On
the Origin of Species

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.

CHAPTER I.
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
Müller 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 Canidæ—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 vertebræ 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 scutellæ 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
Columbidæ 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 Columbidæ, 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 encyclopædia. 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.

CHAPTER II.
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 larvæ 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 Gärtner, 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.

CHAPTER III.
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. Linnæus 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.

CHAPTER IV.
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 larvæ. 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 Leguminosæ, 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 Gärtner, 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 Kölreuter
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 Europæo-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 the 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.

CHAPTER V.
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 Schiödte 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 Schiödte 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 antennæ 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 Encyclopædias 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 Compositæ countenances
this idea; but, in the case of the corolla of the Umbelliferæ, 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 Compositæ 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 Umbelliferæ
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 antennæ. 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 vertebræ 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 Engidæ, 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.

CHAPTER VI.
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 Lemuridæ, 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 aërial 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 branchiæ 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 branchiæ 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 branchiæ 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 branchiæ 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 branchiæ, the whole surface of the body and sack, including
the small frena, serving for respiration. The Balanidæ 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 branchiæ.
Now I think no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchiæ 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 branchiæ, 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 branchiæ 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.

CHAPTER VII.
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
antennæ; 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 antennæ on the
abdomen first of one aphis and then of another; and each aphis, as soon as it
felt the antennæ, 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
Gallinaceæ; 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 Sphegidæ (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 larvæ 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
larvæ. 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 larvæ and pupæ 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 larvæ, 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
larvæ and pupæ 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 pupæ to rear as slaves. I then dug up a small
parcel of the pupæ 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 pupæ 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 pupæ 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 pupæ of F. fusca, whereas they
were much terrified when they came across the pupæ, 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 pupæ.

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 pupæ. 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 larvæ, 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 larvæ. 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 pupæ of other species, if scattered near their nests, it is possible
that pupæ 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 pupæ 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 larvæ
of ichneumonidæ 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.

CHAPTER VIII.
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, Kölreuter and Gärtner, who almost
devoted their lives to this subject, without being deeply impressed with the
high generality of some degree of sterility. Kölreuter 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. Gärtner, also, makes the rule equally
universal; and he disputes the entire fertility of Kölreuter’s ten cases.
But in these and in many other cases, Gärtner 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
Gärtner 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 Gärtner 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 Leguminosæ, in which there is an acknowledged difficulty
in the manipulation) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gärtner 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 Gärtner 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, Kölreuter and Gärtner, 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 Gärtner
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 Gärtner, 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
Gärtner 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
Kölreuter and Gärtner 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 Gärtner. 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 Gärtner 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 Gärtner’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 Gärtner 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
Kölreuter. 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 Kölreuter 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.
Gärtner, 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

Gärtner: 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 Gärtner 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 Gärtner 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. Gärtner 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 Gärtner 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 Gärtner: 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.

Kölreuter, 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. Gärtner, 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 Gärtner 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. Gärtner 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: Gärtner 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. Gärtner 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 Kölreuter.

These alone are the unimportant differences, which Gärtner 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 Gärtner 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.

CHAPTER IX.
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
palæontological 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.