross between any two species is not always governed by
their systematic affinity or degree of resemblance to each other. This
latter statement is clearly proved by reciprocal crosses between the
same two species, for according as the one species or the other is used
as the father or the mother, there is generally some difference,
and occasionally the widest possible difference, in the facility of
effecting an union. The hybrids, moreover, produced from reciprocal
crosses often differ in fertility.

Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded
in nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which
we must suppose it would be equally important to keep from blending
together? Why should the degree of sterility be innately variable in
the individuals of the same species? Why should some species cross with
facility, and yet produce very sterile hybrids; and other species cross
with extreme difficulty, and yet produce fairly fertile hybrids?
Why should there often be so great a difference in the result of a
reciprocal cross between the same two species? Why, it may even be
asked, has the production of hybrids been permitted? to grant to species
the special power of producing hybrids, and then to stop their further
propagation by different degrees of sterility, not strictly related to
the facility of the first union between their parents, seems to be a
strange arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly
to indicate that the sterility both of first crosses and of hybrids is
simply incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature, that, in reciprocal crosses
between two species the male sexual element of the one will often freely
act on the female sexual element of the other, but not in a reversed
direction. It will be advisable to explain a little more fully by an
example what I mean by sterility being incidental on other differences,
and not a specially endowed quality. As the capacity of one plant to be
grafted or budded on another is so entirely unimportant for its welfare
in a state of nature, I presume that no one will suppose that this
capacity is a SPECIALLY endowed quality, but will admit that it is
incidental on differences in the laws of growth of the two plants. We
can sometimes see the reason why one tree will not take on another, from
differences in their rate of growth, in the hardness of their wood, in
the period of the flow or nature of their sap, etc.; but in a multitude
of cases we can assign no reason whatever. Great diversity in the size
of two plants, one being woody and the other herbaceous, one being
evergreen and the other deciduous, and adaptation to widely different
climates, does not always prevent the two grafting together. As in
hybridisation, so with grafting, the capacity is limited by systematic
affinity, for no one has been able to graft trees together belonging to
quite distinct families; and, on the other hand, closely allied species,
and varieties of the same species, can usually, but not invariably,
be grafted with ease. But this capacity, as in hybridisation, is by no
means absolutely governed by systematic affinity. Although many distinct
genera within the same family have been grafted together, in other cases
species of the same genus will not take on each other. The pear can be
grafted far more readily on the quince, which is ranked as a distinct
genus, than on the apple, which is a member of the same genus. Even
different varieties of the pear take with different degrees of facility
on the quince; so do different varieties of the apricot and peach on
certain varieties of the plum.

As Gartner found that there was sometimes an innate difference in
different INDIVIDUALS of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same
two species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.

We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty on
another species, when thus grafted were rendered barren. On the other
hand, certain species of Sorbus, when grafted on other species, yielded
twice as much fruit as when on their own roots. We are reminded by this
latter fact of the extraordinary case of Hippeastrum, Lobelia, etc.,
which seeded much more freely when fertilised with the pollen of
distinct species, than when self-fertilised with their own pollen.

We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male
and female elements in the act of reproduction, yet that there is a
rude degree of parallelism in the results of grafting and of crossing
distinct species. And as we must look at the curious and complex laws
governing the facility with which trees can be grafted on each other
as incidental on unknown differences in their vegetative systems, so I
believe that the still more complex laws governing the facility of
first crosses, are incidental on unknown differences, chiefly in their
reproductive systems. These differences, in both cases, follow to a
certain extent, as might have been expected, systematic affinity, by
which every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed. The facts by no means seem to me
to indicate that the greater or lesser difficulty of either grafting or
crossing together various species has been a special endowment;
although in the case of crossing, the difficulty is as important for the
endurance and stability of specific forms, as in the case of grafting it
is unimportant for their welfare.

CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We may now look a little closer at the probable causes of the sterility
of first crosses and of hybrids. These two cases are fundamentally
different, for, as just remarked, in the union of two pure species the
male and female sexual elements are perfect, whereas in hybrids they are
imperfect. Even in first crosses, the greater or lesser difficulty in
effecting a union apparently depends on several distinct causes. There
must sometimes be a physical impossibility in the male element reaching
the ovule, as would be the case with a plant having a pistil too long
for the pollen-tubes to reach the ovarium. It has also been observed
that when pollen of one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the male element may reach the female
element, but be incapable of causing an embryo to be developed, as seems
to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and
then perish at an early period. This latter alternative has not been
sufficiently attended to; but I believe, from observations communicated
to me by Mr. Hewitt, who has had great experience in hybridising
gallinaceous birds, that the early death of the embryo is a very
frequent cause of sterility in first crosses. I was at first very
unwilling to believe in this view; as hybrids, when once born, are
generally healthy and long-lived, as we see in the case of the common
mule. Hybrids, however, are differently circumstanced before and after
birth: when born and living in a country where their two parents can
live, they are generally placed under suitable conditions of life. But
a hybrid partakes of only half of the nature and constitution of its
mother, and therefore before birth, as long as it is nourished within
its mother's womb or within the egg or seed produced by the mother, it
may be exposed to conditions in some degree unsuitable, and consequently
be liable to perish at an early period; more especially as all very
young beings seem eminently sensitive to injurious or unnatural
conditions of life.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than once
alluded to a large body of facts, which I have collected, showing that
when animals and plants are removed from their natural conditions,
they are extremely liable to have their reproductive systems seriously
affected. This, in fact, is the great bar to the domestication of
animals. Between the sterility thus superinduced and that of hybrids,
there are many points of similarity. In both cases the sterility is
independent of general health, and is often accompanied by excess of
size or great luxuriance. In both cases, the sterility occurs in various
degrees; in both, the male element is the most liable to be affected;
but sometimes the female more than the male. In both, the tendency
goes to a certain extent with systematic affinity, for whole groups
of animals and plants are rendered impotent by the same unnatural
conditions; and whole groups of species tend to produce sterile hybrids.
On the other hand, one species in a group will sometimes resist great
changes of conditions with unimpaired fertility; and certain species in
a group will produce unusually fertile hybrids. No one can tell, till he
tries, whether any particular animal will breed under confinement or any
plant seed freely under culture; nor can he tell, till he tries, whether
any two species of a genus will produce more or less sterile hybrids.
Lastly, when organic beings are placed during several generations under
conditions not natural to them, they are extremely liable to vary,
which is due, as I believe, to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for hybrids in successive generations are
eminently liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of
two species, the reproductive system, independently of the general state
of health, is affected by sterility in a very similar manner. In the
one case, the conditions of life have been disturbed, though often in so
slight a degree as to be inappreciable by us; in the other case, or
that of hybrids, the external conditions have remained the same, but
the organisation has been disturbed by two different structures and
constitutions having been blended into one. For it is scarcely possible
that two organisations should be compounded into one, without some
disturbance occurring in the development, or periodical action, or
mutual relation of the different parts and organs one to another, or to
the conditions of life. When hybrids are able to breed inter se, they
transmit to their offspring from generation to generation the same
compounded organisation, and hence we need not be surprised that their
sterility, though in some degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting
on vague hypotheses, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent. Nor
do I pretend that the foregoing remarks go to the root of the matter:
no explanation is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted to show,
is that in two cases, in some respects allied, sterility is the common
result,--in the one case from the conditions of life having been
disturbed, in the other case from the organisation having been disturbed
by two organisations having been compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends
to an allied yet very different class of facts. It is an old and almost
universal belief, founded, I think, on a considerable body of evidence,
that slight changes in the conditions of life are beneficial to all
living things. We see this acted on by farmers and gardeners in their
frequent exchanges of seed, tubers, etc., from one soil or climate to
another, and back again. During the convalescence of animals, we plainly
see that great benefit is derived from almost any change in the
habits of life. Again, both with plants and animals, there is abundant
evidence, that a cross between very distinct individuals of the same
species, that is between members of different strains or sub-breeds,
gives vigour and fertility to the offspring. I believe, indeed, from
the facts alluded to in our fourth chapter, that a certain amount of
crossing is indispensable even with hermaphrodites; and that close
interbreeding continued during several generations between the nearest
relations, especially if these be kept under the same conditions of
life, always induces weakness and sterility in the progeny.

Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour and
fertility to the offspring. But we have seen that greater changes, or
changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between males
and females which have become widely or specifically different, produce
hybrids which are generally sterile in some degree. I cannot persuade
myself that this parallelism is an accident or an illusion. Both series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.

It may be urged, as a most forcible argument, that there must be some
essential distinction between species and varieties, and that there
must be some error in all the foregoing remarks, inasmuch as varieties,
however much they may differ from each other in external appearance,
cross with perfect facility, and yield perfectly fertile offspring. I
fully admit that this is almost invariably the case. But if we look to
varieties produced under nature, we are immediately involved in hopeless
difficulties; for if two hitherto reputed varieties be found in any
degree sterile together, they are at once ranked by most naturalists
as species. For instance, the blue and red pimpernel, the primrose
and cowslip, which are considered by many of our best botanists as
varieties, are said by Gartner not to be quite fertile when crossed, and
he consequently ranks them as undoubted species. If we thus argue in
a circle, the fertility of all varieties produced under nature will
assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in doubt. For when it is
stated, for instance, that the German Spitz dog unites more easily
than other dogs with foxes, or that certain South American indigenous
domestic dogs do not readily cross with European dogs, the explanation
which will occur to everyone, and probably the true one, is that
these dogs have descended from several aboriginally distinct species.
Nevertheless the perfect fertility of so many domestic varieties,
differing widely from each other in appearance, for instance of the
pigeon or of the cabbage, is a remarkable fact; more especially when we
reflect how many species there are, which, though resembling each
other most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic varieties less
remarkable than at first appears. It can, in the first place, be clearly
shown that mere external dissimilarity between two species does not
determine their greater or lesser degree of sterility when crossed; and
we may apply the same rule to domestic varieties. In the second place,
some eminent naturalists believe that a long course of domestication
tends to eliminate sterility in the successive generations of hybrids,
which were at first only slightly sterile; and if this be so, we surely
ought not to expect to find sterility both appearing and disappearing
under nearly the same conditions of life. Lastly, and this seems to me
by far the most important consideration, new races of animals and plants
are produced under domestication by man's methodical and unconscious
power of selection, for his own use and pleasure: he neither wishes to
select, nor could select, slight differences in the reproductive system,
or other constitutional differences correlated with the reproductive
system. He supplies his several varieties with the same food; treats
them in nearly the same manner, and does not wish to alter their general
habits of life. Nature acts uniformly and slowly during vast periods
of time on the whole organisation, in any way which may be for each
creature's own good; and thus she may, either directly, or more probably
indirectly, through correlation, modify the reproductive system in the
several descendants from any one species. Seeing this difference in the
process of selection, as carried on by man and nature, we need not be
surprised at some difference in the result.

I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it seems to me impossible to
resist the evidence of the existence of a certain amount of sterility in
the few following cases, which I will briefly abstract. The evidence
is at least as good as that from which we believe in the sterility of
a multitude of species. The evidence is, also, derived from hostile
witnesses, who in all other cases consider fertility and sterility as
safe criterions of specific distinction. Gartner kept during several
years a dwarf kind of maize with yellow seeds, and a tall variety with
red seeds, growing near each other in his garden; and although these
plants have separated sexes, they never naturally crossed. He then
fertilised thirteen flowers of the one with the pollen of the other; but
only a single head produced any seed, and this one head produced only
five grains. Manipulation in this case could not have been injurious, as
the plants have separated sexes. No one, I believe, has suspected that
these varieties of maize are distinct species; and it is important to
notice that the hybrid plants thus raised were themselves PERFECTLY
fertile; so that even Gartner did not venture to consider the two
varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like
the maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not; but the
forms experimentised on, are ranked by Sagaret, who mainly founds his
classification by the test of infertility, as varieties.

The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of experiments
made during many years on nine species of Verbascum, by so good an
observer and so hostile a witness, as Gartner: namely, that yellow
and white varieties of the same species of Verbascum when intercrossed
produce less seed, than do either coloured varieties when fertilised
with pollen from their own coloured flowers. Moreover, he asserts that
when yellow and white varieties of one species are crossed with yellow
and white varieties of a DISTINCT species, more seed is produced by the
crosses between the same coloured flowers, than between those which are
differently coloured. Yet these varieties of Verbascum present no other
difference besides the mere colour of the flower; and one variety can
sometimes be raised from the seed of the other.

From observations which I have made on certain varieties of hollyhock, I
am inclined to suspect that they present analogous facts.

Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one variety of the common
tobacco is more fertile, when crossed with a widely distinct species,
than are the other varieties. He experimentised on five forms, which are
commonly reputed to be varieties, and which he tested by the severest
trial, namely, by reciprocal crosses, and he found their mongrel
offspring perfectly fertile. But one of these five varieties, when used
either as father or mother, and crossed with the Nicotiana glutinosa,
always yielded hybrids not so sterile as those which were produced
from the four other varieties when crossed with N. glutinosa. Hence the
reproductive system of this one variety must have been in some manner
and in some degree modified.

From these facts; from the great difficulty of ascertaining the
infertility of varieties in a state of nature, for a supposed variety if
infertile in any degree would generally be ranked as species; from
man selecting only external characters in the production of the most
distinct domestic varieties, and from not wishing or being able to
produce recondite and functional differences in the reproductive system;
from these several considerations and facts, I do not think that the
very general fertility of varieties can be proved to be of universal
occurrence, or to form a fundamental distinction between varieties
and species. The general fertility of varieties does not seem to me
sufficient to overthrow the view which I have taken with respect to
the very general, but not invariable, sterility of first crosses and of
hybrids, namely, that it is not a special endowment, but is incidental
on slowly acquired modifications, more especially in the reproductive
systems of the forms which are crossed.

HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.

Independently of the question of fertility, the offspring of species
when crossed and of varieties when crossed may be compared in several
other respects. Gartner, whose strong wish was to draw a marked line of
distinction between species and varieties, could find very few and,
as it seems to me, quite unimportant differences between the so-called
hybrid offspring of species, and the so-called mongrel offspring of
varieties. And, on the other hand, they agree most closely in very many
important respects.

I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gartner admits that hybrids from
species which have long been cultivated are often variable in the first
generation; and I have myself seen striking instances of this fact.
Gartner further admits that hybrids between very closely allied species
are more variable than those from very distinct species; and this shows
that the difference in the degree of variability graduates away.
When mongrels and the more fertile hybrids are propagated for several
generations an extreme amount of variability in their offspring
is notorious; but some few cases both of hybrids and mongrels long
retaining uniformity of character could be given. The variability,
however, in the successive generations of mongrels is, perhaps, greater
than in hybrids.

This greater variability of mongrels than of hybrids does not seem to me
at all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies in most cases that there has been recent
variability; and therefore we might expect that such variability would
often continue and be super-added to that arising from the mere act of
crossing. The slight degree of variability in hybrids from the first
cross or in the first generation, in contrast with their extreme
variability in the succeeding generations, is a curious fact and
deserves attention. For it bears on and corroborates the view which I
have taken on the cause of ordinary variability; namely, that it is due
to the reproductive system being eminently sensitive to any change in
the conditions of life, being thus often rendered either impotent or at
least incapable of its proper function of producing offspring identical
with the parent-form. Now hybrids in the first generation are descended
from species (excluding those long cultivated) which have not had their
reproductive systems in any way affected, and they are not variable; but
hybrids themselves have their reproductive systems seriously affected,
and their descendants are highly variable.

But to return to our comparison of mongrels and hybrids: Gartner
states that mongrels are more liable than hybrids to revert to either
parent-form; but this, if it be true, is certainly only a difference in
degree. Gartner further insists that when any two species, although
most closely allied to each other, are crossed with a third species,
the hybrids are widely different from each other; whereas if two very
distinct varieties of one species are crossed with another species, the
hybrids do not differ much. But this conclusion, as far as I can make
out, is founded on a single experiment; and seems directly opposed to
the results of several experiments made by Kolreuter.

These alone are the unimportant differences, which Gartner is able to
point out, between hybrid and mongrel plants. On the other hand, the
resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species, follows
according to Gartner the same laws. When two species are crossed,
one has sometimes a prepotent power of impressing its likeness on the
hybrid; and so I believe it to be with varieties of plants. With animals
one variety certainly often has this prepotent power over another
variety. Hybrid plants produced from a reciprocal cross, generally
resemble each other closely; and so it is with mongrels from a
reciprocal cross. Both hybrids and mongrels can be reduced to either
pure parent-form, by repeated crosses in successive generations with
either parent.

These several remarks are apparently applicable to animals; but the
subject is here excessively complicated, partly owing to the existence
of secondary sexual characters; but more especially owing to prepotency
in transmitting likeness running more strongly in one sex than in the
other, both when one species is crossed with another, and when one
variety is crossed with another variety. For instance, I think those
authors are right, who maintain that the ass has a prepotent power over
the horse, so that both the mule and the hinny more resemble the ass
than the horse; but that the prepotency runs more strongly in the
male-ass than in the female, so that the mule, which is the offspring of
the male-ass and mare, is more like an ass, than is the hinny, which is
the offspring of the female-ass and stallion.

Much stress has been laid by some authors on the supposed fact, that
mongrel animals alone are born closely like one of their parents; but
it can be shown that this does sometimes occur with hybrids; yet I grant
much less frequently with hybrids than with mongrels. Looking to the
cases which I have collected of cross-bred animals closely resembling
one parent, the resemblances seem chiefly confined to characters almost
monstrous in their nature, and which have suddenly appeared--such as
albinism, melanism, deficiency of tail or horns, or additional fingers
and toes; and do not relate to characters which have been slowly
acquired by selection. Consequently, sudden reversions to the perfect
character of either parent would be more likely to occur with mongrels,
which are descended from varieties often suddenly produced and
semi-monstrous in character, than with hybrids, which are descended from
species slowly and naturally produced. On the whole I entirely agree
with Dr. Prosper Lucas, who, after arranging an enormous body of facts
with respect to animals, comes to the conclusion, that the laws of
resemblance of the child to its parents are the same, whether the two
parents differ much or little from each other, namely in the union
of individuals of the same variety, or of different varieties, or of
distinct species.

Laying aside the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the
offspring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an astonishing
fact. But it harmonises perfectly with the view that there is no
essential distinction between species and varieties.

SUMMARY OF CHAPTER.

First crosses between forms sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that
the two most careful experimentalists who have ever lived, have come to
diametrically opposite conclusions in ranking forms by this test. The
sterility is innately variable in individuals of the same species, and
is eminently susceptible of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed by several curious and complex laws. It is generally different,
and sometimes widely different, in reciprocal crosses between the same
two species. It is not always equal in degree in a first cross and in
the hybrid produced from this cross.

In the same manner as in grafting trees, the capacity of one species
or variety to take on another, is incidental on generally unknown
differences in their vegetative systems, so in crossing, the greater
or less facility of one species to unite with another, is incidental
on unknown differences in their reproductive systems. There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature,
than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together in
order to prevent them becoming inarched in our forests.

The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances;
in some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which
have had this system and their whole organisation disturbed by being
compounded of two distinct species, seems closely allied to that
sterility which so frequently affects pure species, when their natural
conditions of life have been disturbed. This view is supported by a
parallelism of another kind;--namely, that the crossing of forms only
slightly different is favourable to the vigour and fertility of their
offspring; and that slight changes in the conditions of life are
apparently favourable to the vigour and fertility of all organic beings.
It is not surprising that the degree of difficulty in uniting two
species, and the degree of sterility of their hybrid-offspring should
generally correspond, though due to distinct causes; for both depend
on the amount of difference of some kind between the species which are
crossed. Nor is it surprising that the facility of effecting a first
cross, the fertility of the hybrids produced, and the capacity of being
grafted together--though this latter capacity evidently depends on
widely different circumstances--should all run, to a certain extent,
parallel with the systematic affinity of the forms which are subjected
to experiment; for systematic affinity attempts to express all kinds of
resemblance between all species.

First crosses between forms known to be varieties, or sufficiently alike
to be considered as varieties, and their mongrel offspring, are very
generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable we
are to argue in a circle with respect to varieties in a state of nature;
and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In
all other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels. Finally, then, the facts
briefly given in this chapter do not seem to me opposed to, but even
rather to support the view, that there is no fundamental distinction
between species and varieties.




9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day. On the
nature of extinct intermediate varieties; on their number. On the
vast lapse of time, as inferred from the rate of deposition and of
denudation. On the poorness of our palaeontological collections. On the
intermittence of geological formations. On the absence of intermediate
varieties in any one formation. On the sudden appearance of groups of
species. On their sudden appearance in the lowest known fossiliferous
strata.

In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific forms,
and their not being blended together by innumerable transitional links,
is a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day, under the circumstances apparently
most favourable for their presence, namely on an extensive and
continuous area with graduated physical conditions. I endeavoured to
show, that the life of each species depends in a more important manner
on the presence of other already defined organic forms, than on climate;
and, therefore, that the really governing conditions of life do not
graduate away quite insensibly like heat or moisture. I endeavoured,
also, to show that intermediate varieties, from existing in lesser
numbers than the forms which they connect, will generally be beaten
out and exterminated during the course of further modification and
improvement. The main cause, however, of innumerable intermediate links
not now occurring everywhere throughout nature depends on the very
process of natural selection, through which new varieties continually
take the places of and exterminate their parent-forms. But just in
proportion as this process of extermination has acted on an enormous
scale, so must the number of intermediate varieties, which have
formerly existed on the earth, be truly enormous. Why then is not every
geological formation and every stratum full of such intermediate links?
Geology assuredly does not reveal any such finely graduated organic
chain; and this, perhaps, is the most obvious and gravest objection
which can be urged against my theory. The explanation lies, as I
believe, in the extreme imperfection of the geological record.

In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself, forms DIRECTLY intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons have both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified, that if we had
no historical or indirect evidence regarding their origin, it would not
have been possible to have determined from a mere comparison of their
structure with that of the rock-pigeon, whether they had descended from
this species or from some other allied species, such as C. oenas.

So with natural species, if we look to forms very distinct, for instance
to the horse and tapir, we have no reason to suppose that links ever
existed directly intermediate between them, but between each and an
unknown common parent. The common parent will have had in its whole
organisation much general resemblance to the tapir and to the horse; but
in some points of structure may have differed considerably from both,
even perhaps more than they differ from each other. Hence in all such
cases, we should be unable to recognise the parent-form of any two or
more species, even if we closely compared the structure of the parent
with that of its modified descendants, unless at the same time we had a
nearly perfect chain of the intermediate links.

It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case DIRECT intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount of
change; and the principle of competition between organism and organism,
between child and parent, will render this a very rare event; for in all
cases the new and improved forms of life will tend to supplant the old
and unimproved forms.

By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the varieties of the same species at the
present day; and these parent-species, now generally extinct, have in
their turn been similarly connected with more ancient species; and so on
backwards, always converging to the common ancestor of each great class.
So that the number of intermediate and transitional links, between all
living and extinct species, must have been inconceivably great. But
assuredly, if this theory be true, such have lived upon this earth.

ON THE LAPSE OF TIME.

Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected, that time will not have
sufficed for so great an amount of organic change, all changes having
been effected very slowly through natural selection. It is hardly
possible for me even to recall to the reader, who may not be a practical
geologist, the facts leading the mind feebly to comprehend the lapse of
time. He who can read Sir Charles Lyell's grand work on the Principles
of Geology, which the future historian will recognise as having produced
a revolution in natural science, yet does not admit how incomprehensibly
vast have been the past periods of time, may at once close this volume.
Not that it suffices to study the Principles of Geology, or to read
special treatises by different observers on separate formations, and to
mark how each author attempts to give an inadequate idea of the duration
of each formation or even each stratum. A man must for years examine for
himself great piles of superimposed strata, and watch the sea at work
grinding down old rocks and making fresh sediment, before he can hope to
comprehend anything of the lapse of time, the monuments of which we see
around us.

It is good to wander along lines of sea-coast, when formed of moderately
hard rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat
into them only when they are charged with sand or pebbles; for there
is reason to believe that pure water can effect little or nothing in
wearing away rock. At last the base of the cliff is undermined, huge
fragments fall down, and these remaining fixed, have to be worn away,
atom by atom, until reduced in size they can be rolled about by the
waves, and then are more quickly ground into pebbles, sand, or mud.
But how often do we see along the bases of retreating cliffs rounded
boulders, all thickly clothed by marine productions, showing how little
they are abraded and how seldom they are rolled about! Moreover, if
we follow for a few miles any line of rocky cliff, which is undergoing
degradation, we find that it is only here and there, along a short
length or round a promontory, that the cliffs are at the present time
suffering. The appearance of the surface and the vegetation show that
elsewhere years have elapsed since the waters washed their base.

He who most closely studies the action of the sea on our shores, will,
I believe, be most deeply impressed with the slowness with which rocky
coasts are worn away. The observations on this head by Hugh Miller,
and by that excellent observer Mr. Smith of Jordan Hill, are most
impressive. With the mind thus impressed, let any one examine beds of
conglomerate many thousand feet in thickness, which, though probably
formed at a quicker rate than many other deposits, yet, from being
formed of worn and rounded pebbles, each of which bears the stamp of
time, are good to show how slowly the mass has been accumulated. Let
him remember Lyell's profound remark, that the thickness and extent of
sedimentary formations are the result and measure of the degradation
which the earth's crust has elsewhere suffered. And what an amount of
degradation is implied by the sedimentary deposits of many countries!
Professor Ramsay has given me the maximum thickness, in most cases from
actual measurement, in a few cases from estimate, of each formation in
different parts of Great Britain; and this is the result:--

                                                      Feet

     Palaeozoic strata (not including igneous beds)..57,154.
     Secondary strata................................13,190.
     Tertiary strata..................................2,240.

--making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness
on the Continent. Moreover, between each successive formation, we have,
in the opinion of most geologists, enormously long blank periods.
So that the lofty pile of sedimentary rocks in Britain, gives but an
inadequate idea of the time which has elapsed during their accumulation;
yet what time this must have consumed! Good observers have estimated
that sediment is deposited by the great Mississippi river at the rate
of only 600 feet in a hundred thousand years. This estimate may be quite
erroneous; yet, considering over what wide spaces very fine sediment is
transported by the currents of the sea, the process of accumulation in
any one area must be extremely slow.

But the amount of denudation which the strata have in many places
suffered, independently of the rate of accumulation of the degraded
matter, probably offers the best evidence of the lapse of time. I
remember having been much struck with the evidence of denudation, when
viewing volcanic islands, which have been worn by the waves and pared
all round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their formerly
liquid state, showed at a glance how far the hard, rocky beds had once
extended into the open ocean. The same story is still more plainly told
by faults,--those great cracks along which the strata have been upheaved
on one side, or thrown down on the other, to the height or depth of
thousands of feet; for since the crust cracked, the surface of the land
has been so completely planed down by the action of the sea, that no
trace of these vast dislocations is externally visible.

The Craven fault, for instance, extends for upwards of 30 miles, and
along this line the vertical displacement of the strata has varied
from 600 to 3000 feet. Professor Ramsay has published an account of
a downthrow in Anglesea of 2300 feet; and he informs me that he fully
believes there is one in Merionethshire of 12,000 feet; yet in these
cases there is nothing on the surface to show such prodigious movements;
the pile of rocks on the one or other side having been smoothly swept
away. The consideration of these facts impresses my mind almost in
the same manner as does the vain endeavour to grapple with the idea of
eternity.

I am tempted to give one other case, the well-known one of the
denudation of the Weald. Though it must be admitted that the denudation
of the Weald has been a mere trifle, in comparison with that which has
removed masses of our palaeozoic strata, in parts ten thousand feet
in thickness, as shown in Professor Ramsay's masterly memoir on this
subject. Yet it is an admirable lesson to stand on the North Downs and
to look at the distant South Downs; for, remembering that at no great
distance to the west the northern and southern escarpments meet and
close, one can safely picture to oneself the great dome of rocks which
must have covered up the Weald within so limited a period as since the
latter part of the Chalk formation. The distance from the northern to
the southern Downs is about 22 miles, and the thickness of the several
formations is on an average about 1100 feet, as I am informed by
Professor Ramsay. But if, as some geologists suppose, a range of
older rocks underlies the Weald, on the flanks of which the overlying
sedimentary deposits might have accumulated in thinner masses than
elsewhere, the above estimate would be erroneous; but this source of
doubt probably would not greatly affect the estimate as applied to the
western extremity of the district. If, then, we knew the rate at which
the sea commonly wears away a line of cliff of any given height, we
could measure the time requisite to have denuded the Weald. This, of
course, cannot be done; but we may, in order to form some crude notion
on the subject, assume that the sea would eat into cliffs 500 feet in
height at the rate of one inch in a century. This will at first appear
much too small an allowance; but it is the same as if we were to assume
a cliff one yard in height to be eaten back along a whole line of
coast at the rate of one yard in nearly every twenty-two years. I
doubt whether any rock, even as soft as chalk, would yield at this rate
excepting on the most exposed coasts; though no doubt the degradation
of a lofty cliff would be more rapid from the breakage of the fallen
fragments. On the other hand, I do not believe that any line of coast,
ten or twenty miles in length, ever suffers degradation at the same time
along its whole indented length; and we must remember that almost all
strata contain harder layers or nodules, which from long resisting
attrition form a breakwater at the base. Hence, under ordinary
circumstances, I conclude that for a cliff 500 feet in height, a
denudation of one inch per century for the whole length would be an
ample allowance. At this rate, on the above data, the denudation of the
Weald must have required 306,662,400 years; or say three hundred million
years.

The action of fresh water on the gently inclined Wealden district, when
upraised, could hardly have been great, but it would somewhat reduce the
above estimate. On the other hand, during oscillations of level, which
we know this area has undergone, the surface may have existed for
millions of years as land, and thus have escaped the action of the
sea: when deeply submerged for perhaps equally long periods, it would,
likewise, have escaped the action of the coast-waves. So that in all
probability a far longer period than 300 million years has elapsed since
the latter part of the Secondary period.

I have made these few remarks because it is highly important for us to
gain some notion, however imperfect, of the lapse of years. During each
of these years, over the whole world, the land and the water has
been peopled by hosts of living forms. What an infinite number of
generations, which the mind cannot grasp, must have succeeded each other
in the long roll of years! Now turn to our richest geological museums,
and what a paltry display we behold!

ON THE POORNESS OF OUR PALAEONTOLOGICAL COLLECTIONS.

That our palaeontological collections are very imperfect, is admitted by
every one. The remark of that admirable palaeontologist, the late Edward
Forbes, should not be forgotten, namely, that numbers of our fossil
species are known and named from single and often broken specimens, or
from a few specimens collected on some one spot. Only a small portion
of the surface of the earth has been geologically explored, and no part
with sufficient care, as the important discoveries made every year in
Europe prove. No organism wholly soft can be preserved. Shells and
bones will decay and disappear when left on the bottom of the sea, where
sediment is not accumulating. I believe we are continually taking a
most erroneous view, when we tacitly admit to ourselves that sediment
is being deposited over nearly the whole bed of the sea, at a rate
sufficiently quick to embed and preserve fossil remains. Throughout an
enormously large proportion of the ocean, the bright blue tint of the
water bespeaks its purity. The many cases on record of a formation
conformably covered, after an enormous interval of time, by another
and later formation, without the underlying bed having suffered in the
interval any wear and tear, seem explicable only on the view of the
bottom of the sea not rarely lying for ages in an unaltered condition.
The remains which do become embedded, if in sand or gravel, will when
the beds are upraised generally be dissolved by the percolation of
rain-water. I suspect that but few of the very many animals which live
on the beach between high and low watermark are preserved. For instance,
the several species of the Chthamalinae (a sub-family of sessile
cirripedes) coat the rocks all over the world in infinite numbers: they
are all strictly littoral, with the exception of a single Mediterranean
species, which inhabits deep water and has been found fossil in Sicily,
whereas not one other species has hitherto been found in any tertiary
formation: yet it is now known that the genus Chthamalus existed
during the chalk period. The molluscan genus Chiton offers a partially
analogous case.

With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these
vast periods, with one exception discovered by Sir C. Lyell in the
carboniferous strata of North America. In regard to mammiferous remains,
a single glance at the historical table published in the Supplement to
Lyell's Manual, will bring home the truth, how accidental and rare is
their preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known belonging
to the age of our secondary or palaeozoic formations.

But the imperfection in the geological record mainly results from
another and more important cause than any of the foregoing; namely, from
the several formations being separated from each other by wide intervals
of time. When we see the formations tabulated in written works, or when
we follow them in nature, it is difficult to avoid believing that
they are closely consecutive. But we know, for instance, from Sir R.
Murchison's great work on Russia, what wide gaps there are in that
country between the superimposed formations; so it is in North America,
and in many other parts of the world. The most skilful geologist, if
his attention had been exclusively confined to these large territories,
would never have suspected that during the periods which were blank and
barren in his own country, great piles of sediment, charged with new and
peculiar forms of life, had elsewhere been accumulated. And if in each
separate territory, hardly any idea can be formed of the length of time
which has elapsed between the consecutive formations, we may infer that
this could nowhere be ascertained. The frequent and great changes in the
mineralogical composition of consecutive formations, generally implying
great changes in the geography of the surrounding lands, whence the
sediment has been derived, accords with the belief of vast intervals of
time having elapsed between each formation.

But we can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each
other in close sequence. Scarcely any fact struck me more when examining
many hundred miles of the South American coasts, which have been
upraised several hundred feet within the recent period, than the absence
of any recent deposits sufficiently extensive to last for even a short
geological period. Along the whole west coast, which is inhabited by a
peculiar marine fauna, tertiary beds are so scantily developed, that no
record of several successive and peculiar marine faunas will probably be
preserved to a distant age. A little reflection will explain why along
the rising coast of the western side of South America, no extensive
formations with recent or tertiary remains can anywhere be found, though
the supply of sediment must for ages have been great, from the enormous
degradation of the coast-rocks and from muddy streams entering the
sea. The explanation, no doubt, is, that the littoral and sub-littoral
deposits are continually worn away, as soon as they are brought up by
the slow and gradual rising of the land within the grinding action of
the coast-waves.

We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during subsequent
oscillations of level. Such thick and extensive accumulations of
sediment may be formed in two ways; either, in profound depths of the
sea, in which case, judging from the researches of E. Forbes, we may
conclude that the bottom will be inhabited by extremely few animals, and
the mass when upraised will give a most imperfect record of the forms
of life which then existed; or, sediment may be accumulated to any
thickness and extent over a shallow bottom, if it continue slowly to
subside. In this latter case, as long as the rate of subsidence and
supply of sediment nearly balance each other, the sea will remain
shallow and favourable for life, and thus a fossiliferous formation
thick enough, when upraised, to resist any amount of degradation, may be
formed.

I am convinced that all our ancient formations, which are rich in
fossils, have thus been formed during subsidence. Since publishing my
views on this subject in 1845, I have watched the progress of Geology,
and have been surprised to note how author after author, in treating
of this or that great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only ancient tertiary
formation on the west coast of South America, which has been bulky
enough to resist such degradation as it has as yet suffered, but which
will hardly last to a distant geological age, was certainly deposited
during a downward oscillation of level, and thus gained considerable
thickness.

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, may
have been formed over wide spaces during periods of subsidence, but only
where the supply of sediment was sufficient to keep the sea shallow and
to embed and preserve the remains before they had time to decay. On the
other hand, as long as the bed of the sea remained stationary, THICK
deposits could not have been accumulated in the shallow parts, which are
the most favourable to life. Still less could this have happened during
the alternate periods of elevation; or, to speak more accurately, the
beds which were then accumulated will have been destroyed by being
upraised and brought within the limits of the coast-action.

Thus the geological record will almost necessarily be rendered
intermittent. I feel much confidence in the truth of these views, for
they are in strict accordance with the general principles inculcated
by Sir C. Lyell; and E. Forbes independently arrived at a similar
conclusion.

One remark is here worth a passing notice. During periods of elevation
the area of the land and of the adjoining shoal parts of the sea will
be increased, and new stations will often be formed;--all circumstances
most favourable, as previously explained, for the formation of new
varieties and species; but during such periods there will generally be
a blank in the geological record. On the other hand, during subsidence,
the inhabited area and number of inhabitants will decrease (excepting
the productions on the shores of a continent when first broken up into
an archipelago), and consequently during subsidence, though there will
be much extinction, fewer new varieties or species will be formed; and
it is during these very periods of subsidence, that our great deposits
rich in fossils have been accumulated. Nature may almost be said to have
guarded against the frequent discovery of her transitional or linking
forms.

From the foregoing considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if we
confine our attention to any one formation, it becomes more difficult
to understand, why we do not therein find closely graduated varieties
between the allied species which lived at its commencement and at its
close. Some cases are on record of the same species presenting distinct
varieties in the upper and lower parts of the same formation, but, as
they are rare, they may be here passed over. Although each formation has
indisputably required a vast number of years for its deposition, I can
see several reasons why each should not include a graduated series
of links between the species which then lived; but I can by no
means pretend to assign due proportional weight to the following
considerations.

Although each formation may mark a very long lapse of years, each
perhaps is short compared with the period requisite to change one
species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or thrice
as long as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us coming to any just
conclusion on this head. When we see a species first appearing in the
middle of any formation, it would be rash in the extreme to infer that
it had not elsewhere previously existed. So again when we find a species
disappearing before the uppermost layers have been deposited, it would
be equally rash to suppose that it then became wholly extinct. We forget
how small the area of Europe is compared with the rest of the world;
nor have the several stages of the same formation throughout Europe been
correlated with perfect accuracy.

With marine animals of all kinds, we may safely infer a large amount of
migration during climatal and other changes; and when we see a species
first appearing in any formation, the probability is that it only then
first immigrated into that area. It is well known, for instance, that
several species appeared somewhat earlier in the palaeozoic beds of
North America than in those of Europe; time having apparently been
required for their migration from the American to the European seas. In
examining the latest deposits of various quarters of the world, it has
everywhere been noted, that some few still existing species are common
in the deposit, but have become extinct in the immediately surrounding
sea; or, conversely, that some are now abundant in the neighbouring sea,
but are rare or absent in this particular deposit. It is an excellent
lesson to reflect on the ascertained amount of migration of the
inhabitants of Europe during the Glacial period, which forms only a part
of one whole geological period; and likewise to reflect on the great
changes of level, on the inordinately great change of climate, on the
prodigious lapse of time, all included within this same glacial period.
Yet it may be doubted whether in any quarter of the world, sedimentary
deposits, INCLUDING FOSSIL REMAINS, have gone on accumulating within
the same area during the whole of this period. It is not, for instance,
probable that sediment was deposited during the whole of the glacial
period near the mouth of the Mississippi, within that limit of depth at
which marine animals can flourish; for we know what vast geographical
changes occurred in other parts of America during this space of time.
When such beds as were deposited in shallow water near the mouth of
the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migration of species and to geographical
changes. And in the distant future, a geologist examining these beds,
might be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead
of having been really far greater, that is extending from before the
glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper
and lower parts of the same formation, the deposit must have gone on
accumulating for a very long period, in order to have given sufficient
time for the slow process of variation; hence the deposit will generally
have to be a very thick one; and the species undergoing modification
will have had to live on the same area throughout this whole time.
But we have seen that a thick fossiliferous formation can only be
accumulated during a period of subsidence; and to keep the depth
approximately the same, which is necessary in order to enable the same
species to live on the same space, the supply of sediment must nearly
have counterbalanced the amount of subsidence. But this same movement
of subsidence will often tend to sink the area whence the sediment
is derived, and thus diminish the supply whilst the downward movement
continues. In fact, this nearly exact balancing between the supply of
sediment and the amount of subsidence is probably a rare contingency;
for it has been observed by more than one palaeontologist, that very
thick deposits are usually barren of organic remains, except near their
upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of different mineralogical composition, we may reasonably
suspect that the process of deposition has been much interrupted, as
a change in the currents of the sea and a supply of sediment of a
different nature will generally have been due to geographical changes
requiring much time. Nor will the closest inspection of a formation give
any idea of the time which its deposition has consumed. Many instances
could be given of beds only a few feet in thickness, representing
formations, elsewhere thousands of feet in thickness, and which must
have required an enormous period for their accumulation; yet no one
ignorant of this fact would have suspected the vast lapse of time
represented by the thinner formation. Many cases could be given of the
lower beds of a formation having been upraised, denuded, submerged, and
then re-covered by the upper beds of the same formation,--facts,
showing what wide, yet easily overlooked, intervals have occurred in
its accumulation. In other cases we have the plainest evidence in great
fossilised trees, still standing upright as they grew, of many long
intervals of time and changes of level during the process of deposition,
which would never even have been suspected, had not the trees chanced to
have been preserved: thus, Messrs. Lyell and Dawson found carboniferous
beds 1400 feet thick in Nova Scotia, with ancient root-bearing strata,
one above the other, at no less than sixty-eight different levels.
Hence, when the same species occur at the bottom, middle, and top of a
formation, the probability is that they have not lived on the same
spot during the whole period of deposition, but have disappeared and
reappeared, perhaps many times, during the same geological period.
So that if such species were to undergo a considerable amount of
modification during any one geological period, a section would not
probably include all the fine intermediate gradations which must on
my theory have existed between them, but abrupt, though perhaps very
slight, changes of form.

It is all-important to remember that naturalists have no golden rule
by which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by close intermediate
gradations. And this from the reasons just assigned we can seldom hope
to effect in any one geological section. Supposing B and C to be two
species, and a third, A, to be found in an underlying bed; even if A
were strictly intermediate between B and C, it would simply be ranked as
a third and distinct species, unless at the same time it could be
most closely connected with either one or both forms by intermediate
varieties. Nor should it be forgotten, as before explained, that A
might be the actual progenitor of B and C, and yet might not at all
necessarily be strictly intermediate between them in all points of
structure. So that we might obtain the parent-species and its several
modified descendants from the lower and upper beds of a formation,
and unless we obtained numerous transitional gradations, we should not
recognise their relationship, and should consequently be compelled to
rank them all as distinct species.

It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D'Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on my
theory we ought to find. Moreover, if we look to rather wider intervals,
namely, to distinct but consecutive stages of the same great formation,
we find that the embedded fossils, though almost universally ranked as
specifically different, yet are far more closely allied to each other
than are the species found in more widely separated formations; but to
this subject I shall have to return in the following chapter.

One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, there is reason to
suspect, as we have formerly seen, that their varieties are generally
at first local; and that such local varieties do not spread widely and
supplant their parent-forms until they have been modified and perfected
in some considerable degree. According to this view, the chance of
discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes
are supposed to have been local or confined to some one spot. Most
marine animals have a wide range; and we have seen that with plants it
is those which have the widest range, that oftenest present varieties;
so that with shells and other marine animals, it is probably those
which have had the widest range, far exceeding the limits of the known
geological formations of Europe, which have oftenest given rise, first
to local varieties and ultimately to new species; and this again would
greatly lessen the chance of our being able to trace the stages of
transition in any one geological formation.

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties and thus proved to be the same species, until
many specimens have been collected from many places; and in the case
of fossil species this could rarely be effected by palaeontologists. We
shall, perhaps, best perceive the improbability of our being enabled to
connect species by numerous, fine, intermediate, fossil links, by asking
ourselves whether, for instance, geologists at some future period will
be able to prove, that our different breeds of cattle, sheep, horses,
and dogs have descended from a single stock or from several aboriginal
stocks; or, again, whether certain sea-shells inhabiting the shores
of North America, which are ranked by some conchologists as distinct
species from their European representatives, and by other conchologists
as only varieties, are really varieties or are, as it is called,
specifically distinct. This could be effected only by the future
geologist discovering in a fossil state numerous intermediate
gradations; and such success seems to me improbable in the highest
degree.

Geological research, though it has added numerous species to existing
and extinct genera, and has made the intervals between some few groups
less wide than they otherwise would have been, yet has done scarcely
anything in breaking down the distinction between species, by connecting
them together by numerous, fine, intermediate varieties; and this not
having been effected, is probably the gravest and most obvious of all
the many objections which may be urged against my views. Hence it will
be worth while to sum up the foregoing remarks, under an imaginary
illustration. The Malay Archipelago is of about the size of Europe from
the North Cape to the Mediterranean, and from Britain to Russia; and
therefore equals all the geological formations which have been examined
with any accuracy, excepting those of the United States of America. I
fully agree with Mr. Godwin-Austen, that the present condition of the
Malay Archipelago, with its numerous large islands separated by wide and
shallow seas, probably represents the former state of Europe, when most
of our formations were accumulating. The Malay Archipelago is one of
the richest regions of the whole world in organic beings; yet if all
the species were to be collected which have ever lived there, how
imperfectly would they represent the natural history of the world!

But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an excessively imperfect manner in
the formations which we suppose to be there accumulating. I suspect that
not many of the strictly littoral animals, or of those which lived on
naked submarine rocks, would be embedded; and those embedded in gravel
or sand, would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at a
sufficient rate to protect organic bodies from decay, no remains could
be preserved.

In our archipelago, I believe that fossiliferous formations could be
formed of sufficient thickness to last to an age, as distant in futurity
as the secondary formations lie in the past, only during periods of
subsidence. These periods of subsidence would be separated from each
other by enormous intervals, during which the area would be either
stationary or rising; whilst rising, each fossiliferous formation
would be destroyed, almost as soon as accumulated, by the incessant
coast-action, as we now see on the shores of South America. During the
periods of subsidence there would probably be much extinction of life;
during the periods of elevation, there would be much variation, but the
geological record would then be least perfect.

It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with
a contemporaneous accumulation of sediment, would EXCEED the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not fully
preserved, transitional varieties would merely appear as so many
distinct species. It is, also, probable that each great period of
subsidence would be interrupted by oscillations of level, and that
slight climatal changes would intervene during such lengthy periods; and
in these cases the inhabitants of the archipelago would have to migrate,
and no closely consecutive record of their modifications could be
preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy leads me to believe
that it would be chiefly these far-ranging species which would oftenest
produce new varieties; and the varieties would at first generally
be local or confined to one place, but if possessed of any decided
advantage, or when further modified and improved, they would slowly
spread and supplant their parent-forms. When such varieties returned to
their ancient homes, as they would differ from their former state, in
a nearly uniform, though perhaps extremely slight degree, they would,
according to the principles followed by many palaeontologists, be ranked
as new and distinct species.

If then, there be some degree of truth in these remarks, we have no
right to expect to find in our geological formations, an infinite number
of those fine transitional forms, which on my theory assuredly have
connected all the past and present species of the same group into one
long and branching chain of life. We ought only to look for a few links,
some more closely, some more distantly related to each other; and these
links, let them be ever so close, if found in different stages of the
same formation, would, by most palaeontologists, be ranked as distinct
species. But I do not pretend that I should ever have suspected how poor
a record of the mutations of life, the best preserved geological section
presented, had not the difficulty of our not discovering innumerable
transitional links between the species which appeared at the
commencement and close of each formation, pressed so hardly on my
theory.

ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.

The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists,
for instance, by Agassiz, Pictet, and by none more forcibly than
by Professor Sedgwick, as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging to the same
genera or families, have really started into life all at once, the fact
would be fatal to the theory of descent with slow modification through
natural selection. For the development of a group of forms, all of which
have descended from some one progenitor, must have been an extremely
slow process; and the progenitors must have lived long ages before their
modified descendants. But we continually over-rate the perfection of the
geological record, and falsely infer, because certain genera or families
have not been found beneath a certain stage, that they did not exist
before that stage. We continually forget how large the world is,
compared with the area over which our geological formations have been
carefully examined; we forget that groups of species may elsewhere have
long existed and have slowly multiplied before they invaded the ancient
archipelagoes of Europe and of the United States. We do not make due
allowance for the enormous intervals of time, which have probably
elapsed between our consecutive formations,--longer perhaps in some
cases than the time required for the accumulation of each formation.
These intervals will have given time for the multiplication of species
from some one or some few parent-forms; and in the succeeding formation
such species will appear as if suddenly created.

I may here recall a remark formerly made, namely that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance to fly through the air; but that when this
had been effected, and a few species had thus acquired a great advantage
over other organisms, a comparatively short time would be necessary to
produce many divergent forms, which would be able to spread rapidly and
widely throughout the world.

I will now give a few examples to illustrate these remarks; and to show
how liable we are to error in supposing that whole groups of species
have suddenly been produced. I may recall the well-known fact that in
geological treatises, published not many years ago, the great class
of mammals was always spoken of as having abruptly come in at the
commencement of the tertiary series. And now one of the richest known
accumulations of fossil mammals belongs to the middle of the secondary
series; and one true mammal has been discovered in the new red sandstone
at nearly the commencement of this great series. Cuvier used to urge
that no monkey occurred in any tertiary stratum; but now extinct species
have been discovered in India, South America, and in Europe even as far
back as the eocene stage. The most striking case, however, is that of
the Whale family; as these animals have huge bones, are marine, and
range over the world, the fact of not a single bone of a whale having
been discovered in any secondary formation, seemed fully to justify the
belief that this great and distinct order had been suddenly produced
in the interval between the latest secondary and earliest tertiary
formation. But now we may read in the Supplement to Lyell's 'Manual,'
published in 1858, clear evidence of the existence of whales in the
upper greensand, some time before the close of the secondary period.

I may give another instance, which from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I have
stated that, from the number of existing and extinct tertiary species;
from the extraordinary abundance of the individuals of many species
all over the world, from the Arctic regions to the equator, inhabiting
various zones of depths from the upper tidal limits to 50 fathoms;
from the perfect manner in which specimens are preserved in the oldest
tertiary beds; from the ease with which even a fragment of a valve can
be recognised; from all these circumstances, I inferred that had sessile
cirripedes existed during the secondary periods, they would certainly
have been preserved and discovered; and as not one species had been
discovered in beds of this age, I concluded that this great group had
been suddenly developed at the commencement of the tertiary series. This
was a sore trouble to me, adding as I thought one more instance of the
abrupt appearance of a great group of species. But my work had hardly
been published, when a skilful palaeontologist, M. Bosquet, sent me a
drawing of a perfect specimen of an unmistakeable sessile cirripede,
which he had himself extracted from the chalk of Belgium. And, as if
to make the case as striking as possible, this sessile cirripede was a
Chthamalus, a very common, large, and ubiquitous genus, of which not one
specimen has as yet been found even in any tertiary stratum. Hence we
now positively know that sessile cirripedes existed during the secondary
period; and these cirripedes might have been the progenitors of our many
tertiary and existing species.

The case most frequently insisted on by palaeontologists of the
apparently sudden appearance of a whole group of species, is that of the
teleostean fishes, low down in the Chalk period. This group includes the
large majority of existing species. Lately, Professor Pictet has carried
their existence one sub-stage further back; and some palaeontologists
believe that certain much older fishes, of which the affinities are as
yet imperfectly known, are really teleostean. Assuming, however, that
the whole of them did appear, as Agassiz believes, at the commencement
of the chalk formation, the fact would certainly be highly remarkable;
but I cannot see that it would be an insuperable difficulty on my
theory, unless it could likewise be shown that the species of this group
appeared suddenly and simultaneously throughout the world at this same
period. It is almost superfluous to remark that hardly any fossil-fish
are known from south of the equator; and by running through Pictet's
Palaeontology it will be seen that very few species are known from
several formations in Europe. Some few families of fish now have a
confined range; the teleostean fish might formerly have had a similarly
confined range, and after having been largely developed in some one sea,
might have spread widely. Nor have we any right to suppose that the seas
of the world have always been so freely open from south to north as
they are at present. Even at this day, if the Malay Archipelago were
converted into land, the tropical parts of the Indian Ocean would form
a large and perfectly enclosed basin, in which any great group of marine
animals might be multiplied; and here they would remain confined, until
some of the species became adapted to a cooler climate, and were enabled
to double the southern capes of Africa or Australia, and thus reach
other and distant seas.

From these and similar considerations, but chiefly from our ignorance
of the geology of other countries beyond the confines of Europe and the
United States; and from the revolution in our palaeontological ideas
on many points, which the discoveries of even the last dozen years have
effected, it seems to me to be about as rash in us to dogmatize on the
succession of organic beings throughout the world, as it would be for
a naturalist to land for five minutes on some one barren point in
Australia, and then to discuss the number and range of its productions.

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.

There is another and allied difficulty, which is much graver. I allude
to the manner in which numbers of species of the same group, suddenly
appear in the lowest known fossiliferous rocks. Most of the arguments
which have convinced me that all the existing species of the same group
have descended from one progenitor, apply with nearly equal force to
the earliest known species. For instance, I cannot doubt that all the
Silurian trilobites have descended from some one crustacean, which must
have lived long before the Silurian age, and which probably differed
greatly from any known animal. Some of the most ancient Silurian
animals, as the Nautilus, Lingula, etc., do not differ much from living
species; and it cannot on my theory be supposed, that these old species
were the progenitors of all the species of the orders to which they
belong, for they do not present characters in any degree intermediate
between them. If, moreover, they had been the progenitors of these
orders, they would almost certainly have been long ago supplanted and
exterminated by their numerous and improved descendants.

Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as,
or probably far longer than, the whole interval from the Silurian age to
the present day; and that during these vast, yet quite unknown, periods
of time, the world swarmed with living creatures.

To the question why we do not find records of these vast primordial
periods, I can give no satisfactory answer. Several of the most eminent
geologists, with Sir R. Murchison at their head, are convinced that we
see in the organic remains of the lowest Silurian stratum the dawn of
life on this planet. Other highly competent judges, as Lyell and the
late E. Forbes, dispute this conclusion. We should not forget that only
a small portion of the world is known with accuracy. M. Barrande has
lately added another and lower stage to the Silurian system, abounding
with new and peculiar species. Traces of life have been detected in the
Longmynd beds beneath Barrande's so-called primordial zone. The presence
of phosphatic nodules and bituminous matter in some of the lowest azoic
rocks, probably indicates the former existence of life at these periods.
But the difficulty of understanding the absence of vast piles of
fossiliferous strata, which on my theory no doubt were somewhere
accumulated before the Silurian epoch, is very great. If these most
ancient beds had been wholly worn away by denudation, or obliterated
by metamorphic action, we ought to find only small remnants of the
formations next succeeding them in age, and these ought to be very
generally in a metamorphosed condition. But the descriptions which we
now possess of the Silurian deposits over immense territories in
Russia and in North America, do not support the view, that the older a
formation is, the more it has suffered the extremity of denudation and
metamorphism.

The case at present must remain inexplicable; and may be truly urged as
a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains, which do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that from
first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the existing continents of
Europe and North America. But we do not know what was the state of
things in the intervals between the successive formations; whether
Europe and the United States during these intervals existed as dry
land, or as a submarine surface near land, on which sediment was not
deposited, or again as the bed of an open and unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but not one oceanic island
is as yet known to afford even a remnant of any palaeozoic or secondary
formation. Hence we may perhaps infer, that during the palaeozoic and
secondary periods, neither continents nor continental islands existed
where our oceans now extend; for had they existed there, palaeozoic and
secondary formations would in all probability have been accumulated from
sediment derived from their wear and tear; and would have been at least
partially upheaved by the oscillations of level, which we may fairly
conclude must have intervened during these enormously long periods. If
then we may infer anything from these facts, we may infer that where
our oceans now extend, oceans have extended from the remotest period of
which we have any record; and on the other hand, that where continents
now exist, large tracts of land have existed, subjected no doubt to
great oscillations of level, since the earliest silurian period. The
coloured map appended to my volume on Coral Reefs, led me to conclude
that the great oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But have we any right to assume that things have
thus remained from eternity? Our continents seem to have been formed
by a preponderance, during many oscillations of level, of the force of
elevation; but may not the areas of preponderant movement have changed
in the lapse of ages? At a period immeasurably antecedent to the
silurian epoch, continents may have existed where oceans are now spread
out; and clear and open oceans may have existed where our continents now
stand. Nor should we be justified in assuming that if, for instance, the
bed of the Pacific Ocean were now converted into a continent, we should
there find formations older than the silurian strata, supposing such to
have been formerly deposited; for it might well happen that strata which
had subsided some miles nearer to the centre of the earth, and which
had been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have always
remained nearer to the surface. The immense areas in some parts of the
world, for instance in South America, of bare metamorphic rocks, which
must have been heated under great pressure, have always seemed to me to
require some special explanation; and we may perhaps believe that we see
in these large areas, the many formations long anterior to the silurian
epoch in a completely metamorphosed condition.

The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between the
many species which now exist or have existed; the sudden manner in which
whole groups of species appear in our European formations; the almost
entire absence, as at present known, of fossiliferous formations beneath
the Silurian strata, are all undoubtedly of the gravest nature. We
see this in the plainest manner by the fact that all the most eminent
palaeontologists, namely Cuvier, Owen, Agassiz, Barrande, Falconer,
E. Forbes, etc., and all our greatest geologists, as Lyell, Murchison,
Sedgwick, etc., have unanimously, often vehemently, maintained the
immutability of species. But I have reason to believe that one great
authority, Sir Charles Lyell, from further reflexion entertains grave
doubts on this subject. I feel how rash it is to differ from these great
authorities, to whom, with others, we owe all our knowledge. Those who
think the natural geological record in any degree perfect, and who do
not attach much weight to the facts and arguments of other kinds given
in this volume, will undoubtedly at once reject my theory. For my part,
following out Lyell's metaphor, I look at the natural geological record,
as a history of the world imperfectly kept, and written in a changing
dialect; of this history we possess the last volume alone, relating only
to two or three countries. Of this volume, only here and there a short
chapter has been preserved; and of each page, only here and there a few
lines. Each word of the slowly-changing language, in which the
history is supposed to be written, being more or less different in
the interrupted succession of chapters, may represent the apparently
abruptly changed forms of life, entombed in our consecutive, but widely
separated formations. On this view, the difficulties above discussed are
greatly diminished, or even disappear.




10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

On the slow and successive appearance of new species. On their different
rates of change. Species once lost do not reappear. Groups of species
follow the same general rules in their appearance and disappearance as
do single species. On Extinction. On simultaneous changes in the forms
of life throughout the world. On the affinities of extinct species to
each other and to living species. On the state of development of ancient
forms. On the succession of the same types within the same areas.
Summary of preceding and present chapters.

Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common
view of the immutability of species, or with that of their slow and
gradual modification, through descent and natural selection.

New species have appeared very slowly, one after another, both on the
land and in the waters. Lyell has shown that it is hardly possible to
resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between them, and to
make the percentage system of lost and new forms more gradual. In
some of the most recent beds, though undoubtedly of high antiquity if
measured by years, only one or two species are lost forms, and only one
or two are new forms, having here appeared for the first time, either
locally, or, as far as we know, on the face of the earth. If we may
trust the observations of Philippi in Sicily, the successive changes in
the marine inhabitants of that island have been many and most gradual.
The secondary formations are more broken; but, as Bronn has remarked,
neither the appearance nor disappearance of their many now extinct
species has been simultaneous in each separate formation.

Species of different genera and classes have not changed at the same
rate, or in the same degree. In the oldest tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, in an existing
crocodile associated with many strange and lost mammals and reptiles in
the sub-Himalayan deposits. The Silurian Lingula differs but little from
the living species of this genus; whereas most of the other Silurian
Molluscs and all the Crustaceans have changed greatly. The productions
of the land seem to change at a quicker rate than those of the sea, of
which a striking instance has lately been observed in Switzerland. There
is some reason to believe that organisms, considered high in the scale
of nature, change more quickly than those that are low: though there
are exceptions to this rule. The amount of organic change, as Pictet
has remarked, does not strictly correspond with the succession of our
geological formations; so that between each two consecutive formations,
the forms of life have seldom changed in exactly the same degree. Yet if
we compare any but the most closely related formations, all the species
will be found to have undergone some change. When a species has once
disappeared from the face of the earth, we have reason to believe
that the same identical form never reappears. The strongest apparent
exception to this latter rule, is that of the so-called "colonies" of M.
Barrande, which intrude for a period in the midst of an older formation,
and then allow the pre-existing fauna to reappear; but Lyell's
explanation, namely, that it is a case of temporary migration from a
distinct geographical province, seems to me satisfactory.

These several facts accord well with my theory. I believe in no fixed
law of development, causing all the inhabitants of a country to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be extremely slow. The variability of each species
is quite independent of that of all others. Whether such variability be
taken advantage of by natural selection, and whether the variations be
accumulated to a greater or lesser amount, thus causing a greater or
lesser amount of modification in the varying species, depends on many
complex contingencies,--on the variability being of a beneficial nature,
on the power of intercrossing, on the rate of breeding, on the slowly
changing physical conditions of the country, and more especially on the
nature of the other inhabitants with which the varying species comes
into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or,
if changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and
coleopterous insects of Madeira having come to differ considerably from
their nearest allies on the continent of Europe, whereas the marine
shells and birds have remained unaltered. We can perhaps understand
the apparently quicker rate of change in terrestrial and in more highly
organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When
many of the inhabitants of a country have become modified and improved,
we can understand, on the principle of competition, and on that of the
many all-important relations of organism to organism, that any form
which does not become in some degree modified and improved, will be
liable to be exterminated. Hence we can see why all the species in the
same region do at last, if we look to wide enough intervals of time,
become modified; for those which do not change will become extinct.

In members of the same class the average amount of change, during long
and equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding,
our formations have been almost necessarily accumulated at wide and
irregularly intermittent intervals; consequently the amount of organic
change exhibited by the fossils embedded in consecutive formations
is not equal. Each formation, on this view, does not mark a new and
complete act of creation, but only an occasional scene, taken almost at
hazard, in a slowly changing drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species might
be adapted (and no doubt this has occurred in innumerable instances) to
fill the exact place of another species in the economy of nature, and
thus supplant it; yet the two forms--the old and the new--would not be
identically the same; for both would almost certainly inherit different
characters from their distinct progenitors. For instance, it is just
possible, if our fantail-pigeons were all destroyed, that fanciers, by
striving during long ages for the same object, might make a new breed
hardly distinguishable from our present fantail; but if the parent
rock-pigeon were also destroyed, and in nature we have every reason
to believe that the parent-form will generally be supplanted and
exterminated by its improved offspring, it is quite incredible that a
fantail, identical with the existing breed, could be raised from any
other species of pigeon, or even from the other well-established races
of the domestic pigeon, for the newly-formed fantail would be almost
sure to inherit from its new progenitor some slight characteristic
differences.

Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species,
changing more or less quickly, and in a greater or lesser degree. A
group does not reappear after it has once disappeared; or its existence,
as long as it lasts, is continuous. I am aware that there are some
apparent exceptions to this rule, but the exceptions are surprisingly
few, so few, that E. Forbes, Pictet, and Woodward (though all strongly
opposed to such views as I maintain) admit its truth; and the rule
strictly accords with my theory. For as all the species of the same
group have descended from some one species, it is clear that as long as
any species of the group have appeared in the long succession of ages,
so long must its members have continuously existed, in order to have
generated either new and modified or the same old and unmodified forms.
Species of the genus Lingula, for instance, must have continuously
existed by an unbroken succession of generations, from the lowest
Silurian stratum to the present day.

We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give
an explanation of this fact, which if true would have been fatal to my
views. But such cases are certainly exceptional; the general rule being
a gradual increase in number, till the group reaches its maximum, and
then, sooner or later, it gradually decreases. If the number of
the species of a genus, or the number of the genera of a family, be
represented by a vertical line of varying thickness, crossing the
successive geological formations in which the species are found, the
line will sometimes falsely appear to begin at its lower end, not in a
sharp point, but abruptly; it then gradually thickens upwards, sometimes
keeping for a space of equal thickness, and ultimately thins out in the
upper beds, marking the decrease and final extinction of the species.
This gradual increase in number of the species of a group is strictly
conformable with my theory; as the species of the same genus, and the
genera of the same family, can increase only slowly and progressively;
for the process of modification and the production of a number of allied
forms must be slow and gradual,--one species giving rise first to two
or three varieties, these being slowly converted into species, which in
their turn produce by equally slow steps other species, and so on, like
the branching of a great tree from a single stem, till the group becomes
large.

ON EXTINCTION.

We have as yet spoken only incidentally of the disappearance of species
and of groups of species. On the theory of natural selection the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away at successive periods by catastrophes,
is very generally given up, even by those geologists, as Elie de
Beaumont, Murchison, Barrande, etc., whose general views would naturally
lead them to this conclusion. On the contrary, we have every reason to
believe, from the study of the tertiary formations, that species and
groups of species gradually disappear, one after another, first from one
spot, then from another, and finally from the world. Both single species
and whole groups of species last for very unequal periods; some groups,
as we have seen, having endured from the earliest known dawn of life
to the present day; some having disappeared before the close of the
palaeozoic period. No fixed law seems to determine the length of time
during which any single species or any single genus endures. There is
reason to believe that the complete extinction of the species of a group
is generally a slower process than their production: if the appearance
and disappearance of a group of species be represented, as before, by
a vertical line of varying thickness, the line is found to taper more
gradually at its upper end, which marks the progress of extermination,
than at its lower end, which marks the first appearance and increase
in numbers of the species. In some cases, however, the extermination
of whole groups of beings, as of ammonites towards the close of the
secondary period, has been wonderfully sudden.

The whole subject of the extinction of species has been involved in the
most gratuitous mystery. Some authors have even supposed that as the
individual has a definite length of life, so have species a definite
duration. No one I think can have marvelled more at the extinction of
species, than I have done. When I found in La Plata the tooth of a horse
embedded with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a
very late geological period, I was filled with astonishment; for seeing
that the horse, since its introduction by the Spaniards into South
America, has run wild over the whole country and has increased in
numbers at an unparalleled rate, I asked myself what could so recently
have exterminated the former horse under conditions of life apparently
so favourable. But how utterly groundless was my astonishment! Professor
Owen soon perceived that the tooth, though so like that of the existing
horse, belonged to an extinct species. Had this horse been still
living, but in some degree rare, no naturalist would have felt the least
surprise at its rarity; for rarity is the attribute of a vast number of
species of all classes, in all countries. If we ask ourselves why this
or that species is rare, we answer that something is unfavourable in its
conditions of life; but what that something is, we can hardly ever tell.
On the supposition of the fossil horse still existing as a rare species,
we might have felt certain from the analogy of all other mammals,
even of the slow-breeding elephant, and from the history of the
naturalisation of the domestic horse in South America, that under more
favourable conditions it would in a very few years have stocked the
whole continent. But we could not have told what the unfavourable
conditions were which checked its increase, whether some one or several
contingencies, and at what period of the horse's life, and in what
degree, they severally acted. If the conditions had gone on, however
slowly, becoming less and less favourable, we assuredly should not have
perceived the fact, yet the fossil horse would certainly have become
rarer and rarer, and finally extinct;--its place being seized on by some
more successful competitor.

It is most difficult always to remember that the increase of every
living being is constantly being checked by unperceived injurious
agencies; and that these same unperceived agencies are amply sufficient
to cause rarity, and finally extinction. We see in many cases in the
more recent tertiary formations, that rarity precedes extinction; and we
know that this has been the progress of events with those animals which
have been exterminated, either locally or wholly, through man's agency.
I may repeat what I published in 1845, namely, that to admit that
species generally become rare before they become extinct--to feel no
surprise at the rarity of a species, and yet to marvel greatly when
it ceases to exist, is much the same as to admit that sickness in the
individual is the forerunner of death--to feel no surprise at sickness,
but when the sick man dies, to wonder and to suspect that he died by
some unknown deed of violence.

The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition;
and the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the
less improved varieties in the same neighbourhood; when much improved it
is transported far and near, like our short-horn cattle, and takes the
place of other breeds in other countries. Thus the appearance of new
forms and the disappearance of old forms, both natural and artificial,
are bound together. In certain flourishing groups, the number of new
specific forms which have been produced within a given time is probably
greater than that of the old forms which have been exterminated; but we
know that the number of species has not gone on indefinitely increasing,
at least during the later geological periods, so that looking to later
times we may believe that the production of new forms has caused the
extinction of about the same number of old forms.

The competition will generally be most severe, as formerly explained
and illustrated by examples, between the forms which are most like each
other in all respects. Hence the improved and modified descendants of
a species will generally cause the extermination of the parent-species;
and if many new forms have been developed from any one species, the
nearest allies of that species, i.e. the species of the same genus, will
be the most liable to extermination. Thus, as I believe, a number of
new species descended from one species, that is a new genus, comes to
supplant an old genus, belonging to the same family. But it must often
have happened that a new species belonging to some one group will have
seized on the place occupied by a species belonging to a distinct group,
and thus caused its extermination; and if many allied forms be developed
from the successful intruder, many will have to yield their places; and
it will generally be allied forms, which will suffer from some inherited
inferiority in common. But whether it be species belonging to the same
or to a distinct class, which yield their places to other species which
have been modified and improved, a few of the sufferers may often long
be preserved, from being fitted to some peculiar line of life, or from
inhabiting some distant and isolated station, where they have escaped
severe competition. For instance, a single species of Trigonia, a great
genus of shells in the secondary formations, survives in the Australian
seas; and a few members of the great and almost extinct group of Ganoid
fishes still inhabit our fresh waters. Therefore the utter extinction
of a group is generally, as we have seen, a slower process than its
production.

With respect to the apparently sudden extermination of whole families
or orders, as of Trilobites at the close of the palaeozoic period and
of Ammonites at the close of the secondary period, we must remember what
has been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when by sudden immigration or by unusually
rapid development, many species of a new group have taken possession
of a new area, they will have exterminated in a correspondingly rapid
manner many of the old inhabitants; and the forms which thus yield
their places will commonly be allied, for they will partake of some
inferiority in common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct, accords well with the theory of
natural selection. We need not marvel at extinction; if we must
marvel, let it be at our presumption in imagining for a moment that we
understand the many complex contingencies, on which the existence of
each species depends. If we forget for an instant, that each species
tends to increase inordinately, and that some check is always in action,
yet seldom perceived by us, the whole economy of nature will be utterly
obscured. Whenever we can precisely say why this species is more
abundant in individuals than that; why this species and not another
can be naturalised in a given country; then, and not till then, we may
justly feel surprise why we cannot account for the extinction of this
particular species or group of species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE
WORLD.

Scarcely any palaeontological discovery is more striking than the fact,
that the forms of life change almost simultaneously throughout the
world. Thus our European Chalk formation can be recognised in many
distant parts of the world, under the most different climates, where not
a fragment of the mineral chalk itself can be found; namely, in North
America, in equatorial South America, in Tierra del Fuego, at the
Cape of Good Hope, and in the peninsula of India. For at these distant
points, the organic remains in certain beds present an unmistakeable
degree of resemblance to those of the Chalk. It is not that the same
species are met with; for in some cases not one species is identically
the same, but they belong to the same families, genera, and sections
of genera, and sometimes are similarly characterised in such trifling
points as mere superficial sculpture. Moreover other forms, which are
not found in the Chalk of Europe, but which occur in the formations
either above or below, are similarly absent at these distant points of
the world. In the several successive palaeozoic formations of Russia,
Western Europe and North America, a similar parallelism in the forms of
life has been observed by several authors: so it is, according to Lyell,
with the several European and North American tertiary deposits. Even
if the few fossil species which are common to the Old and New Worlds be
kept wholly out of view, the general parallelism in the successive forms
of life, in the stages of the widely separated palaeozoic and tertiary
periods, would still be manifest, and the several formations could be
easily correlated.

These observations, however, relate to the marine inhabitants of distant
parts of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water change at distant points in
the same parallel manner. We may doubt whether they have thus changed:
if the Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought
to Europe from La Plata, without any information in regard to their
geological position, no one would have suspected that they had coexisted
with still living sea-shells; but as these anomalous monsters coexisted
with the Mastodon and Horse, it might at least have been inferred that
they had lived during one of the latter tertiary stages.

When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year, or
even that it has a very strict geological sense; for if all the marine
animals which live at the present day in Europe, and all those that
lived in Europe during the pleistocene period (an enormously remote
period as measured by years, including the whole glacial epoch), were to
be compared with those now living in South America or in Australia, the
most skilful naturalist would hardly be able to say whether the existing
or the pleistocene inhabitants of Europe resembled most closely those of
the southern hemisphere. So, again, several highly competent observers
believe that the existing productions of the United States are more
closely related to those which lived in Europe during certain later
tertiary stages, than to those which now live here; and if this be so,
it is evident that fossiliferous beds deposited at the present day on
the shores of North America would hereafter be liable to be classed with
somewhat older European beds. Nevertheless, looking to a remotely future
epoch, there can, I think, be little doubt that all the more modern
MARINE formations, namely, the upper pliocene, the pleistocene and
strictly modern beds, of Europe, North and South America, and Australia,
from containing fossil remains in some degree allied, and from not
including those forms which are only found in the older underlying
deposits, would be correctly ranked as simultaneous in a geological
sense.

The fact of the forms of life changing simultaneously, in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM. de Verneuil and d'Archiac. After referring
to the parallelism of the palaeozoic forms of life in various parts
of Europe, they add, "If struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes more or less
local and temporary, but depend on general laws which govern the whole
animal kingdom." M. Barrande has made forcible remarks to precisely the
same effect. It is, indeed, quite futile to look to changes of currents,
climate, or other physical conditions, as the cause of these great
mutations in the forms of life throughout the world, under the most
different climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the present
distribution of organic beings, and find how slight is the relation
between the physical conditions of various countries, and the nature of
their inhabitants.

This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by new varieties arising, which have some
advantage over older forms; and those forms, which are already dominant,
or have some advantage over the other forms in their own country, would
naturally oftenest give rise to new varieties or incipient species; for
these latter must be victorious in a still higher degree in order to be
preserved and to survive. We have distinct evidence on this head, in
the plants which are dominant, that is, which are commonest in their own
homes, and are most widely diffused, having produced the greatest number
of new varieties. It is also natural that the dominant, varying, and
far-spreading species, which already have invaded to a certain extent
the territories of other species, should be those which would have
the best chance of spreading still further, and of giving rise in new
countries to new varieties and species. The process of diffusion
may often be very slow, being dependent on climatal and geographical
changes, or on strange accidents, but in the long run the dominant
forms will generally succeed in spreading. The diffusion would, it
is probable, be slower with the terrestrial inhabitants of distinct
continents than with the marine inhabitants of the continuous sea. We
might therefore expect to find, as we apparently do find, a less strict
degree of parallel succession in the productions of the land than of the
sea.

Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their
existence, would cease. We know not at all precisely what are all the
conditions most favourable for the multiplication of new and dominant
species; but we can, I think, clearly see that a number of individuals,
from giving a better chance of the appearance of favourable variations,
and that severe competition with many already existing forms, would
be highly favourable, as would be the power of spreading into new
territories. A certain amount of isolation, recurring at long intervals
of time, would probably be also favourable, as before explained. One
quarter of the world may have been most favourable for the production
of new and dominant species on the land, and another for those in the
waters of the sea. If two great regions had been for a long period
favourably circumstanced in an equal degree, whenever their inhabitants
met, the battle would be prolonged and severe; and some from one
birthplace and some from the other might be victorious. But in the
course of time, the forms dominant in the highest degree, wherever
produced, would tend everywhere to prevail. As they prevailed, they
would cause the extinction of other and inferior forms; and as these
inferior forms would be allied in groups by inheritance, whole groups
would tend slowly to disappear; though here and there a single member
might long be enabled to survive.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant owing to inheritance, and to having
already had some advantage over their parents or over other species;
these again spreading, varying, and producing new species. The forms
which are beaten and which yield their places to the new and victorious
forms, will generally be allied in groups, from inheriting some
inferiority in common; and therefore as new and improved groups spread
throughout the world, old groups will disappear from the world; and the
succession of forms in both ways will everywhere tend to correspond.

There is one other remark connected with this subject worth making. I
have given my reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of
the sea was either stationary or rising, and likewise when sediment was
not thrown down quickly enough to embed and preserve organic remains.
During these long and blank intervals I suppose that the inhabitants
of each region underwent a considerable amount of modification and
extinction, and that there was much migration from other parts of the
world. As we have reason to believe that large areas are affected by the
same movement, it is probable that strictly contemporaneous formations
have often been accumulated over very wide spaces in the same quarter
of the world; but we are far from having any right to conclude that this
has invariably been the case, and that large areas have invariably been
affected by the same movements. When two formations have been deposited
in two regions during nearly, but not exactly the same period, we should
find in both, from the causes explained in the foregoing paragraphs, the
same general succession in the forms of life; but the species would not
exactly correspond; for there will have been a little more time in
the one region than in the other for modification, extinction, and
immigration.

I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England
and France, is able to draw a close general parallelism between the
successive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult
to account for, considering the proximity of the two areas,--unless,
indeed, it be assumed that an isthmus separated two seas inhabited
by distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds
a surprising amount of difference in the species. If the several
formations in these regions have not been deposited during the same
exact periods,--a formation in one region often corresponding with a
blank interval in the other,--and if in both regions the species
have gone on slowly changing during the accumulation of the several
formations and during the long intervals of time between them; in this
case, the several formations in the two regions could be arranged in
the same order, in accordance with the general succession of the form
of life, and the order would falsely appear to be strictly parallel;
nevertheless the species would not all be the same in the apparently
corresponding stages in the two regions.

ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.

Let us now look to the mutual affinities of extinct and living species.
They all fall into one grand natural system; and this fact is at once
explained on the principle of descent. The more ancient any form is, the
more, as a general rule, it differs from living forms. But, as Buckland
long ago remarked, all fossils can be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the wide intervals between existing genera, families, and orders, cannot
be disputed. For if we confine our attention either to the living or
to the extinct alone, the series is far less perfect than if we combine
both into one general system. With respect to the Vertebrata, whole
pages could be filled with striking illustrations from our great
palaeontologist, Owen, showing how extinct animals fall in between
existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two
most distinct orders of mammals; but Owen has discovered so many fossil
links, that he has had to alter the whole classification of these two
orders; and has placed certain pachyderms in the same sub-order with
ruminants: for example, he dissolves by fine gradations the apparently
wide difference between the pig and the camel. In regard to the
Invertebrata, Barrande, and a higher authority could not be named,
asserts that he is every day taught that palaeozoic animals, though
belonging to the same orders, families, or genera with those living at
the present day, were not at this early epoch limited in such distinct
groups as they now are.

Some writers have objected to any extinct species or group of species
being considered as intermediate between living species or groups. If by
this term it is meant that an extinct form is directly intermediate in
all its characters between two living forms, the objection is probably
valid. But I apprehend that in a perfectly natural classification many
fossil species would have to stand between living species, and some
extinct genera between living genera, even between genera belonging to
distinct families. The most common case, especially with respect to very
distinct groups, such as fish and reptiles, seems to be, that supposing
them to be distinguished at the present day from each other by a
dozen characters, the ancient members of the same two groups would be
distinguished by a somewhat lesser number of characters, so that the two
groups, though formerly quite distinct, at that period made some small
approach to each other.

It is a common belief that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely
separated from each other. This remark no doubt must be restricted
to those groups which have undergone much change in the course of
geological ages; and it would be difficult to prove the truth of
the proposition, for every now and then even a living animal, as the
Lepidosiren, is discovered having affinities directed towards very
distinct groups. Yet if we compare the older Reptiles and Batrachians,
the older Fish, the older Cephalopods, and the eocene Mammals, with the
more recent members of the same classes, we must admit that there is
some truth in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex,
I must request the reader to turn to the diagram in the fourth chapter.
We may suppose that the numbered letters represent genera, and the
dotted lines diverging from them the species in each genus. The diagram
is much too simple, too few genera and too few species being given,
but this is unimportant for us. The horizontal lines may represent
successive geological formations, and all the forms beneath the
uppermost line may be considered as extinct. The three existing genera,
a14, q14, p14, will form a small family; b14 and f14 a closely allied
family or sub-family; and o14, e14, m14, a third family. These three
families, together with the many extinct genera on the several lines of
descent diverging from the parent-form A, will form an order; for all
will have inherited something in common from their ancient and common
progenitor. On the principle of the continued tendency to divergence
of character, which was formerly illustrated by this diagram, the more
recent any form is, the more it will generally differ from its ancient
progenitor. Hence we can understand the rule that the most ancient
fossils differ most from existing forms. We must not, however, assume
that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize
on many and different places in the economy of nature. Therefore it is
quite possible, as we have seen in the case of some Silurian forms,
that a species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.

All the many forms, extinct and recent, descended from A, make, as
before remarked, one order; and this order, from the continued effects
of extinction and divergence of character, has become divided into
several sub-families and families, some of which are supposed to have
perished at different periods, and some to have endured to the present
day.

By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered
at several points low down in the series, the three existing families on
the uppermost line would be rendered less distinct from each other. If,
for instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred,
these three families would be so closely linked together that they
probably would have to be united into one great family, in nearly the
same manner as has occurred with ruminants and pachyderms. Yet he who
objected to call the extinct genera, which thus linked the living
genera of three families together, intermediate in character, would be
justified, as they are intermediate, not directly, but only by a long
and circuitous course through many widely different forms. If many
extinct forms were to be discovered above one of the middle horizontal
lines or geological formations--for instance, above Number VI.--but
none from beneath this line, then only the two families on the left
hand (namely, a14, etc., and b14, etc.) would have to be united into
one family; and the two other families (namely, a14 to f14 now including
five genera, and o14 to m14) would yet remain distinct. These two
families, however, would be less distinct from each other than they were
before the discovery of the fossils. If, for instance, we suppose the
existing genera of the two families to differ from each other by a dozen
characters, in this case the genera, at the early period marked VI.,
would differ by a lesser number of characters; for at this early
stage of descent they have not diverged in character from the common
progenitor of the order, nearly so much as they subsequently diverged.
Thus it comes that ancient and extinct genera are often in some slight
degree intermediate in character between their modified descendants, or
between their collateral relations.

In nature the case will be far more complicated than is represented in
the diagram; for the groups will have been more numerous, they will
have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no right
to expect, except in very rare cases, to fill up wide intervals in the
natural system, and thus unite distinct families or orders. All that we
have a right to expect, is that those groups, which have within known
geological periods undergone much modification, should in the older
formations make some slight approach to each other; so that the older
members should differ less from each other in some of their characters
than do the existing members of the same groups; and this by the
concurrent evidence of our best palaeontologists seems frequently to be
the case.

Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, seem to me explained in a satisfactory
manner. And they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna of any great period
in the earth's history will be intermediate in general character between
that which preceded and that which succeeded it. Thus, the species
which lived at the sixth great stage of descent in the diagram are the
modified offspring of those which lived at the fifth stage, and are the
parents of those which became still more modified at the seventh stage;
hence they could hardly fail to be nearly intermediate in character
between the forms of life above and below. We must, however, allow for
the entire extinction of some preceding forms, and for the coming in of
quite new forms by immigration, and for a large amount of modification,
during the long and blank intervals between the successive formations.
Subject to these allowances, the fauna of each geological period
undoubtedly is intermediate in character, between the preceding and
succeeding faunas. I need give only one instance, namely, the manner
in which the fossils of the Devonian system, when this system was first
discovered, were at once recognised by palaeontologists as intermediate
in character between those of the overlying carboniferous, and
underlying Silurian system. But each fauna is not necessarily exactly
intermediate, as unequal intervals of time have elapsed between
consecutive formations.

It is no real objection to the truth of the statement, that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions
to the rule. For instance, mastodons and elephants, when arranged by Dr.
Falconer in two series, first according to their mutual affinities
and then according to their periods of existence, do not accord in
arrangement. The species extreme in character are not the oldest, or
the most recent; nor are those which are intermediate in character,
intermediate in age. But supposing for an instant, in this and other
such cases, that the record of the first appearance and disappearance
of the species was perfect, we have no reason to believe that forms
successively produced necessarily endure for corresponding lengths of
time: a very ancient form might occasionally last much longer than
a form elsewhere subsequently produced, especially in the case of
terrestrial productions inhabiting separated districts. To compare small
things with great: if the principal living and extinct races of the
domestic pigeon were arranged as well as they could be in serial
affinity, this arrangement would not closely accord with the order
in time of their production, and still less with the order of their
disappearance; for the parent rock-pigeon now lives; and many varieties
between the rock-pigeon and the carrier have become extinct; and
carriers which are extreme in the important character of length of beak
originated earlier than short-beaked tumblers, which are at the opposite
end of the series in this same respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character,
is the fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the
several stages of the chalk formation, though the species are distinct
in each stage. This fact alone, from its generality, seems to have
shaken Professor Pictet in his firm belief in the immutability of
species. He who is acquainted with the distribution of existing species
over the globe, will not attempt to account for the close resemblance of
the distinct species in closely consecutive formations, by the physical
conditions of the ancient areas having remained nearly the same. Let it
be remembered that the forms of life, at least those inhabiting the sea,
have changed almost simultaneously throughout the world, and therefore
under the most different climates and conditions. Consider the
prodigious vicissitudes of climate during the pleistocene period, which
includes the whole glacial period, and note how little the specific
forms of the inhabitants of the sea have been affected.

On the theory of descent, the full meaning of the fact of fossil remains
from closely consecutive formations, though ranked as distinct species,
being closely related, is obvious. As the accumulation of each formation
has often been interrupted, and as long blank intervals have intervened
between successive formations, we ought not to expect to find, as I
attempted to show in the last chapter, in any one or two formations all
the intermediate varieties between the species which appeared at the
commencement and close of these periods; but we ought to find after
intervals, very long as measured by years, but only moderately long
as measured geologically, closely allied forms, or, as they have been
called by some authors, representative species; and these we assuredly
do find. We find, in short, such evidence of the slow and scarcely
sensible mutation of specific forms, as we have a just right to expect
to find.

ON THE STATE OF DEVELOPMENT OF ANCIENT FORMS.

There has been much discussion whether recent forms are more highly
developed than ancient. I will not here enter on this subject, for
naturalists have not as yet defined to each other's satisfaction what is
meant by high and low forms. But in one particular sense the more recent
forms must, on my theory, be higher than the more ancient; for each new
species is formed by having had some advantage in the struggle for life
over other and preceding forms. If under a nearly similar climate, the
eocene inhabitants of one quarter of the world were put into competition
with the existing inhabitants of the same or some other quarter, the
eocene fauna or flora would certainly be beaten and exterminated;
as would a secondary fauna by an eocene, and a palaeozoic fauna by a
secondary fauna. I do not doubt that this process of improvement has
affected in a marked and sensible manner the organisation of the more
recent and victorious forms of life, in comparison with the ancient and
beaten forms; but I can see no way of testing this sort of progress.
Crustaceans, for instance, not the highest in their own class, may have
beaten the highest molluscs. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have
seized on places which must have been previously occupied, we may
believe, if all the animals and plants of Great Britain were set free
in New Zealand, that in the course of time a multitude of British forms
would become thoroughly naturalized there, and would exterminate many
of the natives. On the other hand, from what we see now occurring in New
Zealand, and from hardly a single inhabitant of the southern hemisphere
having become wild in any part of Europe, we may doubt, if all the
productions of New Zealand were set free in Great Britain, whether any
considerable number would be enabled to seize on places now occupied by
our native plants and animals. Under this point of view, the productions
of Great Britain may be said to be higher than those of New Zealand. Yet
the most skilful naturalist from an examination of the species of the
two countries could not have foreseen this result.

Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the
embryological development of recent forms. I must follow Pictet and
Huxley in thinking that the truth of this doctrine is very far from
proved. Yet I fully expect to see it hereafter confirmed, at least in
regard to subordinate groups, which have branched off from each other
within comparatively recent times. For this doctrine of Agassiz accords
well with the theory of natural selection. In a future chapter I
shall attempt to show that the adult differs from its embryo, owing
to variations supervening at a not early age, and being inherited at
a corresponding age. This process, whilst it leaves the embryo almost
unaltered, continually adds, in the course of successive generations,
more and more difference to the adult.

Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the ancient and less modified condition of each animal. This
view may be true, and yet it may never be capable of full proof. Seeing,
for instance, that the oldest known mammals, reptiles, and fish strictly
belong to their own proper classes, though some of these old forms are
in a slight degree less distinct from each other than are the typical
members of the same groups at the present day, it would be vain to look
for animals having the common embryological character of the Vertebrata,
until beds far beneath the lowest Silurian strata are discovered--a
discovery of which the chance is very small.

ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS.

Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even
to an uneducated eye, in the gigantic pieces of armour like those of the
armadillo, found in several parts of La Plata; and Professor Owen has
shown in the most striking manner that most of the fossil mammals,
buried there in such numbers, are related to South American types. This
relationship is even more clearly seen in the wonderful collection of
fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was
so much impressed with these facts that I strongly insisted, in 1839
and 1845, on this "law of the succession of types,"--on "this wonderful
relationship in the same continent between the dead and the living."
Professor Owen has subsequently extended the same generalisation to
the mammals of the Old World. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see
it also in the birds of the caves of Brazil. Mr. Woodward has shown that
the same law holds good with sea-shells, but from the wide distribution
of most genera of molluscs, it is not well displayed by them. Other
cases could be added, as the relation between the extinct and
living land-shells of Madeira; and between the extinct and living
brackish-water shells of the Aralo-Caspian Sea.

Now what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man, who after comparing
the present climate of Australia and of parts of South America under the
same latitude, would attempt to account, on the one hand, by dissimilar
physical conditions for the dissimilarity of the inhabitants of these
two continents, and, on the other hand, by similarity of conditions,
for the uniformity of the same types in each during the later tertiary
periods. Nor can it be pretended that it is an immutable law that
marsupials should have been chiefly or solely produced in Australia; or
that Edentata and other American types should have been solely produced
in South America. For we know that Europe in ancient times was peopled
by numerous marsupials; and I have shown in the publications above
alluded to, that in America the law of distribution of terrestrial
mammals was formerly different from what it now is. North America
formerly partook strongly of the present character of the southern
half of the continent; and the southern half was formerly more closely
allied, than it is at present, to the northern half. In a similar manner
we know from Falconer and Cautley's discoveries, that northern India was
formerly more closely related in its mammals to Africa than it is at
the present time. Analogous facts could be given in relation to the
distribution of marine animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the
same areas, is at once explained; for the inhabitants of each quarter of
the world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed
greatly from those of another continent, so will their modified
descendants still differ in nearly the same manner and degree. But
after very long intervals of time and after great geographical changes,
permitting much inter-migration, the feebler will yield to the more
dominant forms, and there will be nothing immutable in the laws of past
and present distribution.

It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the
sloth, armadillo, and anteater, as their degenerate descendants. This
cannot for an instant be admitted. These huge animals have become wholly
extinct, and have left no progeny. But in the caves of Brazil, there
are many extinct species which are closely allied in size and in other
characters to the species still living in South America; and some of
these fossils may be the actual progenitors of living species. It must
not be forgotten that, on my theory, all the species of the same genus
have descended from some one species; so that if six genera, each having
eight species, be found in one geological formation, and in the next
succeeding formation there be six other allied or representative genera
with the same number of species, then we may conclude that only one
species of each of the six older genera has left modified descendants,
constituting the six new genera. The other seven species of the old
genera have all died out and have left no progeny. Or, which would
probably be a far commoner case, two or three species of two or three
alone of the six older genera will have been the parents of the six new
genera; the other old species and the other whole genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers, as apparently is the case of the Edentata of
South America, still fewer genera and species will have left modified
blood-descendants.

SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.

I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geologically
explored with care; that only certain classes of organic beings have
been largely preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums, is absolutely as
nothing compared with the incalculable number of generations which
must have passed away even during a single formation; that, owing
to subsidence being necessary for the accumulation of fossiliferous
deposits thick enough to resist future degradation, enormous intervals
of time have elapsed between the successive formations; that there has
probably been more extinction during the periods of subsidence, and more
variation during the periods of elevation, and during the latter the
record will have been least perfectly kept; that each single formation
has not been continuously deposited; that the duration of each formation
is, perhaps, short compared with the average duration of specific forms;
that migration has played an important part in the first appearance of
new forms in any one area and formation; that widely ranging species
are those which have varied most, and have oftenest given rise to new
species; and that varieties have at first often been local. All these
causes taken conjointly, must have tended to make the geological record
extremely imperfect, and will to a large extent explain why we do not
find interminable varieties, connecting together all the extinct and
existing forms of life by the finest graduated steps.

He who rejects these views on the nature of the geological record, will
rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the several stages of
the same great formation. He may disbelieve in the enormous intervals
of time which have elapsed between our consecutive formations; he may
overlook how important a part migration must have played, when the
formations of any one great region alone, as that of Europe, are
considered; he may urge the apparent, but often falsely apparent, sudden
coming in of whole groups of species. He may ask where are the remains
of those infinitely numerous organisms which must have existed long
before the first bed of the Silurian system was deposited: I can answer
this latter question only hypothetically, by saying that as far as we
can see, where our oceans now extend they have for an enormous period
extended, and where our oscillating continents now stand they have stood
ever since the Silurian epoch; but that long before that period, the
world may have presented a wholly different aspect; and that the older
continents, formed of formations older than any known to us, may now all
be in a metamorphosed condition, or may lie buried under the ocean.

Passing from these difficulties, all the other great leading facts in
palaeontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it
is that new species come in slowly and successively; how species of
different classes do not necessarily change together, or at the same
rate, or in the same degree; yet in the long run that all undergo
modification to some extent. The extinction of old forms is the almost
inevitable consequence of the production of new forms. We can understand
why when a species has once disappeared it never reappears. Groups of
species increase in numbers slowly, and endure for unequal periods of
time; for the process of modification is necessarily slow, and depends
on many complex contingencies. The dominant species of the larger
dominant groups tend to leave many modified descendants, and thus new
sub-groups and groups are formed. As these are formed, the species of
the less vigorous groups, from their inferiority inherited from a common
progenitor, tend to become extinct together, and to leave no modified
offspring on the face of the earth. But the utter extinction of a whole
group of species may often be a very slow process, from the survival of
a few descendants, lingering in protected and isolated situations. When
a group has once wholly disappeared, it does not reappear; for the link
of generation has been broken.

We can understand how the spreading of the dominant forms of life, which
are those that oftenest vary, will in the long run tend to people the
world with allied, but modified, descendants; and these will generally
succeed in taking the places of those groups of species which are their
inferiors in the struggle for existence. Hence, after long intervals
of time, the productions of the world will appear to have changed
simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together one grand system; for all are connected by
generation. We can understand, from the continued tendency to divergence
of character, why the more ancient a form is, the more it generally
differs from those now living. Why ancient and extinct forms often tend
to fill up gaps between existing forms, sometimes blending two groups
previously classed as distinct into one; but more commonly only bringing
them a little closer together. The more ancient a form is, the more
often, apparently, it displays characters in some degree intermediate
between groups now distinct; for the more ancient a form is, the more
nearly it will be related to, and consequently resemble, the common
progenitor of groups, since become widely divergent. Extinct forms
are seldom directly intermediate between existing forms; but are
intermediate only by a long and circuitous course through many extinct
and very different forms. We can clearly see why the organic remains of
closely consecutive formations are more closely allied to each other,
than are those of remote formations; for the forms are more closely
linked together by generation: we can clearly see why the remains of an
intermediate formation are intermediate in character.

The inhabitants of each successive period in the world's history have
beaten their predecessors in the race for life, and are, in so far,
higher in the scale of nature; and this may account for that vague yet
ill-defined sentiment, felt by many palaeontologists, that organisation
on the whole has progressed. If it should hereafter be proved that
ancient animals resemble to a certain extent the embryos of more recent
animals of the same class, the fact will be intelligible. The succession
of the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is simply explained by
inheritance.

If then the geological record be as imperfect as I believe it to be, and
it may at least be asserted that the record cannot be proved to be much
more perfect, the main objections to the theory of natural selection are
greatly diminished or disappear. On the other hand, all the chief laws
of palaeontology plainly proclaim, as it seems to me, that species have
been produced by ordinary generation: old forms having been supplanted
by new and improved forms of life, produced by the laws of variation
still acting round us, and preserved by Natural Selection.




11. GEOGRAPHICAL DISTRIBUTION.

Present distribution cannot be accounted for by differences in physical
conditions. Importance of barriers. Affinity of the productions of the
same continent. Centres of creation. Means of dispersal, by changes of
climate and of the level of the land, and by occasional means. Dispersal
during the Glacial period co-extensive with the world.

In considering the distribution of organic beings over the face of
the globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be accounted for by their climatal and other physical conditions. Of
late, almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth: for if we exclude the northern parts where the circumpolar land
is almost continuous, all authors agree that one of the most fundamental
divisions in geographical distribution is that between the New and Old
Worlds; yet if we travel over the vast American continent, from the
central parts of the United States to its extreme southern point, we
meet with the most diversified conditions; the most humid districts,
arid deserts, lofty mountains, grassy plains, forests, marshes, lakes,
and great rivers, under almost every temperature. There is hardly a
climate or condition in the Old World which cannot be paralleled in the
New--at least as closely as the same species generally require; for it
is a most rare case to find a group of organisms confined to any small
spot, having conditions peculiar in only a slight degree; for instance,
small areas in the Old World could be pointed out hotter than any in
the New World, yet these are not inhabited by a peculiar fauna or flora.
Notwithstanding this parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25 deg and 35 deg, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the productions
of South America south of lat. 35 deg with those north of 25 deg, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to
the productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the
sea.

A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference of nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions.
We see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude: for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides
of lofty and continuous mountain-ranges, and of great deserts, and
sometimes even of large rivers, we find different productions; though as
mountain chains, deserts, etc., are not as impassable, or likely to have
endured so long as the oceans separating continents, the differences are
very inferior in degree to those characteristic of distinct continents.

Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of
the eastern and western shores of South and Central America; yet these
great faunas are separated only by the narrow, but impassable, isthmus
of Panama. Westward of the shores of America, a wide space of open ocean
extends, with not an island as a halting-place for emigrants; here we
have a barrier of another kind, and as soon as this is passed we meet
in the eastern islands of the Pacific, with another and totally
distinct fauna. So that here three marine faunas range far northward
and southward, in parallel lines not far from each other, under
corresponding climates; but from being separated from each other
by impassable barriers, either of land or open sea, they are wholly
distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places,
until after travelling over a hemisphere we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although hardly one shell, crab or fish is
common to the above-named three approximate faunas of Eastern and
Western America and the eastern Pacific islands, yet many fish range
from the Pacific into the Indian Ocean, and many shells are common to
the eastern islands of the Pacific and the eastern shores of Africa, on
almost exactly opposite meridians of longitude.

A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is
a law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance,
from north to south never fails to be struck by the manner in which
successive groups of beings, specifically distinct, yet clearly related,
replace each other. He hears from closely allied, yet distinct kinds of
birds, notes nearly similar, and sees their nests similarly constructed,
but not quite alike, with eggs coloured in nearly the same manner. The
plains near the Straits of Magellan are inhabited by one species of
Rhea (American ostrich), and northward the plains of La Plata by another
species of the same genus; and not by a true ostrich or emeu, like those
found in Africa and Australia under the same latitude. On these same
plains of La Plata, we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits and belonging to the
same order of Rodents, but they plainly display an American type of
structure. We ascend the lofty peaks of the Cordillera and we find an
alpine species of bizcacha; we look to the waters, and we do not find
the beaver or musk-rat, but the coypu and capybara, rodents of the
American type. Innumerable other instances could be given. If we look
to the islands off the American shore, however much they may differ in
geological structure, the inhabitants, though they may be all peculiar
species, are essentially American. We may look back to past ages, as
shown in the last chapter, and we find American types then prevalent on
the American continent and in the American seas. We see in these facts
some deep organic bond, prevailing throughout space and time, over
the same areas of land and water, and independent of their physical
conditions. The naturalist must feel little curiosity, who is not led to
inquire what this bond is.

This bond, on my theory, is simply inheritance, that cause which alone,
as far as we positively know, produces organisms quite like, or, as we
see in the case of varieties nearly like each other. The dissimilarity
of the inhabitants of different regions may be attributed to
modification through natural selection, and in a quite subordinate
degree to the direct influence of different physical conditions.
The degree of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been effected
with more or less ease, at periods more or less remote;--on the nature
and number of the former immigrants;--and on their action and reaction,
in their mutual struggles for life;--the relation of organism to
organism being, as I have already often remarked, the most important of
all relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in their
own widely-extended homes will have the best chance of seizing on new
places, when they spread into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification, we can understand how it is
that sections of genera, whole genera, and even families are confined to
the same areas, as is so commonly and notoriously the case.

I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent
property, and will be taken advantage of by natural selection, only so
far as it profits the individual in its complex struggle for life,
so the degree of modification in different species will be no uniform
quantity. If, for instance, a number of species, which stand in direct
competition with each other, migrate in a body into a new and afterwards
isolated country, they will be little liable to modification; for
neither migration nor isolation in themselves can do anything. These
principles come into play only by bringing organisms into new relations
with each other, and in a lesser degree with the surrounding physical
conditions. As we have seen in the last chapter that some forms have
retained nearly the same character from an enormously remote geological
period, so certain species have migrated over vast spaces, and have not
become greatly modified.

On these views, it is obvious, that the several species of the same
genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they have descended
from the same progenitor. In the case of those species, which have
undergone during whole geological periods but little modification, there
is not much difficulty in believing that they may have migrated from the
same region; for during the vast geographical and climatal changes which
will have supervened since ancient times, almost any amount of migration
is possible. But in many other cases, in which we have reason to believe
that the species of a genus have been produced within comparatively
recent times, there is great difficulty on this head. It is also obvious
that the individuals of the same species, though now inhabiting distant
and isolated regions, must have proceeded from one spot, where their
parents were first produced: for, as explained in the last chapter, it
is incredible that individuals identically the same should ever have
been produced through natural selection from parents specifically
distinct.

We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are very many cases of
extreme difficulty, in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that
each species was first produced within a single region captivates the
mind. He who rejects it, rejects the vera causa of ordinary generation
with subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a species
is continuous; and when a plant or animal inhabits two points so distant
from each other, or with an interval of such a nature, that the space
could not be easily passed over by migration, the fact is given as
something remarkable and exceptional. The capacity of migrating across
the sea is more distinctly limited in terrestrial mammals, than perhaps
in any other organic beings; and, accordingly, we find no inexplicable
cases of the same mammal inhabiting distant points of the world. No
geologist will feel any difficulty in such cases as Great Britain having
been formerly united to Europe, and consequently possessing the same
quadrupeds. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the
same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal plants
are identically the same at these distant points of the northern and
southern hemispheres? The answer, as I believe, is, that mammals have
not been able to migrate, whereas some plants, from their varied means
of dispersal, have migrated across the vast and broken interspace. The
great and striking influence which barriers of every kind have had on
distribution, is intelligible only on the view that the great majority
of species have been produced on one side alone, and have not been able
to migrate to the other side. Some few families, many sub-families,
very many genera, and a still greater number of sections of genera
are confined to a single region; and it has been observed by several
naturalists, that the most natural genera, or those genera in which the
species are most closely related to each other, are generally local,
or confined to one area. What a strange anomaly it would be, if, when
coming one step lower in the series, to the individuals of the same
species, a directly opposite rule prevailed; and species were not local,
but had been produced in two or more distinct areas!

Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration
and subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur, in which we cannot explain how
the same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the formerly continuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous
and of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many such cases.
But after some preliminary remarks, I will discuss a few of the most
striking classes of facts; namely, the existence of the same species
on the summits of distant mountain-ranges, and at distant points in the
arctic and antarctic regions; and secondly (in the following chapter),
the wide distribution of freshwater productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the
earth's surface, can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatal and geographical changes and
various occasional means of transport, the belief that this has been the
universal law, seems to me incomparably the safest.

In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from
a common progenitor, can have migrated (undergoing modification
during some part of their migration) from the area inhabited by their
progenitor. If it can be shown to be almost invariably the case, that
a region, of which most of its inhabitants are closely related to,
or belong to the same genera with the species of a second region, has
probably received at some former period immigrants from this other
region, my theory will be strengthened; for we can clearly understand,
on the principle of modification, why the inhabitants of a region should
be related to those of another region, whence it has been stocked. A
volcanic island, for instance, upheaved and formed at the distance of a
few hundreds of miles from a continent, would probably receive from it
in the course of time a few colonists, and their descendants, though
modified, would still be plainly related by inheritance to the
inhabitants of the continent. Cases of this nature are common, and are,
as we shall hereafter more fully see, inexplicable on the theory of
independent creation. This view of the relation of species in one region
to those in another, does not differ much (by substituting the word
variety for species) from that lately advanced in an ingenious paper by
Mr. Wallace, in which he concludes, that "every species has come into
existence coincident both in space and time with a pre-existing
closely allied species." And I now know from correspondence, that this
coincidence he attributes to generation with modification.

The previous remarks on "single and multiple centres of creation" do
not directly bear on another allied question,--namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which
never intercross (if such exist), the species, on my theory, must have
descended from a succession of improved varieties, which will never have
blended with other individuals or varieties, but will have supplanted
each other; so that, at each successive stage of modification and
improvement, all the individuals of each variety will have descended
from a single parent. But in the majority of cases, namely, with
all organisms which habitually unite for each birth, or which often
intercross, I believe that during the slow process of modification
the individuals of the species will have been kept nearly uniform by
intercrossing; so that many individuals will have gone on simultaneously
changing, and the whole amount of modification will not have been due,
at each stage, to descent from a single parent. To illustrate what I
mean: our English racehorses differ slightly from the horses of every
other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in selecting and
training many individuals during many generations.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.

MEANS OF DISPERSAL.

Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts. Change
of climate must have had a powerful influence on migration: a region
when its climate was different may have been a high road for migration,
but now be impassable; I shall, however, presently have to discuss this
branch of the subject in some detail. Changes of level in the land must
also have been highly influential: a narrow isthmus now separates two
marine faunas; submerge it, or let it formerly have been submerged, and
the two faunas will now blend or may formerly have blended: where the
sea now extends, land may at a former period have connected islands or
possibly even continents together, and thus have allowed terrestrial
productions to pass from one to the other. No geologist will dispute
that great mutations of level have occurred within the period of
existing organisms. Edward Forbes insisted that all the islands in the
Atlantic must recently have been connected with Europe or Africa, and
Europe likewise with America. Other authors have thus hypothetically
bridged over every ocean, and have united almost every island to some
mainland. If indeed the arguments used by Forbes are to be trusted,
it must be admitted that scarcely a single island exists which has not
recently been united to some continent. This view cuts the Gordian knot
of the dispersal of the same species to the most distant points, and
removes many a difficulty: but to the best of my judgment we are not
authorized in admitting such enormous geographical changes within
the period of existing species. It seems to me that we have abundant
evidence of great oscillations of level in our continents; but not of
such vast changes in their position and extension, as to have united
them within the recent period to each other and to the several
intervening oceanic islands. I freely admit the former existence of many
islands, now buried beneath the sea, which may have served as halting
places for plants and for many animals during their migration. In the
coral-producing oceans such sunken islands are now marked, as I believe,
by rings of coral or atolls standing over them. Whenever it is fully
admitted, as I believe it will some day be, that each species has
proceeded from a single birthplace, and when in the course of time we
know something definite about the means of distribution, we shall be
enabled to speculate with security on the former extension of the land.
But I do not believe that it will ever be proved that within the recent
period continents which are now quite separate, have been continuously,
or almost continuously, united with each other, and with the many
existing oceanic islands. Several facts in distribution,--such as the
great difference in the marine faunas on the opposite sides of almost
every continent,--the close relation of the tertiary inhabitants of
several lands and even seas to their present inhabitants,--a certain
degree of relation (as we shall hereafter see) between the distribution
of mammals and the depth of the sea,--these and other such facts seem to
me opposed to the admission of such prodigious geographical revolutions
within the recent period, as are necessitated on the view advanced
by Forbes and admitted by his many followers. The nature and relative
proportions of the inhabitants of oceanic islands likewise seem to me
opposed to the belief of their former continuity with continents. Nor
does their almost universally volcanic composition favour the admission
that they are the wrecks of sunken continents;--if they had originally
existed as mountain-ranges on the land, some at least of the islands
would have been formed, like other mountain-summits, of granite,
metamorphic schists, old fossiliferous or other such rocks, instead of
consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but
which more properly might be called occasional means of distribution.
I shall here confine myself to plants. In botanical works, this or
that plant is stated to be ill adapted for wide dissemination; but for
transport across the sea, the greater or less facilities may be said to
be almost wholly unknown. Until I tried, with Mr. Berkeley's aid, a
few experiments, it was not even known how far seeds could resist the
injurious action of sea-water. To my surprise I found that out of 87
kinds, 64 germinated after an immersion of 28 days, and a few survived
an immersion of 137 days. For convenience sake I chiefly tried small
seeds, without the capsule or fruit; and as all of these sank in a few
days, they could not be floated across wide spaces of the sea, whether
or not they were injured by the salt-water. Afterwards I tried some
larger fruits, capsules, etc., and some of these floated for a long
time. It is well known what a difference there is in the buoyancy of
green and seasoned timber; and it occurred to me that floods might wash
down plants or branches, and that these might be dried on the banks, and
then by a fresh rise in the stream be washed into the sea. Hence I was
led to dry stems and branches of 94 plants with ripe fruit, and to place
them on sea water. The majority sank quickly, but some which whilst
green floated for a very short time, when dried floated much longer; for
instance, ripe hazel-nuts sank immediately, but when dried, they floated
for 90 days and afterwards when planted they germinated; an asparagus
plant with ripe berries floated for 23 days, when dried it floated
for 85 days, and the seeds afterwards germinated: the ripe seeds of
Helosciadium sank in two days, when dried they floated for above 90
days, and afterwards germinated. Altogether out of the 94 dried plants,
18 floated for above 28 days, and some of the 18 floated for a very much
longer period. So that as 64/87 seeds germinated after an immersion
of 28 days; and as 18/94 plants with ripe fruit (but not all the same
species as in the foregoing experiment) floated, after being dried, for
above 28 days, as far as we may infer anything from these scanty facts,
we may conclude that the seeds of 14/100 plants of any country might be
floated by sea-currents during 28 days, and would retain their power
of germination. In Johnston's Physical Atlas, the average rate of the
several Atlantic currents is 33 miles per diem (some currents running
at the rate of 60 miles per diem); on this average, the seeds of 14/100
plants belonging to one country might be floated across 924 miles of sea
to another country; and when stranded, if blown to a favourable spot by
an inland gale, they would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea,
so that they were alternately wet and exposed to the air like really
floating plants. He tried 98 seeds, mostly different from mine; but he
chose many large fruits and likewise seeds from plants which live
near the sea; and this would have favoured the average length of
their flotation and of their resistance to the injurious action of the
salt-water. On the other hand he did not previously dry the plants or
branches with the fruit; and this, as we have seen, would have caused
some of them to have floated much longer. The result was that 18/98 of
his seeds floated for 42 days, and were then capable of germination. But
I do not doubt that plants exposed to the waves would float for a less
time than those protected from violent movement as in our experiments.
Therefore it would perhaps be safer to assume that the seeds of about
10/100 plants of a flora, after having been dried, could be floated
across a space of sea 900 miles in width, and would then germinate.
The fact of the larger fruits often floating longer than the small,
is interesting; as plants with large seeds or fruit could hardly be
transported by any other means; and Alph. de Candolle has shown that
such plants generally have restricted ranges.

But seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of the
widest oceans; and the natives of the coral-islands in the Pacific,
procure stones for their tools, solely from the roots of drifted trees,
these stones being a valuable royal tax. I find on examination, that
when irregularly shaped stones are embedded in the roots of trees, small
parcels of earth are very frequently enclosed in their interstices and
behind them,--so perfectly that not a particle could be washed away in
the longest transport: out of one small portion of earth thus COMPLETELY
enclosed by wood in an oak about 50 years old, three dicotyledonous
plants germinated: I am certain of the accuracy of this observation.
Again, I can show that the carcasses of birds, when floating on the sea,
sometimes escape being immediately devoured; and seeds of many kinds
in the crops of floating birds long retain their vitality: peas and
vetches, for instance, are killed by even a few days' immersion in
sea-water; but some taken out of the crop of a pigeon, which had
floated on artificial salt-water for 30 days, to my surprise nearly all
germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may I think safely assume that under such circumstances their
rate of flight would often be 35 miles an hour; and some authors have
given a far higher estimate. I have never seen an instance of nutritious
seeds passing through the intestines of a bird; but hard seeds of fruit
will pass uninjured through even the digestive organs of a turkey. In
the course of two months, I picked up in my garden 12 kinds of seeds,
out of the excrement of small birds, and these seemed perfect, and
some of them, which I tried, germinated. But the following fact is more
important: the crops of birds do not secrete gastric juice, and do not
in the least injure, as I know by trial, the germination of seeds;
now after a bird has found and devoured a large supply of food, it is
positively asserted that all the grains do not pass into the gizzard for
12 or even 18 hours. A bird in this interval might easily be blown to
the distance of 500 miles, and hawks are known to look out for tired
birds, and the contents of their torn crops might thus readily get
scattered. Mr. Brent informs me that a friend of his had to give up
flying carrier-pigeons from France to England, as the hawks on the
English coast destroyed so many on their arrival. Some hawks and owls
bolt their prey whole, and after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoological Gardens, include seeds capable of germination. Some seeds of
the oat, wheat, millet, canary, hemp, clover, and beet germinated after
having been from twelve to twenty-one hours in the stomachs of different
birds of prey; and two seeds of beet grew after having been thus
retained for two days and fourteen hours. Freshwater fish, I find, eat
seeds of many land and water plants: fish are frequently devoured by
birds, and thus the seeds might be transported from place to place. I
forced many kinds of seeds into the stomachs of dead fish, and then gave
their bodies to fishing-eagles, storks, and pelicans; these birds after
an interval of many hours, either rejected the seeds in pellets or
passed them in their excrement; and several of these seeds retained
their power of germination. Certain seeds, however, were always killed
by this process.

Although the beaks and feet of birds are generally quite clean, I can
show that earth sometimes adheres to them: in one instance I removed
twenty-two grains of dry argillaceous earth from one foot of a
partridge, and in this earth there was a pebble quite as large as the
seed of a vetch. Thus seeds might occasionally be transported to great
distances; for many facts could be given showing that soil almost
everywhere is charged with seeds. Reflect for a moment on the millions
of quails which annually cross the Mediterranean; and can we doubt that
the earth adhering to their feet would sometimes include a few minute
seeds? But I shall presently have to recur to this subject.

As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, I can
hardly doubt that they must occasionally have transported seeds from
one part to another of the arctic and antarctic regions, as suggested by
Lyell; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of the species
of plants common to Europe, in comparison with the plants of other
oceanic islands nearer to the mainland, and (as remarked by Mr. H. C.
Watson) from the somewhat northern character of the flora in comparison
with the latitude, I suspected that these islands had been partly
stocked by ice-borne seeds, during the Glacial epoch. At my request Sir
C. Lyell wrote to M. Hartung to inquire whether he had observed erratic
boulders on these islands, and he answered that he had found large
fragments of granite and other rocks, which do not occur in the
archipelago. Hence we may safely infer that icebergs formerly landed
their rocky burthens on the shores of these mid-ocean islands, and it
is at least possible that they may have brought thither the seeds of
northern plants.

Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in
action year after year, for centuries and tens of thousands of years,
it would I think be a marvellous fact if many plants had not thus
become widely transported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents of the sea
are not accidental, nor is the direction of prevalent gales of wind.
It should be observed that scarcely any means of transport would carry
seeds for very great distances; for seeds do not retain their vitality
when exposed for a great length of time to the action of seawater; nor
could they be long carried in the crops or intestines of birds. These
means, however, would suffice for occasional transport across tracts of
sea some hundred miles in breadth, or from island to island, or from a
continent to a neighbouring island, but not from one distant continent
to another. The floras of distant continents would not by such means
become mingled in any great degree; but would remain as distinct as we
now see them to be. The currents, from their course, would never bring
seeds from North America to Britain, though they might and do bring
seeds from the West Indies to our western shores, where, if not killed
by so long an immersion in salt-water, they could not endure our
climate. Almost every year, one or two land-birds are blown across
the whole Atlantic Ocean, from North America to the western shores of
Ireland and England; but seeds could be transported by these wanderers
only by one means, namely, in dirt sticking to their feet, which is in
itself a rare accident. Even in this case, how small would the chance
be of a seed falling on favourable soil, and coming to maturity! But it
would be a great error to argue that because a well-stocked island,
like Great Britain, has not, as far as is known (and it would be very
difficult to prove this), received within the last few centuries,
through occasional means of transport, immigrants from Europe or any
other continent, that a poorly-stocked island, though standing more
remote from the mainland, would not receive colonists by similar means.
I do not doubt that out of twenty seeds or animals transported to an
island, even if far less well-stocked than Britain, scarcely more than
one would be so well fitted to its new home, as to become naturalised.
But this, as it seems to me, is no valid argument against what would
be effected by occasional means of transport, during the long lapse of
geological time, whilst an island was being upheaved and formed, and
before it had become fully stocked with inhabitants. On almost bare
land, with few or no destructive insects or birds living there, nearly
every seed, which chanced to arrive, would be sure to germinate and
survive.

DISPERSAL DURING THE GLACIAL PERIOD.

The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where the Alpine
species could not possibly exist, is one of the most striking cases
known of the same species living at distant points, without the apparent
possibility of their having migrated from one to the other. It is indeed
a remarkable fact to see so many of the same plants living on the snowy
regions of the Alps or Pyrenees, and in the extreme northern parts of
Europe; but it is far more remarkable, that the plants on the White
Mountains, in the United States of America, are all the same with those
of Labrador, and nearly all the same, as we hear from Asa Gray, with
those on the loftiest mountains of Europe. Even as long ago as 1747,
such facts led Gmelin to conclude that the same species must have been
independently created at several distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which, as we shall immediately see,
affords a simple explanation of these facts. We have evidence of almost
every conceivable kind, organic and inorganic, that within a very recent
geological period, central Europe and North America suffered under an
Arctic climate. The ruins of a house burnt by fire do not tell their
tale more plainly, than do the mountains of Scotland and Wales, with
their scored flanks, polished surfaces, and perched boulders, of the icy
streams with which their valleys were lately filled. So greatly has the
climate of Europe changed, that in Northern Italy, gigantic moraines,
left by old glaciers, are now clothed by the vine and maize. Throughout
a large part of the United States, erratic boulders, and rocks scored by
drifted icebergs and coast-ice, plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes
more readily, by supposing a new glacial period to come slowly on, and
then pass away, as formerly occurred. As the cold came on, and as each
more southern zone became fitted for arctic beings and ill-fitted for
their former more temperate inhabitants, the latter would be supplanted
and arctic productions would take their places. The inhabitants of the
more temperate regions would at the same time travel southward, unless
they were stopped by barriers, in which case they would perish. The
mountains would become covered with snow and ice, and their former
Alpine inhabitants would descend to the plains. By the time that the
cold had reached its maximum, we should have a uniform arctic fauna and
flora, covering the central parts of Europe, as far south as the Alps
and Pyrenees, and even stretching into Spain. The now temperate regions
of the United States would likewise be covered by arctic plants and
animals, and these would be nearly the same with those of Europe; for
the present circumpolar inhabitants, which we suppose to have everywhere
travelled southward, are remarkably uniform round the world. We may
suppose that the Glacial period came on a little earlier or later in
North America than in Europe, so will the southern migration there have
been a little earlier or later; but this will make no difference in the
final result.

As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending higher and higher, as the warmth increased,
whilst their brethren were pursuing their northern journey. Hence, when
the warmth had fully returned, the same arctic species, which had lately
lived in a body together on the lowlands of the Old and New Worlds,
would be left isolated on distant mountain-summits (having been
exterminated on all lesser heights) and in the arctic regions of both
hemispheres.

Thus we can understand the identity of many plants at points so
immensely remote as on the mountains of the United States and of Europe.
We can thus also understand the fact that the Alpine plants of each
mountain-range are more especially related to the arctic forms living
due north or nearly due north of them: for the migration as the cold
came on, and the re-migration on the returning warmth, will generally
have been due south and north. The Alpine plants, for example, of
Scotland, as remarked by Mr. H. C. Watson, and those of the Pyrenees, as
remarked by Ramond, are more especially allied to the plants of northern
Scandinavia; those of the United States to Labrador; those of the
mountains of Siberia to the arctic regions of that country. These views,
grounded as they are on the perfectly well-ascertained occurrence of a
former Glacial period, seem to me to explain in so satisfactory a manner
the present distribution of the Alpine and Arctic productions of Europe
and America, that when in other regions we find the same species on
distant mountain-summits, we may almost conclude without other evidence,
that a colder climate permitted their former migration across the low
intervening tracts, since become too warm for their existence.

If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe
to have been the case, chiefly from the distribution of the fossil
Gnathodon), then the arctic and temperate productions will at a very
late period have marched a little further north, and subsequently have
retreated to their present homes; but I have met with no satisfactory
evidence with respect to this intercalated slightly warmer period, since
the Glacial period.

The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as
is especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed,
and, in accordance with the principles inculcated in this volume, they
will not have been liable to much modification. But with our Alpine
productions, left isolated from the moment of the returning warmth,
first at the bases and ultimately on the summits of the mountains, the
case will have been somewhat different; for it is not likely that all
the same arctic species will have been left on mountain ranges distant
from each other, and have survived there ever since; they will, also, in
all probability have become mingled with ancient Alpine species, which
must have existed on the mountains before the commencement of the
Glacial epoch, and which during its coldest period will have been
temporarily driven down to the plains; they will, also, have been
exposed to somewhat different climatal influences. Their mutual
relations will thus have been in some degree disturbed; consequently
they will have been liable to modification; and this we find has been
the case; for if we compare the present Alpine plants and animals of the
several great European mountain-ranges, though very many of the species
are identically the same, some present varieties, some are ranked
as doubtful forms, and some few are distinct yet closely allied or
representative species.

In illustrating what, as I believe, actually took place during
the Glacial period, I assumed that at its commencement the arctic
productions were as uniform round the polar regions as they are at the
present day. But the foregoing remarks on distribution apply not only
to strictly arctic forms, but also to many sub-arctic and to some few
northern temperate forms, for some of these are the same on the lower
mountains and on the plains of North America and Europe; and it may be
reasonably asked how I account for the necessary degree of uniformity
of the sub-arctic and northern temperate forms round the world, at the
commencement of the Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds are
separated from each other by the Atlantic Ocean and by the extreme
northern part of the Pacific. During the Glacial period, when the
inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider
spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature.
We have good reason to believe that during the newer Pliocene period,
before the Glacial epoch, and whilst the majority of the inhabitants of
the world were specifically the same as now, the climate was warmer than
at the present day. Hence we may suppose that the organisms now living
under the climate of latitude 60 deg, during the Pliocene period lived
further north under the Polar Circle, in latitude 66 deg-67 deg; and
that the strictly arctic productions then lived on the broken land still
nearer to the pole. Now if we look at a globe, we shall see that under
the Polar Circle there is almost continuous land from western Europe,
through Siberia, to eastern America. And to this continuity of the
circumpolar land, and to the consequent freedom for intermigration
under a more favourable climate, I attribute the necessary amount of
uniformity in the sub-arctic and northern temperate productions of the
Old and New Worlds, at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have
long remained in nearly the same relative position, though subjected
to large, but partial oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some earlier and still
warmer period, such as the older Pliocene period, a large number of
the same plants and animals inhabited the almost continuous circumpolar
land; and that these plants and animals, both in the Old and New Worlds,
began slowly to migrate southwards as the climate became less warm, long
before the commencement of the Glacial period. We now see, as I believe,
their descendants, mostly in a modified condition, in the central parts
of Europe and the United States. On this view we can understand the
relationship, with very little identity, between the productions of
North America and Europe,--a relationship which is most remarkable,
considering the distance of the two areas, and their separation by the
Atlantic Ocean. We can further understand the singular fact remarked on
by several observers, that the productions of Europe and America during
the later tertiary stages were more closely related to each other
than they are at the present time; for during these warmer periods
the northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered
impassable by cold, for the inter-migration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as
the species in common, which inhabited the New and Old Worlds, migrated
south of the Polar Circle, they must have been completely cut off from
each other. This separation, as far as the more temperate productions
are concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great
region with the native American productions, and have had to compete
with them; and in the other great region, with those of the Old
World. Consequently we have here everything favourable for much
modification,--for far more modification than with the Alpine
productions, left isolated, within a much more recent period, on the
several mountain-ranges and on the arctic lands of the two Worlds. Hence
it has come, that when we compare the now living productions of the
temperate regions of the New and Old Worlds, we find very few identical
species (though Asa Gray has lately shown that more plants are identical
than was formerly supposed), but we find in every great class many
forms, which some naturalists rank as geographical races, and others as
distinct species; and a host of closely allied or representative forms
which are ranked by all naturalists as specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in areas completely sundered. Thus, I think, we
can understand the presence of many existing and tertiary representative
forms on the eastern and western shores of temperate North America;
and the still more striking case of many closely allied crustaceans
(as described in Dana's admirable work), of some fish and other marine
animals, in the Mediterranean and in the seas of Japan,--areas now
separated by a continent and by nearly a hemisphere of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of
seas now disjoined, and likewise of the past and present inhabitants of
the temperate lands of North America and Europe, are inexplicable on the
theory of creation. We cannot say that they have been created alike, in
correspondence with the nearly similar physical conditions of the areas;
for if we compare, for instance, certain parts of South America with
the southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their
inhabitants utterly dissimilar.

But we must return to our more immediate subject, the Glacial period.
I am convinced that Forbes's view may be largely extended. In Europe we
have the plainest evidence of the cold period, from the western shores
of Britain to the Oural range, and southward to the Pyrenees. We may
infer, from the frozen mammals and nature of the mountain vegetation,
that Siberia was similarly affected. Along the Himalaya, at points 900
miles apart, glaciers have left the marks of their former low descent;
and in Sikkim, Dr. Hooker saw maize growing on gigantic ancient
moraines. South of the equator, we have some direct evidence of former
glacial action in New Zealand; and the same plants, found on widely
separated mountains in this island, tell the same story. If one account
which has been published can be trusted, we have direct evidence of
glacial action in the south-eastern corner of Australia.

Looking to America; in the northern half, ice-borne fragments of rock
have been observed on the eastern side as far south as lat. 36 deg-37
deg, and on the shores of the Pacific, where the climate is now so
different, as far south as lat. 46 deg; erratic boulders have, also,
been noticed on the Rocky Mountains. In the Cordillera of Equatorial
South America, glaciers once extended far below their present level.
In central Chile I was astonished at the structure of a vast mound of
detritus, about 800 feet in height, crossing a valley of the Andes; and
this I now feel convinced was a gigantic moraine, left far below any
existing glacier. Further south on both sides of the continent, from
lat. 41 deg to the southernmost extremity, we have the clearest evidence
of former glacial action, in huge boulders transported far from their
parent source.

We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have
good evidence in almost every case, that the epoch was included within
the latest geological period. We have, also, excellent evidence, that it
endured for an enormous time, as measured by years, at each point. The
cold may have come on, or have ceased, earlier at one point of the globe
than at another, but seeing that it endured for long at each, and that
it was contemporaneous in a geological sense, it seems to me probable
that it was, during a part at least of the period, actually simultaneous
throughout the world. Without some distinct evidence to the contrary, we
may at least admit as probable that the glacial action was simultaneous
on the eastern and western sides of North America, in the Cordillera
under the equator and under the warmer temperate zones, and on both
sides of the southern extremity of the continent. If this be admitted,
it is difficult to avoid believing that the temperature of the whole
world was at this period simultaneously cooler. But it would suffice for
my purpose, if the temperature was at the same time lower along certain
broad belts of longitude.

On this view of the whole world, or at least of broad longitudinal
belts, having been simultaneously colder from pole to pole, much light
can be thrown on the present distribution of identical and allied
species. In America, Dr. Hooker has shown that between forty and fifty
of the flowering plants of Tierra del Fuego, forming no inconsiderable
part of its scanty flora, are common to Europe, enormously remote as
these two points are; and there are many closely allied species. On
the lofty mountains of equatorial America a host of peculiar species
belonging to European genera occur. On the highest mountains of Brazil,
some few European genera were found by Gardner, which do not exist in
the wide intervening hot countries. So on the Silla of Caraccas
the illustrious Humboldt long ago found species belonging to genera
characteristic of the Cordillera. On the mountains of Abyssinia, several
European forms and some few representatives of the peculiar flora of the
Cape of Good Hope occur. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the
mountains, some few representative European forms are found, which
have not been discovered in the intertropical parts of Africa. On the
Himalaya, and on the isolated mountain-ranges of the peninsula of India,
on the heights of Ceylon, and on the volcanic cones of Java, many plants
occur, either identically the same or representing each other, and
at the same time representing plants of Europe, not found in the
intervening hot lowlands. A list of the genera collected on the loftier
peaks of Java raises a picture of a collection made on a hill in Europe!
Still more striking is the fact that southern Australian forms are
clearly represented by plants growing on the summits of the mountains
of Borneo. Some of these Australian forms, as I hear from Dr. Hooker,
extend along the heights of the peninsula of Malacca, and are thinly
scattered, on the one hand over India and on the other as far north as
Japan.

On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur
on the lowlands; and a long list can be given, as I am informed by
Dr. Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable 'Introduction to the Flora
of New Zealand,' by Dr. Hooker, analogous and striking facts are
given in regard to the plants of that large island. Hence we see that
throughout the world, the plants growing on the more lofty mountains,
and on the temperate lowlands of the northern and southern hemispheres,
are sometimes identically the same; but they are much oftener
specifically distinct, though related to each other in a most remarkable
manner.

This brief abstract applies to plants alone: some strictly analogous
facts could be given on the distribution of terrestrial animals. In
marine productions, similar cases occur; as an example, I may quote a
remark by the highest authority, Professor Dana, that "it is certainly a
wonderful fact that New Zealand should have a closer resemblance in its
crustacea to Great Britain, its antipode, than to any other part of
the world." Sir J. Richardson, also, speaks of the reappearance on the
shores of New Zealand, Tasmania, etc., of northern forms of fish. Dr.
Hooker informs me that twenty-five species of Algae are common to New
Zealand and to Europe, but have not been found in the intermediate
tropical seas.

It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges
of the intertropical regions, are not arctic, but belong to the northern
temperate zones. As Mr. H. C. Watson has recently remarked, "In receding
from polar towards equatorial latitudes, the Alpine or mountain floras
really become less and less arctic." Many of the forms living on
the mountains of the warmer regions of the earth and in the southern
hemisphere are of doubtful value, being ranked by some naturalists as
specifically distinct, by others as varieties; but some are certainly
identical, and many, though closely related to northern forms, must be
ranked as distinct species.

Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that
the whole world, or a large part of it, was during the Glacial period
simultaneously much colder than at present. The Glacial period, as
measured by years, must have been very long; and when we remember over
what vast spaces some naturalised plants and animals have spread within
a few centuries, this period will have been ample for any amount of
migration. As the cold came slowly on, all the tropical plants and other
productions will have retreated from both sides towards the equator,
followed in the rear by the temperate productions, and these by the
arctic; but with the latter we are not now concerned. The tropical
plants probably suffered much extinction; how much no one can say;
perhaps formerly the tropics supported as many species as we see at the
present day crowded together at the Cape of Good Hope, and in parts of
temperate Australia. As we know that many tropical plants and animals
can withstand a considerable amount of cold, many might have escaped
extermination during a moderate fall of temperature, more especially by
escaping into the warmest spots. But the great fact to bear in mind is,
that all tropical productions will have suffered to a certain extent. On
the other hand, the temperate productions, after migrating nearer to
the equator, though they will have been placed under somewhat new
conditions, will have suffered less. And it is certain that many
temperate plants, if protected from the inroads of competitors, can
withstand a much warmer climate than their own. Hence, it seems to
me possible, bearing in mind that the tropical productions were in
a suffering state and could not have presented a firm front against
intruders, that a certain number of the more vigorous and dominant
temperate forms might have penetrated the native ranks and have reached
or even crossed the equator. The invasion would, of course, have been
greatly favoured by high land, and perhaps by a dry climate; for Dr.
Falconer informs me that it is the damp with the heat of the tropics
which is so destructive to perennial plants from a temperate climate. On
the other hand, the most humid and hottest districts will have afforded
an asylum to the tropical natives. The mountain-ranges north-west of the
Himalaya, and the long line of the Cordillera, seem to have afforded two
great lines of invasion: and it is a striking fact, lately communicated
to me by Dr. Hooker, that all the flowering plants, about forty-six in
number, common to Tierra del Fuego and to Europe still exist in North
America, which must have lain on the line of march. But I do not doubt
that some temperate productions entered and crossed even the LOWLANDS of
the tropics at the period when the cold was most intense,--when arctic
forms had migrated some twenty-five degrees of latitude from their
native country and covered the land at the foot of the Pyrenees. At this
period of extreme cold, I believe that the climate under the equator at
the level of the sea was about the same with that now felt there at the
height of six or seven thousand feet. During this the coldest period, I
suppose that large spaces of the tropical lowlands were clothed with a
mingled tropical and temperate vegetation, like that now growing with
strange luxuriance at the base of the Himalaya, as graphically described
by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned,
these temperate forms would naturally ascend the higher mountains, being
exterminated on the lowlands; those which had not reached the equator,
would re-migrate northward or southward towards their former homes; but
the forms, chiefly northern, which had crossed the equator, would travel
still further from their homes into the more temperate latitudes of the
opposite hemisphere. Although we have reason to believe from geological
evidence that the whole body of arctic shells underwent scarcely any
modification during their long southern migration and re-migration
northward, the case may have been wholly different with those intruding
forms which settled themselves on the intertropical mountains, and in
the southern hemisphere. These being surrounded by strangers will have
had to compete with many new forms of life; and it is probable that
selected modifications in their structure, habits, and constitutions
will have profited them. Thus many of these wanderers, though still
plainly related by inheritance to their brethren of the northern or
southern hemispheres, now exist in their new homes as well-marked
varieties or as distinct species.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many
more identical plants and allied forms have apparently migrated from the
north to the south, than in a reversed direction. We see, however, a
few southern vegetable forms on the mountains of Borneo and Abyssinia.
I suspect that this preponderant migration from north to south is due
to the greater extent of land in the north, and to the northern forms
having existed in their own homes in greater numbers, and having
consequently been advanced through natural selection and competition
to a higher stage of perfection or dominating power, than the southern
forms. And thus, when they became commingled during the Glacial period,
the northern forms were enabled to beat the less powerful southern
forms. Just in the same manner as we see at the present day, that very
many European productions cover the ground in La Plata, and in a lesser
degree in Australia, and have to a certain extent beaten the natives;
whereas extremely few southern forms have become naturalised in any part
of Europe, though hides, wool, and other objects likely to carry seeds
have been largely imported into Europe during the last two or three
centuries from La Plata, and during the last thirty or forty years
from Australia. Something of the same kind must have occurred on the
intertropical mountains: no doubt before the Glacial period they were
stocked with endemic Alpine forms; but these have almost everywhere
largely yielded to the more dominant forms, generated in the larger
areas and more efficient workshops of the north. In many islands the
native productions are nearly equalled or even outnumbered by the
naturalised; and if the natives have not been actually exterminated,
their numbers have been greatly reduced, and this is the first stage
towards extinction. A mountain is an island on the land; and the
intertropical mountains before the Glacial period must have been
completely isolated; and I believe that the productions of these islands
on the land yielded to those produced within the larger areas of the
north, just in the same way as the productions of real islands have
everywhere lately yielded to continental forms, naturalised by man's
agency.

I am far from supposing that all difficulties are removed on the view
here given in regard to the range and affinities of the allied species
which live in the northern and southern temperate zones and on the
mountains of the intertropical regions. Very many difficulties remain
to be solved. I do not pretend to indicate the exact lines and means
of migration, or the reason why certain species and not others have
migrated; why certain species have been modified and have given rise to
new groups of forms, and others have remained unaltered. We cannot hope
to explain such facts, until we can say why one species and not another
becomes naturalised by man's agency in a foreign land; why one ranges
twice or thrice as far, and is twice or thrice as common, as another
species within their own homes.

I have said that many difficulties remain to be solved: some of the
most remarkable are stated with admirable clearness by Dr. Hooker in
his botanical works on the antarctic regions. These cannot be here
discussed. I will only say that as far as regards the occurrence of
identical species at points so enormously remote as Kerguelen Land, New
Zealand, and Fuegia, I believe that towards the close of the Glacial
period, icebergs, as suggested by Lyell, have been largely concerned in
their dispersal. But the existence of several quite distinct species,
belonging to genera exclusively confined to the south, at these and
other distant points of the southern hemisphere, is, on my theory of
descent with modification, a far more remarkable case of difficulty. For
some of these species are so distinct, that we cannot suppose that there
has been time since the commencement of the Glacial period for their
migration, and for their subsequent modification to the necessary
degree. The facts seem to me to indicate that peculiar and very distinct
species have migrated in radiating lines from some common centre; and I
am inclined to look in the southern, as in the northern hemisphere, to a
former and warmer period, before the commencement of the Glacial period,
when the antarctic lands, now covered with ice, supported a highly
peculiar and isolated flora. I suspect that before this flora was
exterminated by the Glacial epoch, a few forms were widely dispersed
to various points of the southern hemisphere by occasional means of
transport, and by the aid, as halting-places, of existing and now sunken
islands, and perhaps at the commencement of the Glacial period, by
icebergs. By these means, as I believe, the southern shores of America,
Australia, New Zealand have become slightly tinted by the same peculiar
forms of vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt
one of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the
present distribution both of the same and of allied forms of life can be
explained. The living waters may be said to have flowed during one short
period from the north and from the south, and to have crossed at the
equator; but to have flowed with greater force from the north so as
to have freely inundated the south. As the tide leaves its drift in
horizontal lines, though rising higher on the shores where the tide
rises highest, so have the living waters left their living drift on our
mountain-summits, in a line gently rising from the arctic lowlands to
a great height under the equator. The various beings thus left stranded
may be compared with savage races of man, driven up and surviving in the
mountain-fastnesses of almost every land, which serve as a record,
full of interest to us, of the former inhabitants of the surrounding
lowlands.




12. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions. On the inhabitants of oceanic
islands. Absence of Batrachians and of terrestrial Mammals. On the
relation of the inhabitants of islands to those of the nearest mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently
a still more impassable barrier, that they never would have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to quite different classes, an
enormous range, but allied species prevail in a remarkable manner
throughout the world. I well remember, when first collecting in the
fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability
to wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to fish, I
believe that the same species never occur in the fresh waters of distant
continents. But on the same continent the species often range widely and
almost capriciously; for two river-systems will have some fish in common
and some different. A few facts seem to favour the possibility of their
occasional transport by accidental means; like that of the live fish
not rarely dropped by whirlwinds in India, and the vitality of their
ova when removed from the water. But I am inclined to attribute the
dispersal of fresh-water fish mainly to slight changes within the recent
period in the level of the land, having caused rivers to flow into each
other. Instances, also, could be given of this having occurred during
floods, without any change of level. We have evidence in the loess of
the Rhine of considerable changes of level in the land within a very
recent geological period, and when the surface was peopled by existing
land and fresh-water shells. The wide difference of the fish on opposite
sides of continuous mountain-ranges, which from an early period must
have parted river-systems and completely prevented their inosculation,
seems to lead to this same conclusion. With respect to allied
fresh-water fish occurring at very distant points of the world, no doubt
there are many cases which cannot at present be explained: but some
fresh-water fish belong to very ancient forms, and in such cases
there will have been ample time for great geographical changes, and
consequently time and means for much migration. In the second place,
salt-water fish can with care be slowly accustomed to live in fresh
water; and, according to Valenciennes, there is hardly a single group of
fishes confined exclusively to fresh water, so that we may imagine that
a marine member of a fresh-water group might travel far along the shores
of the sea, and subsequently become modified and adapted to the fresh
waters of a distant land.

Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not
likely to be transported by birds, and they are immediately killed
by sea water, as are the adults. I could not even understand how some
naturalised species have rapidly spread throughout the same country. But
two facts, which I have observed--and no doubt many others remain to be
observed--throw some light on this subject. When a duck suddenly emerges
from a pond covered with duck-weed, I have twice seen these little
plants adhering to its back; and it has happened to me, in removing
a little duck-weed from one aquarium to another, that I have quite
unintentionally stocked the one with fresh-water shells from the other.
But another agency is perhaps more effectual: I suspended a duck's feet,
which might represent those of a bird sleeping in a natural pond, in
an aquarium, where many ova of fresh-water shells were hatching; and
I found that numbers of the extremely minute and just hatched shells
crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched
molluscs, though aquatic in their nature, survived on the duck's feet,
in damp air, from twelve to twenty hours; and in this length of time a
duck or heron might fly at least six or seven hundred miles, and would
be sure to alight on a pool or rivulet, if blown across sea to an
oceanic island or to any other distant point. Sir Charles Lyell also
informs me that a Dyticus has been caught with an Ancylus (a fresh-water
shell like a limpet) firmly adhering to it; and a water-beetle of
the same family, a Colymbetes, once flew on board the 'Beagle,' when
forty-five miles distant from the nearest land: how much farther it
might have flown with a favouring gale no one can tell.

With respect to plants, it has long been known what enormous ranges many
fresh-water and even marsh-species have, both over continents and to the
most remote oceanic islands. This is strikingly shown, as remarked by
Alph. de Candolle, in large groups of terrestrial plants, which have
only a very few aquatic members; for these latter seem immediately to
acquire, as if in consequence, a very wide range. I think favourable
means of dispersal explain this fact. I have before mentioned that earth
occasionally, though rarely, adheres in some quantity to the feet and
beaks of birds. Wading birds, which frequent the muddy edges of ponds,
if suddenly flushed, would be the most likely to have muddy feet.
Birds of this order I can show are the greatest wanderers, and are
occasionally found on the most remote and barren islands in the open
ocean; they would not be likely to alight on the surface of the sea, so
that the dirt would not be washed off their feet; when making land,
they would be sure to fly to their natural fresh-water haunts. I do not
believe that botanists are aware how charged the mud of ponds is with
seeds: I have tried several little experiments, but will here give only
the most striking case: I took in February three table-spoonfuls of
mud from three different points, beneath water, on the edge of a little
pond; this mud when dry weighed only 6 3/4 ounces; I kept it covered
up in my study for six months, pulling up and counting each plant as it
grew; the plants were of many kinds, and were altogether 537 in number;
and yet the viscid mud was all contained in a breakfast cup! Considering
these facts, I think it would be an inexplicable circumstance if
water-birds did not transport the seeds of fresh-water plants to vast
distances, and if consequently the range of these plants was not very
great. The same agency may have come into play with the eggs of some of
the smaller fresh-water animals.

Other and unknown agencies probably have also played a part. I have
stated that fresh-water fish eat some kinds of seeds, though they reject
many other kinds after having swallowed them; even small fish swallow
seeds of moderate size, as of the yellow water-lily and Potamogeton.
Herons and other birds, century after century, have gone on daily
devouring fish; they then take flight and go to other waters, or are
blown across the sea; and we have seen that seeds retain their power
of germination, when rejected in pellets or in excrement, many hours
afterwards. When I saw the great size of the seeds of that fine
water-lily, the Nelumbium, and remembered Alph. de Candolle's remarks
on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr. Hooker, the Nelumbium
luteum) in a heron's stomach; although I do not know the fact, yet
analogy makes me believe that a heron flying to another pond and getting
a hearty meal of fish, would probably reject from its stomach a pellet
containing the seeds of the Nelumbium undigested; or the seeds might be
dropped by the bird whilst feeding its young, in the same way as fish
are known sometimes to be dropped.

In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance,
on a rising islet, it will be unoccupied; and a single seed or egg
will have a good chance of succeeding. Although there will always be a
struggle for life between the individuals of the species, however
few, already occupying any pond, yet as the number of kinds is small,
compared with those on the land, the competition will probably be less
severe between aquatic than between terrestrial species; consequently
an intruder from the waters of a foreign country, would have a better
chance of seizing on a place, than in the case of terrestrial colonists.
We should, also, remember that some, perhaps many, fresh-water
productions are low in the scale of nature, and that we have reason to
believe that such low beings change or become modified less quickly
than the high; and this will give longer time than the average for
the migration of the same aquatic species. We should not forget the
probability of many species having formerly ranged as continuously as
fresh-water productions ever can range, over immense areas, and having
subsequently become extinct in intermediate regions. But the wide
distribution of fresh-water plants and of the lower animals, whether
retaining the same identical form or in some degree modified, I believe
mainly depends on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have large powers of flight,
and naturally travel from one to another and often distant piece of
water. Nature, like a careful gardener, thus takes her seeds from a bed
of a particular nature, and drops them in another equally well fitted
for them.

ON THE INHABITANTS OF OCEANIC ISLANDS.

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty, on the view
that all the individuals both of the same and of allied species have
descended from a single parent; and therefore have all proceeded from a
common birthplace, notwithstanding that in the course of time they have
come to inhabit distant points of the globe. I have already stated that
I cannot honestly admit Forbes's view on continental extensions, which,
if legitimately followed out, would lead to the belief that within the
recent period all existing islands have been nearly or quite joined to
some continent. This view would remove many difficulties, but it would
not, I think, explain all the facts in regard to insular productions. In
the following remarks I shall not confine myself to the mere question of
dispersal; but shall consider some other facts, which bear on the
truth of the two theories of independent creation and of descent with
modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large
size and varied stations of New Zealand, extending over 780 miles of
latitude, and compare its flowering plants, only 750 in number, with
those on an equal area at the Cape of Good Hope or in Australia,
we must, I think, admit that something quite independently of any
difference in physical conditions has caused so great a difference in
number. Even the uniform county of Cambridge has 847 plants, and the
little island of Anglesea 764, but a few ferns and a few introduced
plants are included in these numbers, and the comparison in some other
respects is not quite fair. We have evidence that the barren island of
Ascension aboriginally possessed under half-a-dozen flowering plants;
yet many have become naturalised on it, as they have on New Zealand and
on every other oceanic island which can be named. In St. Helena there is
reason to believe that the naturalised plants and animals have nearly or
quite exterminated many native productions. He who admits the doctrine
of the creation of each separate species, will have to admit, that a
sufficient number of the best adapted plants and animals have not been
created on oceanic islands; for man has unintentionally stocked them
from various sources far more fully and perfectly than has nature.

Although in oceanic islands the number of kinds of inhabitants is
scanty, the proportion of endemic species (i.e. those found nowhere else
in the world) is often extremely large. If we compare, for instance, the
number of the endemic land-shells in Madeira, or of the endemic birds in
the Galapagos Archipelago, with the number found on any continent, and
then compare the area of the islands with that of the continent, we
shall see that this is true. This fact might have been expected on my
theory, for, as already explained, species occasionally arriving after
long intervals in a new and isolated district, and having to compete
with new associates, will be eminently liable to modification, and
will often produce groups of modified descendants. But it by no means
follows, that, because in an island nearly all the species of one class
are peculiar, those of another class, or of another section of the same
class, are peculiar; and this difference seems to depend on the species
which do not become modified having immigrated with facility and in a
body, so that their mutual relations have not been much disturbed. Thus
in the Galapagos Islands nearly every land-bird, but only two out of the
eleven marine birds, are peculiar; and it is obvious that marine birds
could arrive at these islands more easily than land-birds. Bermuda, on
the other hand, which lies at about the same distance from North America
as the Galapagos Islands do from South America, and which has a very
peculiar soil, does not possess one endemic land bird; and we know from
Mr. J. M. Jones's admirable account of Bermuda, that very many North
American birds, during their great annual migrations, visit either
periodically or occasionally this island. Madeira does not possess one
peculiar bird, and many European and African birds are almost every year
blown there, as I am informed by Mr. E. V. Harcourt. So that these two
islands of Bermuda and Madeira have been stocked by birds, which for
long ages have struggled together in their former homes, and have become
mutually adapted to each other; and when settled in their new homes,
each kind will have been kept by the others to their proper places and
habits, and will consequently have been little liable to modification.
Madeira, again, is inhabited by a wonderful number of peculiar
land-shells, whereas not one species of sea-shell is confined to its
shores: now, though we do not know how seashells are dispersed, yet
we can see that their eggs or larvae, perhaps attached to seaweed or
floating timber, or to the feet of wading-birds, might be transported
far more easily than land-shells, across three or four hundred miles of
open sea. The different orders of insects in Madeira apparently present
analogous facts.

Oceanic islands are sometimes deficient in certain classes, and
their places are apparently occupied by the other inhabitants; in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take the place of mammals. In the plants of the Galapagos Islands, Dr.
Hooker has shown that the proportional numbers of the different
orders are very different from what they are elsewhere. Such cases are
generally accounted for by the physical conditions of the islands;
but this explanation seems to me not a little doubtful. Facility of
immigration, I believe, has been at least as important as the nature of
the conditions.

Many remarkable little facts could be given with respect to the
inhabitants of remote islands. For instance, in certain islands not
tenanted by mammals, some of the endemic plants have beautifully hooked
seeds; yet few relations are more striking than the adaptation of hooked
seeds for transportal by the wool and fur of quadrupeds. This
case presents no difficulty on my view, for a hooked seed might be
transported to an island by some other means; and the plant then
becoming slightly modified, but still retaining its hooked seeds,
would form an endemic species, having as useless an appendage as any
rudimentary organ,--for instance, as the shrivelled wings under the
soldered elytra of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only
herbaceous species; now trees, as Alph. de Candolle has shown, generally
have, whatever the cause may be, confined ranges. Hence trees would be
little likely to reach distant oceanic islands; and an herbaceous plant,
though it would have no chance of successfully competing in stature
with a fully developed tree, when established on an island and having to
compete with herbaceous plants alone, might readily gain an advantage
by growing taller and taller and overtopping the other plants. If so,
natural selection would often tend to add to the stature of herbaceous
plants when growing on an island, to whatever order they belonged, and
thus convert them first into bushes and ultimately into trees.

With respect to the absence of whole orders on oceanic islands, Bory St.
Vincent long ago remarked that Batrachians (frogs, toads, newts) have
never been found on any of the many islands with which the great oceans
are studded. I have taken pains to verify this assertion, and I have
found it strictly true. I have, however, been assured that a frog exists
on the mountains of the great island of New Zealand; but I suspect that
this exception (if the information be correct) may be explained through
glacial agency. This general absence of frogs, toads, and newts on
so many oceanic islands cannot be accounted for by their physical
conditions; indeed it seems that islands are peculiarly well fitted for
these animals; for frogs have been introduced into Madeira, the Azores,
and Mauritius, and have multiplied so as to become a nuisance. But as
these animals and their spawn are known to be immediately killed by
sea-water, on my view we can see that there would be great difficulty in
their transportal across the sea, and therefore why they do not exist on
any oceanic island. But why, on the theory of creation, they should not
have been created there, it would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, but have not finished my search; as yet I have not found
a single instance, free from doubt, of a terrestrial mammal (excluding
domesticated animals kept by the natives) inhabiting an island situated
above 300 miles from a continent or great continental island; and
many islands situated at a much less distance are equally barren. The
Falkland Islands, which are inhabited by a wolf-like fox, come nearest
to an exception; but this group cannot be considered as oceanic, as it
lies on a bank connected with the mainland; moreover, icebergs formerly
brought boulders to its western shores, and they may have formerly
transported foxes, as so frequently now happens in the arctic regions.
Yet it cannot be said that small islands will not support small mammals,
for they occur in many parts of the world on very small islands, if
close to a continent; and hardly an island can be named on which our
smaller quadrupeds have not become naturalised and greatly multiplied.
It cannot be said, on the ordinary view of creation, that there has
not been time for the creation of mammals; many volcanic islands are
sufficiently ancient, as shown by the stupendous degradation which they
have suffered and by their tertiary strata: there has also been time
for the production of endemic species belonging to other classes; and on
continents it is thought that mammals appear and disappear at a quicker
rate than other and lower animals. Though terrestrial mammals do not
occur on oceanic islands, aerial mammals do occur on almost every
island. New Zealand possesses two bats found nowhere else in the world:
Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline
and Marianne Archipelagoes, and Mauritius, all possess their peculiar
bats. Why, it may be asked, has the supposed creative force produced
bats and no other mammals on remote islands? On my view this question
can easily be answered; for no terrestrial mammal can be transported
across a wide space of sea, but bats can fly across. Bats have been seen
wandering by day far over the Atlantic Ocean; and two North American
species either regularly or occasionally visit Bermuda, at the distance
of 600 miles from the mainland. I hear from Mr. Tomes, who has specially
studied this family, that many of the same species have enormous ranges,
and are found on continents and on far distant islands. Hence we have
only to suppose that such wandering species have been modified through
natural selection in their new homes in relation to their new position,
and we can understand the presence of endemic bats on islands, with the
absence of all terrestrial mammals.

Besides the absence of terrestrial mammals in relation to the remoteness
of islands from continents, there is also a relation, to a certain
extent independent of distance, between the depth of the sea separating
an island from the neighbouring mainland, and the presence in both of
the same mammiferous species or of allied species in a more or less
modified condition. Mr. Windsor Earl has made some striking observations
on this head in regard to the great Malay Archipelago, which is
traversed near Celebes by a space of deep ocean; and this space
separates two widely distinct mammalian faunas. On either side the
islands are situated on moderately deep submarine banks, and they are
inhabited by closely allied or identical quadrupeds. No doubt some few
anomalies occur in this great archipelago, and there is much difficulty
in forming a judgment in some cases owing to the probable naturalisation
of certain mammals through man's agency; but we shall soon have much
light thrown on the natural history of this archipelago by the admirable
zeal and researches of Mr. Wallace. I have not as yet had time to follow
up this subject in all other quarters of the world; but as far as I have
gone, the relation generally holds good. We see Britain separated by a
shallow channel from Europe, and the mammals are the same on both
sides; we meet with analogous facts on many islands separated by similar
channels from Australia. The West Indian Islands stand on a deeply
submerged bank, nearly 1000 fathoms in depth, and here we find American
forms, but the species and even the genera are distinct. As the amount
of modification in all cases depends to a certain degree on the lapse
of time, and as during changes of level it is obvious that islands
separated by shallow channels are more likely to have been continuously
united within a recent period to the mainland than islands separated
by deeper channels, we can understand the frequent relation between the
depth of the sea and the degree of affinity of the mammalian inhabitants
of islands with those of a neighbouring continent,--an inexplicable
relation on the view of independent acts of creation.

All the foregoing remarks on the inhabitants of oceanic
islands,--namely, the scarcity of kinds--the richness in endemic forms
in particular classes or sections of classes,--the absence of whole
groups, as of batrachians, and of terrestrial mammals notwithstanding
the presence of aerial bats,--the singular proportions of certain
orders of plants,--herbaceous forms having been developed into trees,
etc.,--seem to me to accord better with the view of occasional means of
transport having been largely efficient in the long course of time, than
with the view of all our oceanic islands having been formerly connected
by continuous land with the nearest continent; for on this latter
view the migration would probably have been more complete; and if
modification be admitted, all the forms of life would have been more
equally modified, in accordance with the paramount importance of the
relation of organism to organism.

I do not deny that there are many and grave difficulties in
understanding how several of the inhabitants of the more remote islands,
whether still retaining the same specific form or modified since their
arrival, could have reached their present homes. But the probability of
many islands having existed as halting-places, of which not a wreck now
remains, must not be overlooked. I will here give a single instance of
one of the cases of difficulty. Almost all oceanic islands, even the
most isolated and smallest, are inhabited by land-shells, generally by
endemic species, but sometimes by species found elsewhere. Dr. Aug. A.
Gould has given several interesting cases in regard to the land-shells
of the islands of the Pacific. Now it is notorious that land-shells are
very easily killed by salt; their eggs, at least such as I have tried,
sink in sea-water and are killed by it. Yet there must be, on my view,
some unknown, but highly efficient means for their transportal. Would
the just-hatched young occasionally crawl on and adhere to the feet of
birds roosting on the ground, and thus get transported? It occurred to
me that land-shells, when hybernating and having a membranous diaphragm
over the mouth of the shell, might be floated in chinks of drifted
timber across moderately wide arms of the sea. And I found that several
species did in this state withstand uninjured an immersion in sea-water
during seven days: one of these shells was the Helix pomatia, and after
it had again hybernated I put it in sea-water for twenty days, and it
perfectly recovered. As this species has a thick calcareous operculum,
I removed it, and when it had formed a new membranous one, I immersed it
for fourteen days in sea-water, and it recovered and crawled away:
but more experiments are wanted on this head. The most striking and
important fact for us in regard to the inhabitants of islands, is their
affinity to those of the nearest mainland, without being actually the
same species. Numerous instances could be given of this fact. I will
give only one, that of the Galapagos Archipelago, situated under the
equator, between 500 and 600 miles from the shores of South America.
Here almost every product of the land and water bears the unmistakeable
stamp of the American continent. There are twenty-six land birds,
and twenty-five of these are ranked by Mr. Gould as distinct species,
supposed to have been created here; yet the close affinity of most of
these birds to American species in every character, in their habits,
gestures, and tones of voice, was manifest. So it is with the other
animals, and with nearly all the plants, as shown by Dr. Hooker in
his admirable memoir on the Flora of this archipelago. The naturalist,
looking at the inhabitants of these volcanic islands in the Pacific,
distant several hundred miles from the continent, yet feels that he is
standing on American land. Why should this be so? why should the species
which are supposed to have been created in the Galapagos Archipelago,
and nowhere else, bear so plain a stamp of affinity to those created in
America? There is nothing in the conditions of life, in the geological
nature of the islands, in their height or climate, or in the proportions
in which the several classes are associated together, which resembles
closely the conditions of the South American coast: in fact there is
a considerable dissimilarity in all these respects. On the other hand,
there is a considerable degree of resemblance in the volcanic nature
of the soil, in climate, height, and size of the islands, between
the Galapagos and Cape de Verde Archipelagos: but what an entire and
absolute difference in their inhabitants! The inhabitants of the Cape
de Verde Islands are related to those of Africa, like those of the
Galapagos to America. I believe this grand fact can receive no sort of
explanation on the ordinary view of independent creation; whereas on the
view here maintained, it is obvious that the Galapagos Islands would be
likely to receive colonists, whether by occasional means of transport or
by formerly continuous land, from America; and the Cape de Verde
Islands from Africa; and that such colonists would be liable to
modification;--the principle of inheritance still betraying their
original birthplace.

Many analogous facts could be given: indeed it is an almost universal
rule that the endemic productions of islands are related to those of the
nearest continent, or of other near islands. The exceptions are few, and
most of them can be explained. Thus the plants of Kerguelen Land, though
standing nearer to Africa than to America, are related, and that very
closely, as we know from Dr. Hooker's account, to those of America: but
on the view that this island has been mainly stocked by seeds brought
with earth and stones on icebergs, drifted by the prevailing currents,
this anomaly disappears. New Zealand in its endemic plants is much more
closely related to Australia, the nearest mainland, than to any other
region: and this is what might have been expected; but it is also
plainly related to South America, which, although the next nearest
continent, is so enormously remote, that the fact becomes an anomaly.
But this difficulty almost disappears on the view that both New Zealand,
South America, and other southern lands were long ago partially stocked
from a nearly intermediate though distant point, namely from the
antarctic islands, when they were clothed with vegetation, before the
commencement of the Glacial period. The affinity, which, though
feeble, I am assured by Dr. Hooker is real, between the flora of the
south-western corner of Australia and of the Cape of Good Hope, is a far
more remarkable case, and is at present inexplicable: but this affinity
is confined to the plants, and will, I do not doubt, be some day
explained.

The law which causes the inhabitants of an archipelago, though
specifically distinct, to be closely allied to those of the nearest
continent, we sometimes see displayed on a small scale, yet in a most
interesting manner, within the limits of the same archipelago. Thus the
several islands of the Galapagos Archipelago are tenanted, as I have
elsewhere shown, in a quite marvellous manner, by very closely related
species; so that the inhabitants of each separate island, though mostly
distinct, are related in an incomparably closer degree to each other
than to the inhabitants of any other part of the world. And this is just
what might have been expected on my view, for the islands are situated
so near each other that they would almost certainly receive
immigrants from the same original source, or from each other. But this
dissimilarity between the endemic inhabitants of the islands may be
used as an argument against my views; for it may be asked, how has it
happened in the several islands situated within sight of each other,
having the same geological nature, the same height, climate, etc., that
many of the immigrants should have been differently modified, though
only in a small degree. This long appeared to me a great difficulty: but
it arises in chief part from the deeply-seated error of considering
the physical conditions of a country as the most important for its
inhabitants; whereas it cannot, I think, be disputed that the nature of
the other inhabitants, with which each has to compete, is at least as
important, and generally a far more important element of success. Now
if we look to those inhabitants of the Galapagos Archipelago which are
found in other parts of the world (laying on one side for the moment
the endemic species, which cannot be here fairly included, as we are
considering how they have come to be modified since their arrival), we
find a considerable amount of difference in the several islands. This
difference might indeed have been expected on the view of the islands
having been stocked by occasional means of transport--a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island. Hence when in former times an immigrant
settled on any one or more of the islands, or when it subsequently
spread from one island to another, it would undoubtedly be exposed to
different conditions of life in the different islands, for it would
have to compete with different sets of organisms: a plant, for instance,
would find the best-fitted ground more perfectly occupied by distinct
plants in one island than in another, and it would be exposed to the
attacks of somewhat different enemies. If then it varied, natural
selection would probably favour different varieties in the different
islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some
species spreading widely and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago,
and in a lesser degree in some analogous instances, is that the new
species formed in the separate islands have not quickly spread to the
other islands. But the islands, though in sight of each other, are
separated by deep arms of the sea, in most cases wider than the British
Channel, and there is no reason to suppose that they have at any former
period been continuously united. The currents of the sea are rapid and
sweep across the archipelago, and gales of wind are extraordinarily
rare; so that the islands are far more effectually separated from each
other than they appear to be on a map. Nevertheless a good many species,
both those found in other parts of the world and those confined to the
archipelago, are common to the several islands, and we may infer from
certain facts that these have probably spread from some one island
to the others. But we often take, I think, an erroneous view of the
probability of closely allied species invading each other's territory,
when put into free intercommunication. Undoubtedly if one species has
any advantage whatever over another, it will in a very brief time wholly
or in part supplant it; but if both are equally well fitted for their
own places in nature, both probably will hold their own places and keep
separate for almost any length of time. Being familiar with the fact
that many species, naturalised through man's agency, have spread with
astonishing rapidity over new countries, we are apt to infer that most
species would thus spread; but we should remember that the forms which
become naturalised in new countries are not generally closely allied to
the aboriginal inhabitants, but are very distinct species, belonging in
a large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though
so well adapted for flying from island to island, are distinct on each;
thus there are three closely-allied species of mocking-thrush, each
confined to its own island. Now let us suppose the mocking-thrush
of Chatham Island to be blown to Charles Island, which has its own
mocking-thrush: why should it succeed in establishing itself there?
We may safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid there than can possibly be
reared; and we may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the species
peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston have
communicated to me a remarkable fact bearing on this subject; namely,
that Madeira and the adjoining islet of Porto Santo possess many
distinct but representative land-shells, some of which live in
crevices of stone; and although large quantities of stone are annually
transported from Porto Santo to Madeira, yet this latter island has not
become colonised by the Porto Santo species: nevertheless both islands
have been colonised by some European land-shells, which no doubt had
some advantage over the indigenous species. From these considerations
I think we need not greatly marvel at the endemic and representative
species, which inhabit the several islands of the Galapagos Archipelago,
not having universally spread from island to island. In many other
instances, as in the several districts of the same continent,
pre-occupation has probably played an important part in checking the
commingling of species under the same conditions of life. Thus, the
south-east and south-west corners of Australia have nearly the same
physical conditions, and are united by continuous land, yet they are
inhabited by a vast number of distinct mammals, birds, and plants.

The principle which determines the general character of the fauna
and flora of oceanic islands, namely, that the inhabitants, when not
identically the same, yet are plainly related to the inhabitants of
that region whence colonists could most readily have been derived,--the
colonists having been subsequently modified and better fitted to their
new homes,--is of the widest application throughout nature. We see
this on every mountain, in every lake and marsh. For Alpine species,
excepting in so far as the same forms, chiefly of plants, have spread
widely throughout the world during the recent Glacial epoch, are related
to those of the surrounding lowlands;--thus we have in South America,
Alpine humming-birds, Alpine rodents, Alpine plants, etc., all of
strictly American forms, and it is obvious that a mountain, as it became
slowly upheaved, would naturally be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting
in so far as great facility of transport has given the same general
forms to the whole world. We see this same principle in the blind
animals inhabiting the caves of America and of Europe. Other analogous
facts could be given. And it will, I believe, be universally found to
be true, that wherever in two regions, let them be ever so distant, many
closely allied or representative species occur, there will likewise be
found some identical species, showing, in accordance with the foregoing
view, that at some former period there has been intercommunication or
migration between the two regions. And wherever many closely-allied
species occur, there will be found many forms which some naturalists
rank as distinct species, and some as varieties; these doubtful forms
showing us the steps in the process of modification.

This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way.
Mr. Gould remarked to me long ago, that in those genera of birds which
range over the world, many of the species have very wide ranges. I
can hardly doubt that this rule is generally true, though it would be
difficult to prove it. Amongst mammals, we see it strikingly displayed
in Bats, and in a lesser degree in the Felidae and Canidae. We see it,
if we compare the distribution of butterflies and beetles. So it is with
most fresh-water productions, in which so many genera range over the
world, and many individual species have enormous ranges. It is not meant
that in world-ranging genera all the species have a wide range, or even
that they have on an AVERAGE a wide range; but only that some of the
species range very widely; for the facility with which widely-ranging
species vary and give rise to new forms will largely determine their
average range. For instance, two varieties of the same species inhabit
America and Europe, and the species thus has an immense range; but, if
the variation had been a little greater, the two varieties would have
been ranked as distinct species, and the common range would have been
greatly reduced. Still less is it meant, that a species which apparently
has the capacity of crossing barriers and ranging widely, as in the case
of certain powerfully-winged birds, will necessarily range widely; for
we should never forget that to range widely implies not only the power
of crossing barriers, but the more important power of being victorious
in distant lands in the struggle for life with foreign associates. But
on the view of all the species of a genus having descended from a single
parent, though now distributed to the most remote points of the world,
we ought to find, and I believe as a general rule we do find, that some
at least of the species range very widely; for it is necessary that the
unmodified parent should range widely, undergoing modification during
its diffusion, and should place itself under diverse conditions
favourable for the conversion of its offspring, firstly into new
varieties and ultimately into new species.

In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off from
a common parent at a remote epoch; so that in such cases there will
have been ample time for great climatal and geographical changes and for
accidents of transport; and consequently for the migration of some of
the species into all quarters of the world, where they may have become
slightly modified in relation to their new conditions. There is, also,
some reason to believe from geological evidence that organisms low in
the scale within each great class, generally change at a slower rate
than the higher forms; and consequently the lower forms will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with the seeds and eggs of many low forms
being very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately
been admirably discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms is, the more widely it is
apt to range.

The relations just discussed,--namely, low and slowly-changing
organisms ranging more widely than the high,--some of the species of
widely-ranging genera themselves ranging widely,--such facts, as alpine,
lacustrine, and marsh productions being related (with the exceptions
before specified) to those on the surrounding low lands and dry lands,
though these stations are so different--the very close relation of the
distinct species which inhabit the islets of the same archipelago,--and
especially the striking relation of the inhabitants of each whole
archipelago or island to those of the nearest mainland,--are, I think,
utterly inexplicable on the ordinary view of the independent creation
of each species, but are explicable on the view of colonisation from the
nearest and readiest source, together with the subsequent modification
and better adaptation of the colonists to their new homes.

SUMMARY OF LAST AND PRESENT CHAPTERS.

In these chapters I have endeavoured to show, that if we make due
allowance for our ignorance of the full effects of all the changes of
climate and of the level of the land, which have certainly occurred
within the recent period, and of other similar changes which may have
occurred within the same period; if we remember how profoundly ignorant
we are with respect to the many and curious means of occasional
transport,--a subject which has hardly ever been properly experimentised
on; if we bear in mind how often a species may have ranged continuously
over a wide area, and then have become extinct in the intermediate
tracts, I think the difficulties in believing that all the individuals
of the same species, wherever located, have descended from the same
parents, are not insuperable. And we are led to this conclusion, which
has been arrived at by many naturalists under the designation of single
centres of creation, by some general considerations, more especially
from the importance of barriers and from the analogical distribution of
sub-genera, genera, and families.

With respect to the distinct species of the same genus, which on my
theory must have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life change most slowly, enormous periods of time being thus granted for
their migration, I do not think that the difficulties are insuperable;
though they often are in this case, and in that of the individuals of
the same species, extremely grave.

As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected
the whole world, or at least great meridional belts. As showing how
diversified are the means of occasional transport, I have discussed at
some little length the means of dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long
course of time the individuals of the same species, and likewise of
allied species, have proceeded from some one source; then I think all
the grand leading facts of geographical distribution are explicable on
the theory of migration (generally of the more dominant forms of life),
together with subsequent modification and the multiplication of new
forms. We can thus understand the high importance of barriers, whether
of land or water, which separate our several zoological and botanical
provinces. We can thus understand the localisation of sub-genera,
genera, and families; and how it is that under different latitudes, for
instance in South America, the inhabitants of the plains and mountains,
of the forests, marshes, and deserts, are in so mysterious a manner
linked together by affinity, and are likewise linked to the extinct
beings which formerly inhabited the same continent. Bearing in mind
that the mutual relations of organism to organism are of the highest
importance, we can see why two areas having nearly the same physical
conditions should often be inhabited by very different forms of
life; for according to the length of time which has elapsed since
new inhabitants entered one region; according to the nature of the
communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each
other and with the aborigines; and according as the immigrants were
capable of varying more or less rapidly, there would ensue in different
regions, independently of their physical conditions, infinitely
diversified conditions of life,--there would be an almost endless amount
of organic action and reaction,--and we should find, as we do find,
some groups of beings greatly, and some only slightly modified,--some
developed in great force, some existing in scanty numbers--in the
different great geographical provinces of the world.

On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings, even within the same class,
should have all its species endemic, and another group should have all
its species common to other quarters of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands possess
their own peculiar species of aerial mammals or bats. We can see why
there should be some relation between the presence of mammals, in a more
or less modified condition, and the depth of the sea between an island
and the mainland. We can clearly see why all the inhabitants of an
archipelago, though specifically distinct on the several islets, should
be closely related to each other, and likewise be related, but less
closely, to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two areas, however
distant from each other, there should be a correlation, in the presence
of identical species, of varieties, of doubtful species, and of distinct
but representative species.

As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space: the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species and
group of species is continuous in time; for the exceptions to the rule
are so few, that they may fairly be attributed to our not having as
yet discovered in an intermediate deposit the forms which are therein
absent, but which occur above and below: so in space, it certainly is
the general rule that the area inhabited by a single species, or by a
group of species, is continuous; and the exceptions, which are not rare,
may, as I have attempted to show, be accounted for by migration at
some former period under different conditions or by occasional means of
transport, and by the species having become extinct in the intermediate
tracts. Both in time and space, species and groups of species have their
points of maximum development. Groups of species, belonging either to a
certain period of time, or to a certain area, are often characterised by
trifling characters in common, as of sculpture or colour. In looking
to the long succession of ages, as in now looking to distant provinces
throughout the world, we find that some organisms differ little, whilst
others belonging to a different class, or to a different order, or even
only to a different family of the same order, differ greatly. In both
time and space the lower members of each class generally change less
than the higher; but there are in both cases marked exceptions to the
rule. On my theory these several relations throughout time and space
are intelligible; for whether we look to the forms of life which have
changed during successive ages within the same quarter of the world, or
to those which have changed after having migrated into distant quarters,
in both cases the forms within each class have been connected by the
same bond of ordinary generation; and the more nearly any two forms are
related in blood, the nearer they will generally stand to each other in
time and space; in both cases the laws of variation have been the same,
and modifications have been accumulated by the same power of natural
selection.




13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS.

CLASSIFICATION, groups subordinate to groups. Natural system. Rules and
difficulties in classification, explained on the theory of descent
with modification. Classification of varieties. Descent always used in
classification. Analogical or adaptive characters. Affinities, general,
complex and radiating. Extinction separates and defines groups.
MORPHOLOGY, between members of the same class, between parts of the same
individual. EMBRYOLOGY, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age. RUDIMENTARY
ORGANS; their origin explained. Summary.

From the first dawn of life, all organic beings are found to resemble
each other in descending degrees, so that they can be classed in groups
under groups. This classification is evidently not arbitrary like the
grouping of the stars in constellations. The existence of groups would
have been of simple signification, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on flesh,
another on vegetable matter, and so on; but the case is widely different
in nature; for it is notorious how commonly members of even the same
subgroup have different habits. In our second and fourth chapters, on
Variation and on Natural Selection, I have attempted to show that it is
the widely ranging, the much diffused and common, that is the dominant
species belonging to the larger genera, which vary most. The varieties,
or incipient species, thus produced ultimately become converted, as I
believe, into new and distinct species; and these, on the principle
of inheritance, tend to produce other new and dominant species.
Consequently the groups which are now large, and which generally include
many dominant species, tend to go on increasing indefinitely in size.
I further attempted to show that from the varying descendants of each
species trying to occupy as many and as different places as possible in
the economy of nature, there is a constant tendency in their characters
to diverge. This conclusion was supported by looking at the great
diversity of the forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in naturalisation.

I attempted also to show that there is a constant tendency in the forms
which are increasing in number and diverging in character, to supplant
and exterminate the less divergent, the less improved, and preceding
forms. I request the reader to turn to the diagram illustrating the
action, as formerly explained, of these several principles; and he
will see that the inevitable result is that the modified descendants
proceeding from one progenitor become broken up into groups subordinate
to groups. In the diagram each letter on the uppermost line may
represent a genus including several species; and all the genera on this
line form together one class, for all have descended from one ancient
but unseen parent, and, consequently, have inherited something in
common. But the three genera on the left hand have, on this same
principle, much in common, and form a sub-family, distinct from that
including the next two genera on the right hand, which diverged from a
common parent at the fifth stage of descent. These five genera have also
much, though less, in common; and they form a family distinct from
that including the three genera still further to the right hand, which
diverged at a still earlier period. And all these genera, descended from
(A), form an order distinct from the genera descended from (I). So that
we here have many species descended from a single progenitor grouped
into genera; and the genera are included in, or subordinate to,
sub-families, families, and orders, all united into one class. Thus, the
grand fact in natural history of the subordination of group under group,
which, from its familiarity, does not always sufficiently strike us, is
in my judgment fully explained.

Naturalists try to arrange the species, genera, and families in each
class, on what is called the Natural System. But what is meant by
this system? Some authors look at it merely as a scheme for arranging
together those living objects which are most alike, and for separating
those which are most unlike; or as an artificial means for enunciating,
as briefly as possible, general propositions,--that is, by one sentence
to give the characters common, for instance, to all mammals, by another
those common to all carnivora, by another those common to the dog-genus,
and then by adding a single sentence, a full description is given
of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant
by the Natural System; they believe that it reveals the plan of the
Creator; but unless it be specified whether order in time or space,
or what else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Such expressions as that
famous one of Linnaeus, and which we often meet with in a more or less
concealed form, that the characters do not make the genus, but that
the genus gives the characters, seem to imply that something more is
included in our classification, than mere resemblance. I believe that
something more is included; and that propinquity of descent,--the only
known cause of the similarity of organic beings,--is the bond, hidden as
it is by various degrees of modification, which is partially revealed to
us by our classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme for
enunciating general propositions and of placing together the forms most
like each other. It might have been thought (and was in ancient times
thought) that those parts of the structure which determined the habits
of life, and the general place of each being in the economy of nature,
would be of very high importance in classification. Nothing can be more
false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These
resemblances, though so intimately connected with the whole life of the
being, are ranked as merely "adaptive or analogical characters;" but to
the consideration of these resemblances we shall have to recur. It
may even be given as a general rule, that the less any part of the
organisation is concerned with special habits, the more important it
becomes for classification. As an instance: Owen, in speaking of the
dugong, says, "The generative organs being those which are most remotely
related to the habits and food of an animal, I have always regarded as
affording very clear indications of its true affinities. We are least
likely in the modifications of these organs to mistake a merely adaptive
for an essential character." So with plants, how remarkable it is that
the organs of vegetation, on which their whole life depends, are of
little signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!

We must not, therefore, in classifying, trust to resemblances in parts
of the organisation, however important they may be for the welfare of
the being in relation to the outer world. Perhaps from this cause it has
partly arisen, that almost all naturalists lay the greatest stress on
resemblances in organs of high vital or physiological importance. No
doubt this view of the classificatory importance of organs which
are important is generally, but by no means always, true. But their
importance for classification, I believe, depends on their greater
constancy throughout large groups of species; and this constancy depends
on such organs having generally been subjected to less change in the
adaptation of the species to their conditions of life. That the
mere physiological importance of an organ does not determine its
classificatory value, is almost shown by the one fact, that in allied
groups, in which the same organ, as we have every reason to suppose, has
nearly the same physiological value, its classificatory value is widely
different. No naturalist can have worked at any group without being
struck with this fact; and it has been most fully acknowledged in the
writings of almost every author. It will suffice to quote the highest
authority, Robert Brown, who in speaking of certain organs in the
Proteaceae, says their generic importance, "like that of all their
parts, not only in this but, as I apprehend, in every natural family,
is very unequal, and in some cases seems to be entirely lost." Again in
another work he says, the genera of the Connaraceae "differ in having
one or more ovaria, in the existence or absence of albumen, in the
imbricate or valvular aestivation. Any one of these characters singly
is frequently of more than generic importance, though here even when
all taken together they appear insufficient to separate Cnestis from
Connarus." To give an example amongst insects, in one great division
of the Hymenoptera, the antennae, as Westwood has remarked, are most
constant in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no one
probably will say that the antennae in these two divisions of the same
order are of unequal physiological importance. Any number of instances
could be given of the varying importance for classification of the same
important organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of high value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants,
and certain rudimentary bones of the leg, are highly serviceable in
exhibiting the close affinity between Ruminants and Pachyderms. Robert
Brown has strongly insisted on the fact that the rudimentary florets are
of the highest importance in the classification of the Grasses.

Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which
are universally admitted as highly serviceable in the definition of
whole groups. For instance, whether or not there is an open passage from
the nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles--the inflection of the
angle of the jaws in Marsupials--the manner in which the wings of
insects are folded--mere colour in certain Algae--mere pubescence on
parts of the flower in grasses--the nature of the dermal covering, as
hair or feathers, in the Vertebrata. If the Ornithorhynchus had been
covered with feathers instead of hair, this external and trifling
character would, I think, have been considered by naturalists as
important an aid in determining the degree of affinity of this strange
creature to birds and reptiles, as an approach in structure in any one
internal and important organ.

The importance, for classification, of trifling characters, mainly
depends on their being correlated with several other characters of more
or less importance. The value indeed of an aggregate of characters is
very evident in natural history. Hence, as has often been remarked, a
species may depart from its allies in several characters, both of high
physiological importance and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence, also, it has been
found, that a classification founded on any single character,
however important that may be, has always failed; for no part of the
organisation is universally constant. The importance of an aggregate of
characters, even when none are important, alone explains, I think, that
saying of Linnaeus, that the characters do not give the genus, but
the genus gives the characters; for this saying seems founded on an
appreciation of many trifling points of resemblance, too slight to be
defined. Certain plants, belonging to the Malpighiaceae, bear perfect
and degraded flowers; in the latter, as A. de Jussieu has remarked, "the
greater number of the characters proper to the species, to the genus,
to the family, to the class, disappear, and thus laugh at our
classification." But when Aspicarpa produced in France, during several
years, only degraded flowers, departing so wonderfully in a number
of the most important points of structure from the proper type of the
order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained amongst the Malpighiaceae. This case
seems to me well to illustrate the spirit with which our classifications
are sometimes necessarily founded.

Practically when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in
defining a group, or in allocating any particular species. If they find
a character nearly uniform, and common to a great number of forms, and
not common to others, they use it as one of high value; if common to
some lesser number, they use it as of subordinate value. This principle
has been broadly confessed by some naturalists to be the true one; and
by none more clearly than by that excellent botanist, Aug. St. Hilaire.
If certain characters are always found correlated with others, though
no apparent bond of connexion can be discovered between them, especial
value is set on them. As in most groups of animals, important organs,
such as those for propelling the blood, or for aerating it, or those for
propagating the race, are found nearly uniform, they are considered as
highly serviceable in classification; but in some groups of animals all
these, the most important vital organs, are found to offer characters of
quite subordinate value.

We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications of
course include all ages of each species. But it is by no means obvious,
on the ordinary view, why the structure of the embryo should be more
important for this purpose than that of the adult, which alone plays its
full part in the economy of nature. Yet it has been strongly urged
by those great naturalists, Milne Edwards and Agassiz, that embryonic
characters are the most important of any in the classification of
animals; and this doctrine has very generally been admitted as true.
The same fact holds good with flowering plants, of which the two main
divisions have been founded on characters derived from the embryo,--on
the number and position of the embryonic leaves or cotyledons, and on
the mode of development of the plumule and radicle. In our discussion
on embryology, we shall see why such characters are so valuable, on the
view of classification tacitly including the idea of descent.

Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of characters
common to all birds; but in the case of crustaceans, such definition has
hitherto been found impossible. There are crustaceans at the opposite
ends of the series, which have hardly a character in common; yet the
species at both ends, from being plainly allied to others, and these to
others, and so onwards, can be recognised as unequivocally belonging to
this, and to no other class of the Articulata.

Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity
of this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and genera,
they seem to be, at least at present, almost arbitrary. Several of the
best botanists, such as Mr. Bentham and others, have strongly insisted
on their arbitrary value. Instances could be given amongst plants and
insects, of a group of forms, first ranked by practised naturalists as
only a genus, and then raised to the rank of a sub-family or family; and
this has been done, not because further research has detected important
structural differences, at first overlooked, but because numerous
allied species, with slightly different grades of difference, have been
subsequently discovered.

All the foregoing rules and aids and difficulties in classification
are explained, if I do not greatly deceive myself, on the view that
the natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, and not some unknown plan of creation, or the
enunciation of general propositions, and the mere putting together and
separating objects more or less alike.

But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to
the other groups, must be strictly genealogical in order to be natural;
but that the AMOUNT of difference in the several branches or groups,
though allied in the same degree in blood to their common progenitor,
may differ greatly, being due to the different degrees of modification
which they have undergone; and this is expressed by the forms being
ranked under different genera, families, sections, or orders. The reader
will best understand what is meant, if he will take the trouble of
referring to the diagram in the fourth chapter. We will suppose the
letters A to L to represent allied genera, which lived during the
Silurian epoch, and these have descended from a species which existed at
an unknown anterior period. Species of three of these genera (A, F, and
I) have transmitted modified descendants to the present day, represented
by the fifteen genera (a14 to z14) on the uppermost horizontal line. Now
all these modified descendants from a single species, are represented as
related in blood or descent to the same degree; they may metaphorically
be called cousins to the same millionth degree; yet they differ widely
and in different degrees from each other. The forms descended from A,
now broken up into two or three families, constitute a distinct order
from those descended from I, also broken up into two families. Nor can
the existing species, descended from A, be ranked in the same genus with
the parent A; or those from I, with the parent I. But the existing genus
F14 may be supposed to have been but slightly modified; and it will
then rank with the parent-genus F; just as some few still living organic
beings belong to Silurian genera. So that the amount or value of the
differences between organic beings all related to each other in the same
degree in blood, has come to be widely different. Nevertheless their
genealogical ARRANGEMENT remains strictly true, not only at the present
time, but at each successive period of descent. All the modified
descendants from A will have inherited something in common from their
common parent, as will all the descendants from I; so will it be with
each subordinate branch of descendants, at each successive period. If,
however, we choose to suppose that any of the descendants of A or of
I have been so much modified as to have more or less completely lost
traces of their parentage, in this case, their places in a natural
classification will have been more or less completely lost,--as
sometimes seems to have occurred with existing organisms. All the
descendants of the genus F, along its whole line of descent, are
supposed to have been but little modified, and they yet form a single
genus. But this genus, though much isolated, will still occupy its
proper intermediate position; for F originally was intermediate in
character between A and I, and the several genera descended from these
two genera will have inherited to a certain extent their characters.
This natural arrangement is shown, as far as is possible on paper, in
the diagram, but in much too simple a manner. If a branching diagram had
not been used, and only the names of the groups had been written in a
linear series, it would have been still less possible to have given a
natural arrangement; and it is notoriously not possible to represent in
a series, on a flat surface, the affinities which we discover in nature
amongst the beings of the same group. Thus, on the view which I hold,
the natural system is genealogical in its arrangement, like a pedigree;
but the degrees of modification which the different groups have
undergone, have to be expressed by ranking them under different
so-called genera, sub-families, families, sections, orders, and classes.

It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford the
best classification of the various languages now spoken throughout the
world; and if all extinct languages, and all intermediate and slowly
changing dialects, had to be included, such an arrangement would, I
think, be the only possible one. Yet it might be that some very ancient
language had altered little, and had given rise to few new languages,
whilst others (owing to the spreading and subsequent isolation and
states of civilisation of the several races, descended from a common
race) had altered much, and had given rise to many new languages and
dialects. The various degrees of difference in the languages from the
same stock, would have to be expressed by groups subordinate to
groups; but the proper or even only possible arrangement would still be
genealogical; and this would be strictly natural, as it would connect
together all languages, extinct and modern, by the closest affinities,
and would give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification
of varieties, which are believed or known to have descended from one
species. These are grouped under species, with sub-varieties under
varieties; and with our domestic productions, several other grades of
difference are requisite, as we have seen with pigeons. The origin
of the existence of groups subordinate to groups, is the same with
varieties as with species, namely, closeness of descent with various
degrees of modification. Nearly the same rules are followed in
classifying varieties, as with species. Authors have insisted on the
necessity of classing varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pine-apple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the swedish
and common turnips together, though the esculent and thickened stems
are so similar. Whatever part is found to be most constant, is used in
classing varieties: thus the great agriculturist Marshall says the horns
are very useful for this purpose with cattle, because they are less
variable than the shape or colour of the body, etc.; whereas with sheep
the horns are much less serviceable, because less constant. In classing
varieties, I apprehend if we had a real pedigree, a genealogical
classification would be universally preferred; and it has been attempted
by some authors. For we might feel sure, whether there had been more
or less modification, the principle of inheritance would keep the forms
together which were allied in the greatest number of points. In tumbler
pigeons, though some sub-varieties differ from the others in the
important character of having a longer beak, yet all are kept together
from having the common habit of tumbling; but the short-faced breed has
nearly or quite lost this habit; nevertheless, without any reasoning
or thinking on the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects. If it could be
proved that the Hottentot had descended from the Negro, I think he would
be classed under the Negro group, however much he might differ in colour
and other important characters from negroes.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade,
or that of a species, the two sexes; and how enormously these sometimes
differ in the most important characters, is known to every naturalist:
scarcely a single fact can be predicated in common of the males and
hermaphrodites of certain cirripedes, when adult, and yet no one dreams
of separating them. The naturalist includes as one species the several
larval stages of the same individual, however much they may differ from
each other and from the adult; as he likewise includes the so-called
alternate generations of Steenstrup, which can only in a technical sense
be considered as the same individual. He includes monsters; he includes
varieties, not solely because they closely resemble the parent-form, but
because they are descended from it. He who believes that the cowslip
is descended from the primrose, or conversely, ranks them together as
a single species, and gives a single definition. As soon as three
Orchidean forms (Monochanthus, Myanthus, and Catasetum), which had
previously been ranked as three distinct genera, were known to be
sometimes produced on the same spike, they were immediately included as
a single species. But it may be asked, what ought we to do, if it could
be proved that one species of kangaroo had been produced, by a long
course of modification, from a bear? Ought we to rank this one
species with bears, and what should we do with the other species?
The supposition is of course preposterous; and I might answer by the
argumentum ad hominem, and ask what should be done if a perfect kangaroo
were seen to come out of the womb of a bear? According to all analogy,
it would be ranked with bears; but then assuredly all the other species
of the kangaroo family would have to be classed under the bear genus.
The whole case is preposterous; for where there has been close descent
in common, there will certainly be close resemblance or affinity.

As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvae
are sometimes extremely different; and as it has been used in classing
varieties which have undergone a certain, and sometimes a considerable
amount of modification, may not this same element of descent have been
unconsciously used in grouping species under genera, and genera under
higher groups, though in these cases the modification has been greater
in degree, and has taken a longer time to complete? I believe it has
thus been unconsciously used; and only thus can I understand the several
rules and guides which have been followed by our best systematists. We
have no written pedigrees; we have to make out community of descent by
resemblances of any kind. Therefore we choose those characters which,
as far as we can judge, are the least likely to have been modified
in relation to the conditions of life to which each species has been
recently exposed. Rudimentary structures on this view are as good as, or
even sometimes better than, other parts of the organisation. We care not
how trifling a character may be--let it be the mere inflection of
the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers--if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for
its presence in so many forms with such different habits, only by its
inheritance from a common parent. We may err in this respect in regard
to single points of structure, but when several characters, let them
be ever so trifling, occur together throughout a large group of beings
having different habits, we may feel almost sure, on the theory of
descent, that these characters have been inherited from a common
ancestor. And we know that such correlated or aggregated characters have
especial value in classification.

We can understand why a species or a group of species may depart, in
several of its most important characteristics, from its allies, and yet
be safely classed with them. This may be safely done, and is often
done, as long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet if these extreme forms
are connected together by a chain of intermediate groups, we may at
once infer their community of descent, and we put them all into the same
class. As we find organs of high physiological importance--those
which serve to preserve life under the most diverse conditions of
existence--are generally the most constant, we attach especial value to
them; but if these same organs, in another group or section of a
group, are found to differ much, we at once value them less in
our classification. We shall hereafter, I think, clearly see why
embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play in
classing large and widely-distributed genera, because all the species of
the same genus, inhabiting any distinct and isolated region, have in all
probability descended from the same parents.

We can understand, on these views, the very important distinction
between real affinities and analogical or adaptive resemblances.
Lamarck first called attention to this distinction, and he has been ably
followed by Macleay and others. The resemblance, in the shape of the
body and in the fin-like anterior limbs, between the dugong, which is a
pachydermatous animal, and the whale, and between both these mammals and
fishes, is analogical. Amongst insects there are innumerable instances:
thus Linnaeus, misled by external appearances, actually classed an
homopterous insect as a moth. We see something of the same kind even
in our domestic varieties, as in the thickened stems of the common and
swedish turnip. The resemblance of the greyhound and racehorse is hardly
more fanciful than the analogies which have been drawn by some authors
between very distinct animals. On my view of characters being of real
importance for classification, only in so far as they reveal descent, we
can clearly understand why analogical or adaptive character, although of
the utmost importance to the welfare of the being, are almost valueless
to the systematist. For animals, belonging to two most distinct lines
of descent, may readily become adapted to similar conditions, and thus
assume a close external resemblance; but such resemblances will not
reveal--will rather tend to conceal their blood-relationship to their
proper lines of descent. We can also understand the apparent paradox,
that the very same characters are analogical when one class or order is
compared with another, but give true affinities when the members of the
same class or order are compared one with another: thus the shape of
the body and fin-like limbs are only analogical when whales are compared
with fishes, being adaptations in both classes for swimming through the
water; but the shape of the body and fin-like limbs serve as characters
exhibiting true affinity between the several members of the whale
family; for these cetaceans agree in so many characters, great and
small, that we cannot doubt that they have inherited their general shape
of body and structure of limbs from a common ancestor. So it is with
fishes.

As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,--to
inhabit for instance the three elements of land, air, and water,--we can
perhaps understand how it is that a numerical parallelism has sometimes
been observed between the sub-groups in distinct classes. A naturalist,
struck by a parallelism of this nature in any one class, by arbitrarily
raising or sinking the value of the groups in other classes (and all our
experience shows that this valuation has hitherto been arbitrary), could
easily extend the parallelism over a wide range; and thus the septenary,
quinary, quaternary, and ternary classifications have probably arisen.

As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages, which made the groups to which
they belong large and their parents dominant, they are almost sure to
spread widely, and to seize on more and more places in the economy
of nature. The larger and more dominant groups thus tend to go on
increasing in size; and they consequently supplant many smaller and
feebler groups. Thus we can account for the fact that all organisms,
recent and extinct, are included under a few great orders, under still
fewer classes, and all in one great natural system. As showing how
few the higher groups are in number, and how widely spread they are
throughout the world, the fact is striking, that the discovery of
Australia has not added a single insect belonging to a new order; and
that in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three orders of small size.

In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character
during the long-continued process of modification, how it is that the
more ancient forms of life often present characters in some slight
degree intermediate between existing groups. A few old and intermediate
parent-forms having occasionally transmitted to the present day
descendants but little modified, will give to us our so-called osculant
or aberrant groups. The more aberrant any form is, the greater must be
the number of connecting forms which on my theory have been exterminated
and utterly lost. And we have some evidence of aberrant forms having
suffered severely from extinction, for they are generally represented by
extremely few species; and such species as do occur are generally very
distinct from each other, which again implies extinction. The genera
Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented by a dozen species instead of by
a single one; but such richness in species, as I find after some
investigation, does not commonly fall to the lot of aberrant genera. We
can, I think, account for this fact only by looking at aberrant forms
as failing groups conquered by more successful competitors, with a
few members preserved by some unusual coincidence of favourable
circumstances.

Mr. Waterhouse has remarked that, when a member belonging to one group
of animals exhibits an affinity to a quite distinct group, this affinity
in most cases is general and not special: thus, according to Mr.
Waterhouse, of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this order, its
relations are general, and not to any one marsupial species more than
to another. As the points of affinity of the bizcacha to Marsupials are
believed to be real and not merely adaptive, they are due on my theory
to inheritance in common. Therefore we must suppose either that all
Rodents, including the bizcacha, branched off from some very ancient
Marsupial, which will have had a character in some degree intermediate
with respect to all existing Marsupials; or that both Rodents and
Marsupials branched off from a common progenitor, and that both groups
have since undergone much modification in divergent directions.
On either view we may suppose that the bizcacha has retained, by
inheritance, more of the character of its ancient progenitor than have
other Rodents; and therefore it will not be specially related to any one
existing Marsupial, but indirectly to all or nearly all Marsupials, from
having partially retained the character of their common progenitor, or
of an early member of the group. On the other hand, of all Marsupials,
as Mr. Waterhouse has remarked, the phascolomys resembles most nearly,
not any one species, but the general order of Rodents. In this case,
however, it may be strongly suspected that the resemblance is only
analogical, owing to the phascolomys having become adapted to habits
like those of a Rodent. The elder De Candolle has made nearly similar
observations on the general nature of the affinities of distinct orders
of plants.

On the principle of the multiplication and gradual divergence in
character of the species descended from a common parent, together with
their retention by inheritance of some characters in common, we can
understand the excessively complex and radiating affinities by which all
the members of the same family or higher group are connected together.
For the common parent of a whole family of species, now broken up by
extinction into distinct groups and sub-groups, will have transmitted
some of its characters, modified in various ways and degrees, to all;
and the several species will consequently be related to each other by
circuitous lines of affinity of various lengths (as may be seen in the
diagram so often referred to), mounting up through many predecessors.
As it is difficult to show the blood-relationship between the
numerous kindred of any ancient and noble family, even by the aid of a
genealogical tree, and almost impossible to do this without this aid,
we can understand the extraordinary difficulty which naturalists have
experienced in describing, without the aid of a diagram, the various
affinities which they perceive between the many living and extinct
members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account even for the
distinctness of whole classes from each other--for instance, of birds
from all other vertebrate animals--by the belief that many ancient forms
of life have been utterly lost, through which the early progenitors of
birds were formerly connected with the early progenitors of the other
vertebrate classes. There has been less entire extinction of the forms
of life which once connected fishes with batrachians. There has been
still less in some other classes, as in that of the Crustacea, for here
the most wonderfully diverse forms are still tied together by a long,
but broken, chain of affinities. Extinction has only separated groups:
it has by no means made them; for if every form which has ever lived
on this earth were suddenly to reappear, though it would be
quite impossible to give definitions by which each group could be
distinguished from other groups, as all would blend together by steps
as fine as those between the finest existing varieties, nevertheless
a natural classification, or at least a natural arrangement, would be
possible. We shall see this by turning to the diagram: the letters, A
to L, may represent eleven Silurian genera, some of which have produced
large groups of modified descendants. Every intermediate link between
these eleven genera and their primordial parent, and every intermediate
link in each branch and sub-branch of their descendants, may be supposed
to be still alive; and the links to be as fine as those between the
finest varieties. In this case it would be quite impossible to give any
definition by which the several members of the several groups could be
distinguished from their more immediate parents; or these parents from
their ancient and unknown progenitor. Yet the natural arrangement in the
diagram would still hold good; and, on the principle of inheritance, all
the forms descended from A, or from I, would have something in common.
In a tree we can specify this or that branch, though at the actual fork
the two unite and blend together. We could not, as I have said, define
the several groups; but we could pick out types, or forms, representing
most of the characters of each group, whether large or small, and thus
give a general idea of the value of the differences between them. This
is what we should be driven to, if we were ever to succeed in collecting
all the forms in any class which have lived throughout all time
and space. We shall certainly never succeed in making so perfect a
collection: nevertheless, in certain classes, we are tending in this
direction; and Milne Edwards has lately insisted, in an able paper, on
the high importance of looking to types, whether or not we can separate
and define the groups to which such types belong.

Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction
and divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages, although having few characters in common,
under one species; we use descent in classing acknowledged varieties,
however different they may be from their parent; and I believe this
element of descent is the hidden bond of connexion which naturalists
have sought under the term of the Natural System. On this idea of the
natural system being, in so far as it has been perfected, genealogical
in its arrangement, with the grades of difference between the
descendants from a common parent, expressed by the terms genera,
families, orders, etc., we can understand the rules which we are
compelled to follow in our classification. We can understand why we
value certain resemblances far more than others; why we are permitted to
use rudimentary and useless organs, or others of trifling physiological
importance; why, in comparing one group with a distinct group, we
summarily reject analogical or adaptive characters, and yet use these
same characters within the limits of the same group. We can clearly see
how it is that all living and extinct forms can be grouped together
in one great system; and how the several members of each class
are connected together by the most complex and radiating lines of
affinities. We shall never, probably, disentangle the inextricable web
of affinities between the members of any one class; but when we have
a distinct object in view, and do not look to some unknown plan of
creation, we may hope to make sure but slow progress.

MORPHOLOGY.

We have seen that the members of the same class, independently of
their habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general name of Morphology. This is the most interesting department
of natural history, and may be said to be its very soul. What can be
more curious than that the hand of a man, formed for grasping, that of a
mole for digging, the leg of the horse, the paddle of the porpoise, and
the wing of the bat, should all be constructed on the same pattern, and
should include the same bones, in the same relative positions? Geoffroy
St. Hilaire has insisted strongly on the high importance of relative
connexion in homologous organs: the parts may change to almost any
extent in form and size, and yet they always remain connected together
in the same order. We never find, for instance, the bones of the arm and
forearm, or of the thigh and leg, transposed. Hence the same names can
be given to the homologous bones in widely different animals. We see the
same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a
sphinx-moth, the curious folded one of a bee or bug, and the great jaws
of a beetle?--yet all these organs, serving for such different purposes,
are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine
of final causes. The hopelessness of the attempt has been expressly
admitted by Owen in his most interesting work on the 'Nature of Limbs.'
On the ordinary view of the independent creation of each being, we can
only say that so it is;--that it has so pleased the Creator to construct
each animal and plant.

The explanation is manifest on the theory of the natural selection of
successive slight modifications,--each modification being profitable
in some way to the modified form, but often affecting by correlation of
growth other parts of the organisation. In changes of this nature, there
will be little or no tendency to modify the original pattern, or to
transpose parts. The bones of a limb might be shortened and widened to
any extent, and become gradually enveloped in thick membrane, so as to
serve as a fin; or a webbed foot might have all its bones, or certain
bones, lengthened to any extent, and the membrane connecting them
increased to any extent, so as to serve as a wing: yet in all this great
amount of modification there will be no tendency to alter the framework
of bones or the relative connexion of the several parts. If we suppose
that the ancient progenitor, the archetype as it may be called, of all
mammals, had its limbs constructed on the existing general pattern,
for whatever purpose they served, we can at once perceive the plain
signification of the homologous construction of the limbs throughout the
whole class. So with the mouths of insects, we have only to suppose that
their common progenitor had an upper lip, mandibles, and two pair
of maxillae, these parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in structure
and function of the mouths of insects. Nevertheless, it is conceivable
that the general pattern of an organ might become so much obscured as to
be finally lost, by the atrophy and ultimately by the complete abortion
of certain parts, by the soldering together of other parts, and by the
doubling or multiplication of others,--variations which we know to be
within the limits of possibility. In the paddles of the extinct gigantic
sea-lizards, and in the mouths of certain suctorial crustaceans, the
general pattern seems to have been thus to a certain extent obscured.

There is another and equally curious branch of the present subject;
namely, the comparison not of the same part in different members of a
class, but of the different parts or organs in the same individual.
Most physiologists believe that the bones of the skull are homologous
with--that is correspond in number and in relative connexion with--the
elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in each member of the vertebrate and articulate classes
are plainly homologous. We see the same law in comparing the wonderfully
complex jaws and legs in crustaceans. It is familiar to almost every
one, that in a flower the relative position of the sepals, petals,
stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves,
arranged in a spire. In monstrous plants, we often get direct evidence
of the possibility of one organ being transformed into another; and we
can actually see in embryonic crustaceans and in many other animals, and
in flowers, that organs, which when mature become extremely different,
are at an early stage of growth exactly alike.

How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and such
extraordinarily shaped pieces of bone? As Owen has remarked, the
benefit derived from the yielding of the separate pieces in the act of
parturition of mammals, will by no means explain the same construction
in the skulls of birds. Why should similar bones have been created in
the formation of the wing and leg of a bat, used as they are for such
totally different purposes? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths?
Why should the sepals, petals, stamens, and pistils in any individual
flower, though fitted for such widely different purposes, be all
constructed on the same pattern?

On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae
bearing certain processes and appendages; in the articulata, we see the
body divided into a series of segments, bearing external appendages;
and in flowering plants, we see a series of successive spiral whorls of
leaves. An indefinite repetition of the same part or organ is the common
characteristic (as Owen has observed) of all low or little-modified
forms; therefore we may readily believe that the unknown progenitor of
the vertebrata possessed many vertebrae; the unknown progenitor of
the articulata, many segments; and the unknown progenitor of flowering
plants, many spiral whorls of leaves. We have formerly seen that
parts many times repeated are eminently liable to vary in number and
structure; consequently it is quite probable that natural selection,
during a long-continued course of modification, should have seized on
a certain number of the primordially similar elements, many times
repeated, and have adapted them to the most diverse purposes. And as
the whole amount of modification will have been effected by slight
successive steps, we need not wonder at discovering in such parts or
organs, a certain degree of fundamental resemblance, retained by the
strong principle of inheritance.

In the great class of molluscs, though we can homologise the parts of
one species with those of another and distinct species, we can indicate
but few serial homologies; that is, we are seldom enabled to say that
one part or organ is homologous with another in the same individual. And
we can understand this fact; for in molluscs, even in the lowest members
of the class, we do not find nearly so much indefinite repetition of
any one part, as we find in the other great classes of the animal and
vegetable kingdoms.

Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae: the jaws of crabs as metamorphosed legs; the stamens and
pistils of flowers as metamorphosed leaves; but it would in these cases
probably be more correct, as Professor Huxley has remarked, to speak
of both skull and vertebrae, both jaws and legs, etc.,--as having been
metamorphosed, not one from the other, but from some common element.
Naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent,
primordial organs of any kind--vertebrae in the one case and legs in the
other--have actually been modified into skulls or jaws. Yet so strong
is the appearance of a modification of this nature having occurred,
that naturalists can hardly avoid employing language having this plain
signification. On my view these terms may be used literally; and the
wonderful fact of the jaws, for instance, of a crab retaining numerous
characters, which they would probably have retained through inheritance,
if they had really been metamorphosed during a long course of descent
from true legs, or from some simple appendage, is explained.

EMBRYOLOGY.

It has already been casually remarked that certain organs in the
individual, which when mature become widely different and serve for
different purposes, are in the embryo exactly alike. The embryos, also,
of distinct animals within the same class are often strikingly similar:
a better proof of this cannot be given, than a circumstance mentioned
by Agassiz, namely, that having forgotten to ticket the embryo of some
vertebrate animal, he cannot now tell whether it be that of a mammal,
bird, or reptile. The vermiform larvae of moths, flies, beetles, etc.,
resemble each other much more closely than do the mature insects; but
in the case of larvae, the embryos are active, and have been adapted
for special lines of life. A trace of the law of embryonic resemblance,
sometimes lasts till a rather late age: thus birds of the same genus,
and of closely allied genera, often resemble each other in their first
and second plumage; as we see in the spotted feathers in the thrush
group. In the cat tribe, most of the species are striped or spotted
in lines; and stripes can be plainly distinguished in the whelp of
the lion. We occasionally though rarely see something of this kind in
plants: thus the embryonic leaves of the ulex or furze, and the first
leaves of the phyllodineous acaceas, are pinnate or divided like the
ordinary leaves of the leguminosae.

The points of structure, in which the embryos of widely different
animals of the same class resemble each other, often have no direct
relation to their conditions of existence. We cannot, for instance,
suppose that in the embryos of the vertebrata the peculiar loop-like
course of the arteries near the branchial slits are related to similar
conditions,--in the young mammal which is nourished in the womb of its
mother, in the egg of the bird which is hatched in a nest, and in the
spawn of a frog under water. We have no more reason to believe in such
a relation, than we have to believe that the same bones in the hand of
a man, wing of a bat, and fin of a porpoise, are related to similar
conditions of life. No one will suppose that the stripes on the whelp
of a lion, or the spots on the young blackbird, are of any use to these
animals, or are related to the conditions to which they are exposed.

The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. From such special adaptations,
the similarity of the larvae or active embryos of allied animals is
sometimes much obscured; and cases could be given of the larvae of two
species, or of two groups of species, differing quite as much, or
even more, from each other than do their adult parents. In most cases,
however, the larvae, though active, still obey more or less closely the
law of common embryonic resemblance. Cirripedes afford a good instance
of this: even the illustrious Cuvier did not perceive that a barnacle
was, as it certainly is, a crustacean; but a glance at the larva shows
this to be the case in an unmistakeable manner. So again the two main
divisions of cirripedes, the pedunculated and sessile, which differ
widely in external appearance, have larvae in all their several stages
barely distinguishable.

The embryo in the course of development generally rises in organisation:
I use this expression, though I am aware that it is hardly possible to
define clearly what is meant by the organisation being higher or lower.
But no one probably will dispute that the butterfly is higher than the
caterpillar. In some cases, however, the mature animal is generally
considered as lower in the scale than the larva, as with certain
parasitic crustaceans. To refer once again to cirripedes: the larvae in
the first stage have three pairs of legs, a very simple single eye, and
a probosciformed mouth, with which they feed largely, for they increase
much in size. In the second stage, answering to the chrysalis stage of
butterflies, they have six pairs of beautifully constructed natatory
legs, a pair of magnificent compound eyes, and extremely complex
antennae; but they have a closed and imperfect mouth, and cannot feed:
their function at this stage is, to search by their well-developed
organs of sense, and to reach by their active powers of swimming, a
proper place on which to become attached and to undergo their final
metamorphosis. When this is completed they are fixed for life: their
legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes
are now reconverted into a minute, single, and very simple eye-spot.
In this last and complete state, cirripedes may be considered as
either more highly or more lowly organised than they were in the larval
condition. But in some genera the larvae become developed either into
hermaphrodites having the ordinary structure, or into what I have called
complemental males: and in the latter, the development has assuredly
been retrograde; for the male is a mere sack, which lives for a short
time, and is destitute of mouth, stomach, or other organ of importance,
excepting for reproduction.

We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or
the fin of a porpoise, should not have been sketched out with all the
parts in proper proportion, as soon as any structure became visible in
the embryo. And in some whole groups of animals and in certain members
of other groups, the embryo does not at any period differ widely from
the adult: thus Owen has remarked in regard to cuttle-fish, "there is no
metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed;" and again in spiders, "there
is nothing worthy to be called a metamorphosis." The larvae of insects,
whether adapted to the most diverse and active habits, or quite
inactive, being fed by their parents or placed in the midst of proper
nutriment, yet nearly all pass through a similar worm-like stage of
development; but in some few cases, as in that of Aphis, if we look to
the admirable drawings by Professor Huxley of the development of this
insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology,--namely
the very general, but not universal difference in structure between the
embryo and the adult;--of parts in the same individual embryo, which
ultimately become very unlike and serve for diverse purposes, being
at this early period of growth alike;--of embryos of different species
within the same class, generally, but not universally, resembling each
other;--of the structure of the embryo not being closely related to its
conditions of existence, except when the embryo becomes at any period
of life active and has to provide for itself;--of the embryo apparently
having sometimes a higher organisation than the mature animal, into
which it is developed. I believe that all these facts can be explained,
as follows, on the view of descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily
appear at an equally early period. But we have little evidence on
this head--indeed the evidence rather points the other way; for it is
notorious that breeders of cattle, horses, and various fancy animals,
cannot positively tell, until some time after the animal has been born,
what its merits or form will ultimately turn out. We see this plainly in
our own children; we cannot always tell whether the child will be tall
or short, or what its precise features will be. The question is not, at
what period of life any variation has been caused, but at what period
it is fully displayed. The cause may have acted, and I believe generally
has acted, even before the embryo is formed; and the variation may be
due to the male and female sexual elements having been affected by
the conditions to which either parent, or their ancestors, have been
exposed. Nevertheless an effect thus caused at a very early period, even
before the formation of the embryo, may appear late in life; as when
an hereditary disease, which appears in old age alone, has been
communicated to the offspring from the reproductive element of one
parent. Or again, as when the horns of cross-bred cattle have been
affected by the shape of the horns of either parent. For the welfare of
a very young animal, as long as it remains in its mother's womb, or in
the egg, or as long as it is nourished and protected by its parent,
it must be quite unimportant whether most of its characters are fully
acquired a little earlier or later in life. It would not signify, for
instance, to a bird which obtained its food best by having a long beak,
whether or not it assumed a beak of this particular length, as long as
it was fed by its parents. Hence, I conclude, that it is quite possible,
that each of the many successive modifications, by which each species
has acquired its present structure, may have supervened at a not very
early period of life; and some direct evidence from our domestic animals
supports this view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared at an
extremely early period.

I have stated in the first chapter, that there is some evidence to
render it probable, that at whatever age any variation first appears
in the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages, for
instance, peculiarities in the caterpillar, cocoon, or imago states of
the silk-moth; or, again, in the horns of almost full-grown cattle. But
further than this, variations which, for all that we can see, might have
appeared earlier or later in life, tend to appear at a corresponding
age in the offspring and parent. I am far from meaning that this is
invariably the case; and I could give a good many cases of variations
(taking the word in the largest sense) which have supervened at an
earlier age in the child than in the parent.

These two principles, if their truth be admitted, will, I believe,
explain all the above specified leading facts in embryology. But first
let us look at a few analogous cases in domestic varieties. Some authors
who have written on Dogs, maintain that the greyhound and bulldog,
though appearing so different, are really varieties most closely allied,
and have probably descended from the same wild stock; hence I was
curious to see how far their puppies differed from each other: I was
told by breeders that they differed just as much as their parents, and
this, judging by the eye, seemed almost to be the case; but on actually
measuring the old dogs and their six-days old puppies, I found that
the puppies had not nearly acquired their full amount of proportional
difference. So, again, I was told that the foals of cart and race-horses
differed as much as the full-grown animals; and this surprised me
greatly, as I think it probable that the difference between these two
breeds has been wholly caused by selection under domestication; but
having had careful measurements made of the dam and of a three-days old
colt of a race and heavy cart-horse, I find that the colts have by no
means acquired their full amount of proportional difference.

As the evidence appears to me conclusive, that the several domestic
breeds of Pigeon have descended from one wild species, I compared young
pigeons of various breeds, within twelve hours after being hatched; I
carefully measured the proportions (but will not here give details) of
the beak, width of mouth, length of nostril and of eyelid, size of
feet and length of leg, in the wild stock, in pouters, fantails, runts,
barbs, dragons, carriers, and tumblers. Now some of these birds, when
mature, differ so extraordinarily in length and form of beak, that
they would, I cannot doubt, be ranked in distinct genera, had they been
natural productions. But when the nestling birds of these several breeds
were placed in a row, though most of them could be distinguished from
each other, yet their proportional differences in the above specified
several points were incomparably less than in the full-grown birds. Some
characteristic points of difference--for instance, that of the width
of mouth--could hardly be detected in the young. But there was one
remarkable exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild rock-pigeon and of the other
breeds, in all its proportions, almost exactly as much as in the adult
state.

The two principles above given seem to me to explain these facts in
regard to the later embryonic stages of our domestic varieties. Fanciers
select their horses, dogs, and pigeons, for breeding, when they are
nearly grown up: they are indifferent whether the desired qualities
and structures have been acquired earlier or later in life, if the
full-grown animal possesses them. And the cases just given, more
especially that of pigeons, seem to show that the characteristic
differences which give value to each breed, and which have been
accumulated by man's selection, have not generally first appeared at
an early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced tumbler,
which when twelve hours old had acquired its proper proportions,
proves that this is not the universal rule; for here the characteristic
differences must either have appeared at an earlier period than usual,
or, if not so, the differences must have been inherited, not at the
corresponding, but at an earlier age.

Now let us apply these facts and the above two principles--which latter,
though not proved true, can be shown to be in some degree probable--to
species in a state of nature. Let us take a genus of birds, descended
on my theory from some one parent-species, and of which the several new
species have become modified through natural selection in accordance
with their diverse habits. Then, from the many slight successive steps
of variation having supervened at a rather late age, and having been
inherited at a corresponding age, the young of the new species of our
supposed genus will manifestly tend to resemble each other much more
closely than do the adults, just as we have seen in the case of
pigeons. We may extend this view to whole families or even classes. The
fore-limbs, for instance, which served as legs in the parent-species,
may become, by a long course of modification, adapted in one descendant
to act as hands, in another as paddles, in another as wings; and on the
above two principles--namely of each successive modification supervening
at a rather late age, and being inherited at a corresponding late
age--the fore-limbs in the embryos of the several descendants of the
parent-species will still resemble each other closely, for they will not
have been modified. But in each individual new species, the embryonic
fore-limbs will differ greatly from the fore-limbs in the mature animal;
the limbs in the latter having undergone much modification at a rather
late period of life, and having thus been converted into hands, or
paddles, or wings. Whatever influence long-continued exercise or use on
the one hand, and disuse on the other, may have in modifying an organ,
such influence will mainly affect the mature animal, which has come
to its full powers of activity and has to gain its own living; and the
effects thus produced will be inherited at a corresponding mature age.
Whereas the young will remain unmodified, or be modified in a lesser
degree, by the effects of use and disuse.

In certain cases the successive steps of variation might supervene, from
causes of which we are wholly ignorant, at a very early period of life,
or each step might be inherited at an earlier period than that at which
it first appeared. In either case (as with the short-faced tumbler) the
young or embryo would closely resemble the mature parent-form. We have
seen that this is the rule of development in certain whole groups of
animals, as with cuttle-fish and spiders, and with a few members of the
great class of insects, as with Aphis. With respect to the final cause
of the young in these cases not undergoing any metamorphosis, or closely
resembling their parents from their earliest age, we can see that this
would result from the two following contingencies; firstly, from the
young, during a course of modification carried on for many generations,
having to provide for their own wants at a very early stage of
development, and secondly, from their following exactly the same habits
of life with their parents; for in this case, it would be indispensable
for the existence of the species, that the child should be modified at
a very early age in the same manner with its parents, in accordance with
their similar habits. Some further explanation, however, of the embryo
not undergoing any metamorphosis is perhaps requisite. If, on the other
hand, it profited the young to follow habits of life in any degree
different from those of their parent, and consequently to be constructed
in a slightly different manner, then, on the principle of inheritance at
corresponding ages, the active young or larvae might easily be rendered
by natural selection different to any conceivable extent from their
parents. Such differences might, also, become correlated with successive
stages of development; so that the larvae, in the first stage, might
differ greatly from the larvae in the second stage, as we have seen to
be the case with cirripedes. The adult might become fitted for sites or
habits, in which organs of locomotion or of the senses, etc., would be
useless; and in this case the final metamorphosis would be said to be
retrograde.

As all the organic beings, extinct and recent, which have ever lived on
this earth have to be classed together, and as all have been connected
by the finest gradations, the best, or indeed, if our collections were
nearly perfect, the only possible arrangement, would be genealogical.
Descent being on my view the hidden bond of connexion which naturalists
have been seeking under the term of the natural system. On this view
we can understand how it is that, in the eyes of most naturalists, the
structure of the embryo is even more important for classification than
that of the adult. For the embryo is the animal in its less modified
state; and in so far it reveals the structure of its progenitor. In
two groups of animal, however much they may at present differ from each
other in structure and habits, if they pass through the same or similar
embryonic stages, we may feel assured that they have both descended from
the same or nearly similar parents, and are therefore in that degree
closely related. Thus, community in embryonic structure reveals
community of descent. It will reveal this community of descent, however
much the structure of the adult may have been modified and obscured; we
have seen, for instance, that cirripedes can at once be recognised by
their larvae as belonging to the great class of crustaceans. As the
embryonic state of each species and group of species partially shows us
the structure of their less modified ancient progenitors, we can clearly
see why ancient and extinct forms of life should resemble the embryos of
their descendants,--our existing species. Agassiz believes this to be a
law of nature; but I am bound to confess that I only hope to see the
law hereafter proved true. It can be proved true in those cases alone in
which the ancient state, now supposed to be represented in many embryos,
has not been obliterated, either by the successive variations in a long
course of modification having supervened at a very early age, or by the
variations having been inherited at an earlier period than that at which
they first appeared. It should also be borne in mind, that the supposed
law of resemblance of ancient forms of life to the embryonic stages of
recent forms, may be true, but yet, owing to the geological record not
extending far enough back in time, may remain for a long period, or for
ever, incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are
second in importance to none in natural history, are explained on the
principle of slight modifications not appearing, in the many descendants
from some one ancient progenitor, at a very early period in the life of
each, though perhaps caused at the earliest, and being inherited at a
corresponding not early period. Embryology rises greatly in interest,
when we thus look at the embryo as a picture, more or less obscured, of
the common parent-form of each great class of animals.

RUDIMENTARY, ATROPHIED, OR ABORTED ORGANS.

Organs or parts in this strange condition, bearing the stamp of
inutility, are extremely common throughout nature. For instance,
rudimentary mammae are very general in the males of mammals: I presume
that the "bastard-wing" in birds may be safely considered as a digit
in a rudimentary state: in very many snakes one lobe of the lungs is
rudimentary; in other snakes there are rudiments of the pelvis and hind
limbs. Some of the cases of rudimentary organs are extremely curious;
for instance, the presence of teeth in foetal whales, which when grown
up have not a tooth in their heads; and the presence of teeth, which
never cut through the gums, in the upper jaws of our unborn calves. It
has even been stated on good authority that rudiments of teeth can be
detected in the beaks of certain embryonic birds. Nothing can be plainer
than that wings are formed for flight, yet in how many insects do we see
wings so reduced in size as to be utterly incapable of flight, and not
rarely lying under wing-cases, firmly soldered together!

The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same
species) resembling each other most closely in all respects, one
of which will have full-sized wings, and another mere rudiments of
membrane; and here it is impossible to doubt, that the rudiments
represent wings. Rudimentary organs sometimes retain their potentiality,
and are merely not developed: this seems to be the case with the mammae
of male mammals, for many instances are on record of these organs having
become well developed in full-grown males, and having secreted milk. So
again there are normally four developed and two rudimentary teats in
the udders of the genus Bos, but in our domestic cows the two sometimes
become developed and give milk. In individual plants of the same
species the petals sometimes occur as mere rudiments, and sometimes in
a well-developed state. In plants with separated sexes, the male flowers
often have a rudiment of a pistil; and Kolreuter found that by crossing
such male plants with an hermaphrodite species, the rudiment of the
pistil in the hybrid offspring was much increased in size; and this
shows that the rudiment and the perfect pistil are essentially alike in
nature.

An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules protected in the ovarium at
its base. The pistil consists of a stigma supported on the style; but in
some Compositae, the male florets, which of course cannot be fecundated,
have a pistil, which is in a rudimentary state, for it is not crowned
with a stigma; but the style remains well developed, and is clothed with
hairs as in other compositae, for the purpose of brushing the pollen out
of the surrounding anthers. Again, an organ may become rudimentary for
its proper purpose, and be used for a distinct object: in certain fish
the swim-bladder seems to be rudimentary for its proper function of
giving buoyancy, but has become converted into a nascent breathing organ
or lung. Other similar instances could be given.

Rudimentary organs in the individuals of the same species are very
liable to vary in degree of development and in other respects. Moreover,
in closely allied species, the degree to which the same organ has been
rendered rudimentary occasionally differs much. This latter fact is well
exemplified in the state of the wings of the female moths in certain
groups. Rudimentary organs may be utterly aborted; and this implies,
that we find in an animal or plant no trace of an organ, which analogy
would lead us to expect to find, and which is occasionally found
in monstrous individuals of the species. Thus in the snapdragon
(antirrhinum) we generally do not find a rudiment of a fifth stamen; but
this may sometimes be seen. In tracing the homologies of the same
part in different members of a class, nothing is more common, or more
necessary, than the use and discovery of rudiments. This is well shown
in the drawings given by Owen of the bones of the leg of the horse, ox,
and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the embryo,
but afterwards wholly disappear. It is also, I believe, a universal
rule, that a rudimentary part or organ is of greater size relatively to
the adjoining parts in the embryo, than in the adult; so that the organ
at this early age is less rudimentary, or even cannot be said to be in
any degree rudimentary. Hence, also, a rudimentary organ in the adult,
is often said to have retained its embryonic condition.

I have now given the leading facts with respect to rudimentary organs.
In reflecting on them, every one must be struck with astonishment: for
the same reasoning power which tells us plainly that most parts and
organs are exquisitely adapted for certain purposes, tells us with equal
plainness that these rudimentary or atrophied organs, are imperfect and
useless. In works on natural history rudimentary organs are generally
said to have been created "for the sake of symmetry," or in order "to
complete the scheme of nature;" but this seems to me no explanation,
merely a restatement of the fact. Would it be thought sufficient to
say that because planets revolve in elliptic courses round the sun,
satellites follow the same course round the planets, for the sake of
symmetry, and to complete the scheme of nature? An eminent physiologist
accounts for the presence of rudimentary organs, by supposing that they
serve to excrete matter in excess, or injurious to the system; but can
we suppose that the minute papilla, which often represents the pistil
in male flowers, and which is formed merely of cellular tissue, can thus
act? Can we suppose that the formation of rudimentary teeth which are
subsequently absorbed, can be of any service to the rapidly growing
embryonic calf by the excretion of precious phosphate of lime? When a
man's fingers have been amputated, imperfect nails sometimes appear on
the stumps: I could as soon believe that these vestiges of nails have
appeared, not from unknown laws of growth, but in order to excrete horny
matter, as that the rudimentary nails on the fin of the manatee were
formed for this purpose.

On my view of descent with modification, the origin of rudimentary
organs is simple. We have plenty of cases of rudimentary organs in our
domestic productions,--as the stump of a tail in tailless breeds,--the
vestige of an ear in earless breeds,--the reappearance of minute
dangling horns in hornless breeds of cattle, more especially, according
to Youatt, in young animals,--and the state of the whole flower in the
cauliflower. We often see rudiments of various parts in monsters. But
I doubt whether any of these cases throw light on the origin of
rudimentary organs in a state of nature, further than by showing that
rudiments can be produced; for I doubt whether species under nature ever
undergo abrupt changes. I believe that disuse has been the main agency;
that it has led in successive generations to the gradual reduction of
various organs, until they have become rudimentary,--as in the case of
the eyes of animals inhabiting dark caverns, and of the wings of birds
inhabiting oceanic islands, which have seldom been forced to take
flight, and have ultimately lost the power of flying. Again, an organ
useful under certain conditions, might become injurious under others,
as with the wings of beetles living on small and exposed islands; and in
this case natural selection would continue slowly to reduce the organ,
until it was rendered harmless and rudimentary.

Any change in function, which can be effected by insensibly small steps,
is within the power of natural selection; so that an organ rendered,
during changed habits of life, useless or injurious for one purpose,
might easily be modified and used for another purpose. Or an organ
might be retained for one alone of its former functions. An organ, when
rendered useless, may well be variable, for its variations cannot be
checked by natural selection. At whatever period of life disuse or
selection reduces an organ, and this will generally be when the being
has come to maturity and to its full powers of action, the principle
of inheritance at corresponding ages will reproduce the organ in its
reduced state at the same age, and consequently will seldom affect or
reduce it in the embryo. Thus we can understand the greater relative
size of rudimentary organs in the embryo, and their lesser relative size
in the adult. But if each step of the process of reduction were to
be inherited, not at the corresponding age, but at an extremely early
period of life (as we have good reason to believe to be possible) the
rudimentary part would tend to be wholly lost, and we should have a case
of complete abortion. The principle, also, of economy, explained in a
former chapter, by which the materials forming any part or structure, if
not useful to the possessor, will be saved as far as is possible, will
probably often come into play; and this will tend to cause the entire
obliteration of a rudimentary organ.

As the presence of rudimentary organs is thus due to the tendency
in every part of the organisation, which has long existed, to
be inherited--we can understand, on the genealogical view of
classification, how it is that systematists have found rudimentary
parts as useful as, or even sometimes more useful than, parts of high
physiological importance. Rudimentary organs may be compared with the
letters in a word, still retained in the spelling, but become useless
in the pronunciation, but which serve as a clue in seeking for its
derivation. On the view of descent with modification, we may conclude
that the existence of organs in a rudimentary, imperfect, and useless
condition, or quite aborted, far from presenting a strange difficulty,
as they assuredly do on the ordinary doctrine of creation, might
even have been anticipated, and can be accounted for by the laws of
inheritance.

SUMMARY.

In this chapter I have attempted to show, that the subordination of
group to group in all organisms throughout all time; that the nature of
the relationship, by which all living and extinct beings are united by
complex, radiating, and circuitous lines of affinities into one
grand system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters, if
constant and prevalent, whether of high vital importance, or of the most
trifling importance, or, as in rudimentary organs, of no importance; the
wide opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules;--all naturally follow
on the view of the common parentage of those forms which are considered
by naturalists as allied, together with their modification through
natural selection, with its contingencies of extinction and divergence
of character. In considering this view of classification, it should be
borne in mind that the element of descent has been universally used in
ranking together the sexes, ages, and acknowledged varieties of the same
species, however different they may be in structure. If we extend the
use of this element of descent,--the only certainly known cause of
similarity in organic beings,--we shall understand what is meant by the
natural system: it is genealogical in its attempted arrangement,
with the grades of acquired difference marked by the terms varieties,
species, genera, families, orders, and classes.

On this same view of descent with modification, all the great facts in
Morphology become intelligible,--whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on
the same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily
or generally supervening at a very early period of life, and being
inherited at a corresponding period, we can understand the great leading
facts in Embryology; namely, the resemblance in an individual embryo of
the homologous parts, which when matured will become widely different
from each other in structure and function; and the resemblance in
different species of a class of the homologous parts or organs, though
fitted in the adult members for purposes as different as possible.
Larvae are active embryos, which have become specially modified in
relation to their habits of life, through the principle of modifications
being inherited at corresponding ages. On this same principle--and
bearing in mind, that when organs are reduced in size, either from
disuse or selection, it will generally be at that period of life when
the being has to provide for its own wants, and bearing in mind how
strong is the principle of inheritance--the occurrence of rudimentary
organs and their final abortion, present to us no inexplicable
difficulties; on the contrary, their presence might have been even
anticipated. The importance of embryological characters and of
rudimentary organs in classification is intelligible, on the view that
an arrangement is only so far natural as it is genealogical.

Finally, the several classes of facts which have been considered in
this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera, and families of organic beings, with which this world
is peopled, have all descended, each within its own class or group, from
common parents, and have all been modified in the course of descent,
that I should without hesitation adopt this view, even if it were
unsupported by other facts or arguments.




14. RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural Selection.
Recapitulation of the general and special circumstances in its favour.
Causes of the general belief in the immutability of species. How far the
theory of natural selection may be extended. Effects of its adoption on
the study of Natural history. Concluding remarks.

As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I
have endeavoured to give to them their full force. Nothing at first can
appear more difficult to believe than that the more complex organs and
instincts should have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable
slight variations, each good for the individual possessor. Nevertheless,
this difficulty, though appearing to our imagination insuperably great,
cannot be considered real if we admit the following propositions,
namely,--that gradations in the perfection of any organ or instinct,
which we may consider, either do now exist or could have existed, each
good of its kind,--that all organs and instincts are, in ever so slight
a degree, variable,--and, lastly, that there is a struggle for existence
leading to the preservation of each profitable deviation of structure or
instinct. The truth of these propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially amongst
broken and failing groups of organic beings; but we see so many strange
gradations in nature, as is proclaimed by the canon, "Natura non facit
saltum," that we ought to be extremely cautious in saying that any organ
or instinct, or any whole being, could not have arrived at its present
state by many graduated steps. There are, it must be admitted, cases of
special difficulty on the theory of natural selection; and one of the
most curious of these is the existence of two or three defined castes
of workers or sterile females in the same community of ants; but I have
attempted to show how this difficulty can be mastered.

With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter,
which seem to me conclusively to show that this sterility is no more
a special endowment than is the incapacity of two trees to be grafted
together, but that it is incidental on constitutional differences in the
reproductive systems of the intercrossed species. We see the truth of
this conclusion in the vast difference in the result, when the same two
species are crossed reciprocally; that is, when one species is first
used as the father and then as the mother.

The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either
their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the varieties which have been
experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.

The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds
are rendered in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life, we need
not feel surprise at hybrids being in some degree sterile, for their
constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is supported
by another parallel, but directly opposite, class of facts; namely, that
the vigour and fertility of all organic beings are increased by slight
changes in their conditions of life, and that the offspring of slightly
modified forms or varieties acquire from being crossed increased vigour
and fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of
life and crosses between less modified forms, increase fertility.

Turning to geographical distribution, the difficulties encountered
on the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, must have descended from common parents; and
therefore, in however distant and isolated parts of the world they are
now found, they must in the course of successive generations have passed
from some one part to the others. We are often wholly unable even to
conjecture how this could have been effected. Yet, as we have reason to
believe that some species have retained the same specific form for very
long periods, enormously long as measured by years, too much stress
ought not to be laid on the occasional wide diffusion of the same
species; for during very long periods of time there will always be a
good chance for wide migration by many means. A broken or interrupted
range may often be accounted for by the extinction of the species in
the intermediate regions. It cannot be denied that we are as yet very
ignorant of the full extent of the various climatal and geographical
changes which have affected the earth during modern periods; and
such changes will obviously have greatly facilitated migration. As an
example, I have attempted to show how potent has been the influence
of the Glacial period on the distribution both of the same and of
representative species throughout the world. We are as yet profoundly
ignorant of the many occasional means of transport. With respect to
distinct species of the same genus inhabiting very distant and isolated
regions, as the process of modification has necessarily been slow,
all the means of migration will have been possible during a very long
period; and consequently the difficulty of the wide diffusion of species
of the same genus is in some degree lessened.

As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species
in each group by gradations as fine as our present varieties, it may be
asked, Why do we not see these linking forms all around us? Why are
not all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover DIRECTLY connecting links
between them, but only between each and some extinct and supplanted
form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life change
insensibly in going from a district occupied by one species into another
district occupied by a closely allied species, we have no just right to
expect often to find intermediate varieties in the intermediate zone.
For we have reason to believe that only a few species are undergoing
change at any one period; and all changes are slowly effected. I have
also shown that the intermediate varieties which will at first probably
exist in the intermediate zones, will be liable to be supplanted by the
allied forms on either hand; and the latter, from existing in greater
numbers, will generally be modified and improved at a quicker rate than
the intermediate varieties, which exist in lesser numbers; so that
the intermediate varieties will, in the long run, be supplanted and
exterminated.

On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why
is not every geological formation charged with such links? Why does
not every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? We meet with no such
evidence, and this is the most obvious and forcible of the many
objections which may be urged against my theory. Why, again, do whole
groups of allied species appear, though certainly they often falsely
appear, to have come in suddenly on the several geological stages? Why
do we not find great piles of strata beneath the Silurian system, stored
with the remains of the progenitors of the Silurian groups of fossils?
For certainly on my theory such strata must somewhere have been
deposited at these ancient and utterly unknown epochs in the world's
history.

I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect. The
number of specimens in all our museums is absolutely as nothing compared
with the countless generations of countless species which certainly have
existed. We should not be able to recognise a species as the parent
of any one or more species if we were to examine them ever so closely,
unless we likewise possessed many of the intermediate links between
their past or parent and present states; and these many links we
could hardly ever expect to discover, owing to the imperfection of the
geological record. Numerous existing doubtful forms could be named which
are probably varieties; but who will pretend that in future ages so
many fossil links will be discovered, that naturalists will be able
to decide, on the common view, whether or not these doubtful forms are
varieties? As long as most of the links between any two species are
unknown, if any one link or intermediate variety be discovered, it will
simply be classed as another and distinct species. Only a small portion
of the world has been geologically explored. Only organic beings of
certain classes can be preserved in a fossil condition, at least in any
great number. Widely ranging species vary most, and varieties are often
at first local,--both causes rendering the discovery of intermediate
links less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved; and
when they do spread, if discovered in a geological formation, they will
appear as if suddenly created there, and will be simply classed as new
species. Most formations have been intermittent in their accumulation;
and their duration, I am inclined to believe, has been shorter than the
average duration of specific forms. Successive formations are separated
from each other by enormous blank intervals of time; for fossiliferous
formations, thick enough to resist future degradation, can be
accumulated only where much sediment is deposited on the subsiding bed
of the sea. During the alternate periods of elevation and of stationary
level the record will be blank. During these latter periods there will
probably be more variability in the forms of life; during periods of
subsidence, more extinction.

With respect to the absence of fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in the
ninth chapter. That the geological record is imperfect all will admit;
but that it is imperfect to the degree which I require, few will be
inclined to admit. If we look to long enough intervals of time, geology
plainly declares that all species have changed; and they have changed in
the manner which my theory requires, for they have changed slowly and
in a graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to
each other, than are the fossils from formations distant from each other
in time.

Such is the sum of the several chief objections and difficulties
which may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them. I
have felt these difficulties far too heavily during many years to doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant;
nor do we know how ignorant we are. We do not know all the possible
transitional gradations between the simplest and the most perfect
organs; it cannot be pretended that we know all the varied means
of Distribution during the long lapse of years, or that we know how
imperfect the Geological Record is. Grave as these several difficulties
are, in my judgment they do not overthrow the theory of descent with
modification.

Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the reproductive
system being eminently susceptible to changes in the conditions of life;
so that this system, when not rendered impotent, fails to reproduce
offspring exactly like the parent-form. Variability is governed by many
complex laws,--by correlation of growth, by use and disuse, and by
the direct action of the physical conditions of life. There is
much difficulty in ascertaining how much modification our domestic
productions have undergone; but we may safely infer that the amount has
been large, and that modifications can be inherited for long periods.
As long as the conditions of life remain the same, we have reason to
believe that a modification, which has already been inherited for many
generations, may continue to be inherited for an almost infinite number
of generations. On the other hand we have evidence that variability,
when it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently
domesticated productions.

Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation, and causes variability. But man can and does select
the variations given to him by nature, and thus accumulate them in any
desired manner. He thus adapts animals and plants for his own benefit or
pleasure. He may do this methodically, or he may do it unconsciously by
preserving the individuals most useful to him at the time, without
any thought of altering the breed. It is certain that he can largely
influence the character of a breed by selecting, in each successive
generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been
the great agency in the production of the most distinct and useful
domestic breeds. That many of the breeds produced by man have to a large
extent the character of natural species, is shown by the inextricable
doubts whether very many of them are varieties or aboriginal species.

There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature.
In the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is
common to all organic beings. This high rate of increase is proved by
calculation, by the effects of a succession of peculiar seasons, and by
the results of naturalisation, as explained in the third chapter. More
individuals are born than can possibly survive. A grain in the balance
will determine which individual shall live and which shall die,--which
variety or species shall increase in number, and which shall decrease,
or finally become extinct. As the individuals of the same species
come in all respects into the closest competition with each other, the
struggle will generally be most severe between them; it will be almost
equally severe between the varieties of the same species, and next in
severity between the species of the same genus. But the struggle will
often be very severe between beings most remote in the scale of nature.
The slightest advantage in one being, at any age or during any season,
over those with which it comes into competition, or better adaptation
in however slight a degree to the surrounding physical conditions, will
turn the balance.

With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of defence,
or on the charms of the males; and the slightest advantage will lead to
victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected that organic beings would have varied
under nature, in the same way as they generally have varied under the
changed conditions of domestication. And if there be any variability
under nature, it would be an unaccountable fact if natural selection
had not come into play. It has often been asserted, but the assertion is
quite incapable of proof, that the amount of variation under nature is
a strictly limited quantity. Man, though acting on external characters
alone and often capriciously, can produce within a short period a
great result by adding up mere individual differences in his domestic
productions; and every one admits that there are at least individual
differences in species under nature. But, besides such differences, all
naturalists have admitted the existence of varieties, which they think
sufficiently distinct to be worthy of record in systematic works. No one
can draw any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and sub-species,
and species. Let it be observed how naturalists differ in the rank
which they assign to the many representative forms in Europe and North
America.

If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any way
useful to beings, under their excessively complex relations of life,
would be preserved, accumulated, and inherited? Why, if man can by
patience select variations most useful to himself, should nature fail in
selecting variations useful, under changing conditions of life, to her
living products? What limit can be put to this power, acting during long
ages and rigidly scrutinising the whole constitution, structure, and
habits of each creature,--favouring the good and rejecting the bad? I
can see no limit to this power, in slowly and beautifully adapting
each form to the most complex relations of life. The theory of natural
selection, even if we looked no further than this, seems to me to be in
itself probable. I have already recapitulated, as fairly as I could,
the opposed difficulties and objections: now let us turn to the special
facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can
see why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation,
and varieties which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that in each region
where many species of a genus have been produced, and where they now
flourish, these same species should present many varieties; for where
the manufactory of species has been active, we might expect, as a
general rule, to find it still in action; and this is the case if
varieties be incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of the larger
genera apparently have restricted ranges, and they are clustered in
little groups round other species--in which respects they resemble
varieties. These are strange relations on the view of each species
having been independently created, but are intelligible if all species
first existed as varieties.

As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of each
species will be enabled to increase by so much the more as they become
more diversified in habits and structure, so as to be enabled to seize
on many and widely different places in the economy of nature, there
will be a constant tendency in natural selection to preserve the most
divergent offspring of any one species. Hence during a long-continued
course of modification, the slight differences, characteristic of
varieties of the same species, tend to be augmented into the greater
differences characteristic of species of the same genus. New and
improved varieties will inevitably supplant and exterminate the older,
less improved and intermediate varieties; and thus species are rendered
to a large extent defined and distinct objects. Dominant species
belonging to the larger groups tend to give birth to new and dominant
forms; so that each large group tends to become still larger, and at
the same time more divergent in character. But as all groups cannot thus
succeed in increasing in size, for the world would not hold them, the
more dominant groups beat the less dominant. This tendency in the large
groups to go on increasing in size and diverging in character, together
with the almost inevitable contingency of much extinction, explains the
arrangement of all the forms of life, in groups subordinate to groups,
all within a few great classes, which we now see everywhere around us,
and which has prevailed throughout all time. This grand fact of the
grouping of all organic beings seems to me utterly inexplicable on the
theory of creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification;
it can act only by very short and slow steps. Hence the canon of "Natura
non facit saltum," which every fresh addition to our knowledge tends to
make more strictly correct, is on this theory simply intelligible. We
can plainly see why nature is prodigal in variety, though niggard in
innovation. But why this should be a law of nature if each species has
been independently created, no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have
been created to prey on insects on the ground; that upland geese, which
never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic insects;
and that a petrel should have been created with habits and structure
fitting it for the life of an auk or grebe! and so on in endless other
cases. But on the view of each species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill-occupied place in nature,
these facts cease to be strange, or perhaps might even have been
anticipated.

As natural selection acts by competition, it adapts the inhabitants
of each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of
any one country, although on the ordinary view supposed to have been
specially created and adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as far as we can
judge, absolutely perfect; and if some of them be abhorrent to our ideas
of fitness. We need not marvel at the sting of the bee causing the bee's
own death; at drones being produced in such vast numbers for one
single act, and being then slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fir-trees; at the instinctive hatred
of the queen bee for her own fertile daughters; at ichneumonidae feeding
within the live bodies of caterpillars; and at other such cases. The
wonder indeed is, on the theory of natural selection, that more cases of
the want of absolute perfection have not been observed.

The complex and little known laws governing variation are the same, as
far as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone,
they occasionally assume some of the characters of the species proper
to that zone. In both varieties and species, use and disuse seem to have
produced some effect; for it is difficult to resist this conclusion
when we look, for instance, at the logger-headed duck, which has wings
incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, which is occasionally
blind, and then at certain moles, which are habitually blind and have
their eyes covered with skin; or when we look at the blind animals
inhabiting the dark caves of America and Europe. In both varieties and
species correlation of growth seems to have played a most important
part, so that when one part has been modified other parts are
necessarily modified. In both varieties and species reversions to
long-lost characters occur. How inexplicable on the theory of creation
is the occasional appearance of stripes on the shoulder and legs of the
several species of the horse-genus and in their hybrids! How simply is
this fact explained if we believe that these species have descended from
a striped progenitor, in the same manner as the several domestic breeds
of pigeon have descended from the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created,
why should the specific characters, or those by which the species of
the same genus differ from each other, be more variable than the generic
characters in which they all agree? Why, for instance, should the colour
of a flower be more likely to vary in any one species of a genus, if
the other species, supposed to have been created independently, have
differently coloured flowers, than if all the species of the genus have
the same coloured flowers? If species are only well-marked varieties,
of which the characters have become in a high degree permanent, we can
understand this fact; for they have already varied since they branched
off from a common progenitor in certain characters, by which they have
come to be specifically distinct from each other; and therefore these
same characters would be more likely still to be variable than the
generic characters which have been inherited without change for an
enormous period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in any one species of a genus,
and therefore, as we may naturally infer, of great importance to the
species, should be eminently liable to variation; but, on my view, this
part has undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But
a part may be developed in the most unusual manner, like the wing of a
bat, and yet not be more variable than any other structure, if the part
be common to many subordinate forms, that is, if it has been inherited
for a very long period; for in this case it will have been rendered
constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We
can thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I have
attempted to show how much light the principle of gradation throws
on the admirable architectural powers of the hive-bee. Habit no doubt
sometimes comes into play in modifying instincts; but it certainly is
not indispensable, as we see, in the case of neuter insects, which leave
no progeny to inherit the effects of long-continued habit. On the view
of all the species of the same genus having descended from a common
parent, and having inherited much in common, we can understand how it is
that allied species, when placed under considerably different conditions
of life, yet should follow nearly the same instincts; why the thrush of
South America, for instance, lines her nest with mud like our British
species. On the view of instincts having been slowly acquired through
natural selection we need not marvel at some instincts being apparently
not perfect and liable to mistakes, and at many instincts causing other
animals to suffer.

If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws
in their degrees and kinds of resemblance to their parents,--in being
absorbed into each other by successive crosses, and in other such
points,--as do the crossed offspring of acknowledged varieties. On
the other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary
laws.

If we admit that the geological record is imperfect in an extreme
degree, then such facts as the record gives, support the theory of
descent with modification. New species have come on the stage slowly and
at successive intervals; and the amount of change, after equal intervals
of time, is widely different in different groups. The extinction of
species and of whole groups of species, which has played so conspicuous
a part in the history of the organic world, almost inevitably follows on
the principle of natural selection; for old forms will be supplanted
by new and improved forms. Neither single species nor groups of species
reappear when the chain of ordinary generation has once been broken. The
gradual diffusion of dominant forms, with the slow modification of their
descendants, causes the forms of life, after long intervals of time, to
appear as if they had changed simultaneously throughout the world.
The fact of the fossil remains of each formation being in some degree
intermediate in character between the fossils in the formations above
and below, is simply explained by their intermediate position in the
chain of descent. The grand fact that all extinct organic beings belong
to the same system with recent beings, falling either into the same or
into intermediate groups, follows from the living and the extinct being
the offspring of common parents. As the groups which have descended
from an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can see
why the more ancient a fossil is, the oftener it stands in some degree
intermediate between existing and allied groups. Recent forms are
generally looked at as being, in some vague sense, higher than ancient
and extinct forms; and they are in so far higher as the later and more
improved forms have conquered the older and less improved organic beings
in the struggle for life. Lastly, the law of the long endurance of
allied forms on the same continent,--of marsupials in Australia, of
edentata in America, and other such cases,--is intelligible, for within
a confined country, the recent and the extinct will naturally be allied
by descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading
facts in Distribution. We can see why there should be so striking a
parallelism in the distribution of organic beings throughout space, and
in their geological succession throughout time; for in both cases the
beings have been connected by the bond of ordinary generation, and the
means of modification have been the same. We see the full meaning of the
wonderful fact, which must have struck every traveller, namely, that on
the same continent, under the most diverse conditions, under heat and
cold, on mountain and lowland, on deserts and marshes, most of the
inhabitants within each great class are plainly related; for they will
generally be descendants of the same progenitors and early colonists.
On this same principle of former migration, combined in most cases with
modification, we can understand, by the aid of the Glacial period, the
identity of some few plants, and the close alliance of many others,
on the most distant mountains, under the most different climates; and
likewise the close alliance of some of the inhabitants of the sea in
the northern and southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the same physical
conditions of life, we need feel no surprise at their inhabitants
being widely different, if they have been for a long period completely
separated from each other; for as the relation of organism to organism
is the most important of all relations, and as the two areas will have
received colonists from some third source or from each other, at various
periods and in different proportions, the course of modification in the
two areas will inevitably be different.

On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that
many should be peculiar. We can clearly see why those animals which
cannot cross wide spaces of ocean, as frogs and terrestrial mammals,
should not inhabit oceanic islands; and why, on the other hand, new and
peculiar species of bats, which can traverse the ocean, should so often
be found on islands far distant from any continent. Such facts as the
presence of peculiar species of bats, and the absence of all other
mammals, on oceanic islands, are utterly inexplicable on the theory of
independent acts of creation.

The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the
same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species common to both still exist. Wherever many closely
allied yet distinct species occur, many doubtful forms and varieties of
the same species likewise occur. It is a rule of high generality that
the inhabitants of each area are related to the inhabitants of the
nearest source whence immigrants might have been derived. We see this in
nearly all the plants and animals of the Galapagos archipelago, of Juan
Fernandez, and of the other American islands being related in the most
striking manner to the plants and animals of the neighbouring American
mainland; and those of the Cape de Verde archipelago and other African
islands to the African mainland. It must be admitted that these facts
receive no explanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group,
and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies
of extinction and divergence of character. On these same principles
we see how it is, that the mutual affinities of the species and genera
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for classification;--why
adaptive characters, though of paramount importance to the being, are
of hardly any importance in classification; why characters derived from
rudimentary parts, though of no service to the being, are often of high
classificatory value; and why embryological characters are the most
valuable of all. The real affinities of all organic beings are due
to inheritance or community of descent. The natural system is a
genealogical arrangement, in which we have to discover the lines of
descent by the most permanent characters, however slight their vital
importance may be.

The framework of bones being the same in the hand of a man, wing of
a bat, fin of the porpoise, and leg of the horse,--the same number of
vertebrae forming the neck of the giraffe and of the elephant,--and
innumerable other such facts, at once explain themselves on the theory
of descent with slow and slight successive modifications. The similarity
of pattern in the wing and leg of a bat, though used for such different
purpose,--in the jaws and legs of a crab,--in the petals, stamens, and
pistils of a flower, is likewise intelligible on the view of the
gradual modification of parts or organs, which were alike in the early
progenitor of each class. On the principle of successive variations
not always supervening at an early age, and being inherited at a
corresponding not early period of life, we can clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
and arteries running in loops, like those in a fish which has to breathe
the air dissolved in water, by the aid of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often tend to reduce
an organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the
meaning of rudimentary organs. But disuse and selection will generally
act on each creature, when it has come to maturity and has to play its
full part in the struggle for existence, and will thus have little power
of acting on an organ during early life; hence the organ will not be
much reduced or rendered rudimentary at this early age. The calf, for
instance, has inherited teeth, which never cut through the gums of the
upper jaw, from an early progenitor having well-developed teeth; and we
may believe, that the teeth in the mature animal were reduced, during
successive generations, by disuse or by the tongue and palate having
been fitted by natural selection to browse without their aid; whereas in
the calf, the teeth have been left untouched by selection or disuse,
and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each
organic being and each separate organ having been specially created, how
utterly inexplicable it is that parts, like the teeth in the embryonic
calf or like the shrivelled wings under the soldered wing-covers of some
beetles, should thus so frequently bear the plain stamp of inutility!
Nature may be said to have taken pains to reveal, by rudimentary organs
and by homologous structures, her scheme of modification, which it seems
that we wilfully will not understand.

I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability
of species? It cannot be asserted that organic beings in a state of
nature are subject to no variation; it cannot be proved that the amount
of variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief that species
were immutable productions was almost unavoidable as long as the history
of the world was thought to be of short duration; and now that we have
acquired some idea of the lapse of time, we are too apt to assume,
without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.

But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the
term of a hundred million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an almost infinite
number of generations.

Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under
such expressions as the "plan of creation," "unity of design," etc., and
to think that we give an explanation when we only restate a fact. Any
one whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts
will certainly reject my theory. A few naturalists, endowed with
much flexibility of mind, and who have already begun to doubt on the
immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who will
be able to view both sides of the question with impartiality. Whoever
is led to believe that species are mutable will do good service by
conscientiously expressing his conviction; for only thus can the load of
prejudice by which this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that
a multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that
a multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority of
naturalists, and which consequently have every external characteristic
feature of true species,--they admit that these have been produced by
variation, but they refuse to extend the same view to other and very
slightly different forms. Nevertheless they do not pretend that they
can define, or even conjecture, which are the created forms of life, and
which are those produced by secondary laws. They admit variation as a
vera causa in one case, they arbitrarily reject it in another, without
assigning any distinction in the two cases. The day will come when this
will be given as a curious illustration of the blindness of preconceived
opinion. These authors seem no more startled at a miraculous act of
creation than at an ordinary birth. But do they really believe that at
innumerable periods in the earth's history certain elemental atoms have
been commanded suddenly to flash into living tissues? Do they believe
that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants
created as eggs or seed, or as full grown? and in the case of mammals,
were they created bearing the false marks of nourishment from the
mother's womb? Although naturalists very properly demand a full
explanation of every difficulty from those who believe in the mutability
of species, on their own side they ignore the whole subject of the first
appearance of species in what they consider reverent silence.

It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we may consider, by so much the arguments fall away
in force. But some arguments of the greatest weight extend very far.
All the members of whole classes can be connected together by chains of
affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up
very wide intervals between existing orders. Organs in a rudimentary
condition plainly show that an early progenitor had the organ in a
fully developed state; and this in some instances necessarily implies
an enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at an
embryonic age the species closely resemble each other. Therefore I
cannot doubt that the theory of descent with modification embraces all
the members of the same class. I believe that animals have descended
from at most only four or five progenitors, and plants from an equal or
lesser number.

Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see
this even in so trifling a circumstance as that the same poison often
similarly affects plants and animals; or that the poison secreted by
the gall-fly produces monstrous growths on the wild rose or oak-tree.
Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some
one primordial form, into which life was first breathed. When the views
entertained in this volume on the origin of species, or when analogous
views are generally admitted, we can dimly foresee that there will be a
considerable revolution in natural history. Systematists will be able
to pursue their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be in essence
a species. This I feel sure, and I speak after experience, will be no
slight relief. The endless disputes whether or not some fifty species
of British brambles are true species will cease. Systematists will
have only to decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a
far more essential consideration than it is at present; for differences,
however slight, between any two forms, if not blended by intermediate
gradations, are looked at by most naturalists as sufficient to raise
both forms to the rank of species. Hereafter we shall be compelled to
acknowledge that the only distinction between species and well-marked
varieties is, that the latter are known, or believed, to be connected
at the present day by intermediate gradations, whereas species
were formerly thus connected. Hence, without quite rejecting the
consideration of the present existence of intermediate gradations
between any two forms, we shall be led to weigh more carefully and to
value higher the actual amount of difference between them. It is quite
possible that forms now generally acknowledged to be merely varieties
may hereafter be thought worthy of specific names, as with the primrose
and cowslip; and in this case scientific and common language will come
into accordance. In short, we shall have to treat species in the same
manner as those naturalists treat genera, who admit that genera are
merely artificial combinations made for convenience. This may not be a
cheering prospect; but we shall at least be freed from the vain search
for the undiscovered and undiscoverable essence of the term species.

The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a history; when we contemplate every complex
structure and instinct as the summing up of many contrivances, each
useful to the possessor, nearly in the same way as when we look at
any great mechanical invention as the summing up of the labour, the
experience, the reason, and even the blunders of numerous workmen; when
we thus view each organic being, how far more interesting, I speak from
experience, will the study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects
of use and disuse, on the direct action of external conditions, and so
forth. The study of domestic productions will rise immensely in value.
A new variety raised by man will be a far more important and interesting
subject for study than one more species added to the infinitude of
already recorded species. Our classifications will come to be, as far as
they can be so made, genealogies; and will then truly give what may be
called the plan of creation. The rules for classifying will no doubt
become simpler when we have a definite object in view. We possess no
pedigrees or armorial bearings; and we have to discover and trace
the many diverging lines of descent in our natural genealogies, by
characters of any kind which have long been inherited. Rudimentary
organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species, which are called aberrant,
and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal
to us the structure, in some degree obscured, of the prototypes of each
great class.

When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not
very remote period descended from one parent, and have migrated
from some one birthplace; and when we better know the many means
of migration, then, by the light which geology now throws, and will
continue to throw, on former changes of climate and of the level of the
land, we shall surely be enabled to trace in an admirable manner
the former migrations of the inhabitants of the whole world. Even at
present, by comparing the differences of the inhabitants of the sea
on the opposite sides of a continent, and the nature of the various
inhabitants of that continent in relation to their apparent means of
immigration, some light can be thrown on ancient geography.

The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must not
be looked at as a well-filled museum, but as a poor collection made
at hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between
the successive stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We must be
cautious in attempting to correlate as strictly contemporaneous
two formations, which include few identical species, by the general
succession of their forms of life. As species are produced and
exterminated by slowly acting and still existing causes, and not
by miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism,--the improvement of
one being entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the lapse of actual
time. A number of species, however, keeping in a body might remain for a
long period unchanged, whilst within this same period, several of these
species, by migrating into new countries and coming into competition
with foreign associates, might become modified; so that we must not
overrate the accuracy of organic change as a measure of time. During
early periods of the earth's history, when the forms of life were
probably fewer and simpler, the rate of change was probably slower; and
at the first dawn of life, when very few forms of the simplest structure
existed, the rate of change may have been slow in an extreme degree. The
whole history of the world, as at present known, although of a length
quite incomprehensible by us, will hereafter be recognised as a mere
fragment of time, compared with the ages which have elapsed since
the first creature, the progenitor of innumerable extinct and living
descendants, was created.

In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation, that of the
necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants
of the world should have been due to secondary causes, like those
determining the birth and death of the individual. When I view all
beings not as special creations, but as the lineal descendants of some
few beings which lived long before the first bed of the Silurian system
was deposited, they seem to me to become ennobled. Judging from the
past, we may safely infer that not one living species will transmit its
unaltered likeness to a distant futurity. And of the species now living
very few will transmit progeny of any kind to a far distant futurity;
for the manner in which all organic beings are grouped, shows that the
greater number of species of each genus, and all the species of many
genera, have left no descendants, but have become utterly extinct. We
can so far take a prophetic glance into futurity as to foretel that it
will be the common and widely-spread species, belonging to the larger
and dominant groups, which will ultimately prevail and procreate new
and dominant species. As all the living forms of life are the lineal
descendants of those which lived long before the Silurian epoch, we may
feel certain that the ordinary succession by generation has never once
been broken, and that no cataclysm has desolated the whole world.
Hence we may look with some confidence to a secure future of equally
inappreciable length. And as natural selection works solely by and for
the good of each being, all corporeal and mental endowments will tend to
progress towards perfection.

It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different
from each other, and dependent on each other in so complex a manner,
have all been produced by laws acting around us. These laws, taken in
the largest sense, being Growth with Reproduction; Inheritance which is
almost implied by reproduction; Variability from the indirect and direct
action of the external conditions of life, and from use and disuse; a
Ratio of Increase so high as to lead to a Struggle for Life, and as a
consequence to Natural Selection, entailing Divergence of Character and
the Extinction of less-improved forms. Thus, from the war of nature,
from famine and death, the most exalted object which we are capable
of conceiving, namely, the production of the higher animals, directly
follows. There is grandeur in this view of life, with its several
powers, having been originally breathed into a few forms or into one;
and that, whilst this planet has gone cycling on according to the fixed
law of gravity, from so simple a beginning endless forms most beautiful
and most wonderful have been, and are being, evolved.