Charles Darwin.

The origin of species online

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sterility in their modified descendants when crossed. This elimination of
sterility apparently follows from the same cause which allows our domestic
animals to breed freely under diversified circumstances; and this again ap-
parently follows from their having been gradually accustomed to frequent
changes in their conditions of life.

A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring. On the
one side, there is good reason to believe that slight changes in the condi-
tions of life give vigor and fertility to all organic beings. We know also that
a cross between the distinct individuals of the same variety, and between dis-
tinct varieties, increases the number of their offspring, and certainly gives to
them increased size and vigor. This is chiefly owing to the forms which are
crossed having been exposed to somewhat different conditions of life; for I
have ascertained by a laborious series of experiments that if all the individ-
uals of the same variety be subjected during several generations to the same
conditions, the good derived from crossing is often much diminished or
w^holly disappears. This is one side of the case. On the other side, we know
that species which have long been exposed to nearly uniform conditions,
when they are subjected under confinement to new and greatly changed con-
ditions, either perish, or if they survive, are rendered sterile, though retain-
ing perfect health. This does not occur, or only in a very slight degree, with
our domesticated productions, which have long been exposed to fluctuating
conditions. Hence when we find that hybrids produced by a cross between
two distinct species are few in number, owing to their perishing soon after
conception or at a very early age, or if surviving that they are rendered more
or less sterile, it seems highly probable that this result is due to their having
been in fact subjected to a great change in their conditions of life, from being
compounded of two distinct organizations. He who will explain in a definite
manner why, for instance, an elephant or a fox will not breed under confine-
ment in its native country, whilst the domestic pig or dog will breed freely
under the most diversified conditions, will at the same time be able to give a
definite answer to the question why two distinct species, when crossed, as
well as their hybrid offspring, are generally rendered more or less sterile.


while two domesticated varieties when crossed and their mongrel offspring
are perfectly fertile.

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

As according to the theory of natural selection an interminable number
of intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our existing varieties, it may be asked,
Why do we not see these linking forms all around us? Why are not all organic
beings blended together in an inextricable chaos? With respect to existing
forms, we should remember that we have no right to expect (excepting in
rare cases) to discover directly connecting links between them, but only be-
tween each and some extinct and supplanted form. Even on a wide area,
which has during a long period remained continuous, and of which the cli-
matic and other conditions of life change insensibly in proceeding from a
district occupied by one species into another district occupied by a closely
allied species, we have no just right to expect often to find intermediate
varieties in the intermediate zones. For we have reason to believe that only
a few species of a genus ever undergo change; the other species becoming
utterly extinct and leaving no modified progeny. Of the species which do
change, only a few within the same country change at the same time; and
all modifications are slowly effected. I have also shown that the intermediate
varieties which probably at first existed in the intermediate zones, would be
liable to be supplanted by the allied forms on either hand; for the latter, from


existing in greater numbers, would generally be modified and improved at a
quicker rate than the intermediate varieties, which existed in lesser numbers ;
so that the intermediate varieties would, in the long run, be supplanted and

On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each suc-
cessive period between the extinct and still older species, why is not every
geological formation charged with such links? Why does not every col-
lection of fossil remains afford plain evidence of the gradation and muta-
tion of the forms of life? Although geological research has undoubtedly re-
vealed the former existence of many links, bringing numerous forms of life
much closer together, it does not yield the infinitely many fine gradations
between past and present species required on the theory, and this is the most
obvious of the many objections which may be urged against it. Why, again,
do whole groups of allied species appear, though this appearance is often
false, to have come in suddenly on the successive geological stages? Although
we now know that organic beings appeared on this globe, at a period incal-
culably remote, long before the lowest bed of the Cambrian system was de-
posited, why do we not find beneath this system great piles of strata stored
with the remains of the progenitors of the Cambrian fossils? For on the
theory, such strata must somewhere have been deposited at these ancient and
utterly unknown epochs of the world's history.

I can answer these questions and objections only on the supposition that
the geological record is far more imperfect than most geologists believe. The
number of specimens in all our museums is absolutely as nothing compared
with the countless generations of countless species which have certainly ex-
isted. The parent form of any two or more species would not be in all its
characters directly intermediate between its modified offspring, any more
than the rock-pigeon is directly intermediate in crop and tail between its
descendants, the pouter and fantail pigeons. We should not be able to recog-
nize a species as the parent of another and modified species, if we were to
examine the two ever so closely, unless we possessed most of the intermediate
links; and owing to the imperfection of the geological record, we have no just
right to expect to find so many links. If two or three, or even more linking
forms were discovered, they would simply be ranked by many naturalists as
so many new species, more especially if found in different geological sub-
stages, let their difTerences be ever so slight. Numerous existing doubtful
forms could be named which are probably varieties; but who will pretend
that in future ages so many fossil links will be discovered, that naturalists will
be able to decide whether or not these doubtful forms ought to be called
varieties? Only a small portion of the world has been geologically explored.
Only organic beings of certain classes can be preserved in a fossil condition,
at least in any great number. Many species when once formed never undergo
any further change, but become extinct without leaving modified descend-
ants; and the periods during which species have undergone modification,


though long as measured by years, have probably been short in comparison
with the periods during which they retained the same form. It is the domi-
nant and widely ranging species which vary most frequently and vary most,
and varieties are often at first local — both causes rendering the discovery of
intermediate links in any one formation less likely. Local varieties will not
spread into other and distant regions until they are considerably modified
and improved ; and when they have spread, and are discovered in a geological
formation, they appear as if suddenly created there, and will be simply classed
as new species. Most formations have been intermittent in their accumula-
tion, and their duration has probably been shorter than the average duration
of specific forms. Successive formations are in most cases separated from each
other by blank intervals of time of great length, for fossiliferous formations
thick enough to resist future degradation can, as a general rule, be accumu-
lated only where much sediment is deposited on the subsiding bed of the sea.
During the alternate periods of elevation and of stationary level, the record
will generally be blank. During these latter periods there will probably be
more variability in the forms of life; during periods of subsidence, more ex-

With respect to the absence of strata rich in fossils beneath the Cambrian
formation, I can recur only to the hypothesis given in the tenth chapter;
namely, that though our continents and oceans have endured for an enor-
mous period in nearly their present relative positions, we have no reason to
assume that this has always been the case; consequently formations much
older than any now known may lie buried beneath the great oceans. With
respect to the lapse of time not having been sufficient since our planet was
consolidated for the assumed amount of organic change, and this objection,
as urged by Sir William Thompson, is probably one of the gravest as yet ad-
vanced, I can only say, firstly, that we do not know at what rate species
change, as measured by years, and secondly, that many philosophers are not
as yet willing to admit that we know enough of the constitution of the uni-
verse and of the interior of our globe to speculate with safety on its past du-

That the geological record is imperfect, all will admit; but that it is im-
perfect to the degree required by our theory, few will be inclined to admit.
If we look to long enough intervals of time, geology plainly declares that
species have all changed ; and they have changed in the manner required by
the theory, for they have changed slowly and in a graduated manner. We
clearly see this in the fossil remains from consecutive formations invariably
being much more closely related to each other than are the fossils from widely
separated formations.

Such is the sum. of the several chief objections and difficulties which may
be justly urged against the theory; and I have now briefly recapitulated the
answers and explanations which, as far as I can see, may be given. I have felt
these difficulties far too heavily during many years to doubt their weight.
But it deserves especial notice that the more important objections relate to
questions on which we are confessedly ignorant; nor do we know how igno-


rant we are. We do not know all the possible transitional gradations between
the simplest and the most perfect organs; it cannot be pretended that we
know all the varied means of Distribution during the long lapse of years, or
that we know how imperfect is the Geological Record. Serious as these sev-
eral objections are, in my judgment they are by no means sufficient to over-
throw the theory of descent with subsequent modification.

Now let us turn to the other side of the argument. Under domestication
we see much variability caused, or at least excited, by changed conditions of
life; but often in so obscure a manner, that we are tempted to consider the
variations as spontaneous. Variability is governed by many complex laws, by
correlated growth, compensation, the increased use and disuse of parts, and
the definite action of the surrounding conditions. There is much difficulty in
ascertaining how largely our domestic productions have been modified; but
we may safely infer that the amount has been large, and that modifications
can be inherited for long periods. As long as the conditions of. life remain the
same, we have reason to believe that a modification, which has already been
inherited for many generations, may continue to be inherited for an almost
infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not cease under domestica-
tion for a very long period; nor do we know that it ever ceases, for new
varieties are still occasionally produced by our oldest domesticated produc-

Variability is not actually caused by man ; he only unintentionally exposes
organic beings to new conditions of life, and then nature acts on the organiza-
tion and causes it to vary. But man can and does select the variations given to
him by nature, and thus accumulates them in any desired manner. He thus
adapts animals and plants for his own benefit or pleasure. He may do this
methodically, or he raay do it unconsciously by preserving the individuals
most useful or pleasing to him without any intention of altering the breed. It
is certain that he can largely influence the character of a breed by selecting,
in each successive generation, individual differences so slight as to be inap-
preciable except by an educated eye. This unconscious process of selection
has been the great agency in the formation of the most distinct and useful
domestic breeds. That many breeds produced by man have to a large extent
the character of natural species, is shown by the inextricable doubts whether
many of them are varieties or aboriginally distinct species.

There is no reason why the principles which have acted so efficiently under
domestication should not have acted under nature. In the survival of favored
individuals and races, during the constantly recurrent Struggle for Existence,
we see a powerful and ever-acting form of Selection. The struggle for ex-
istence inevitably follows from the high geometrical ratio of increase which
is common to all organic beings. This high rate of increase is proved by cal-
culation — by the rapid increase of many animals and plants during a suc-
cession of peculiar seasons, and when naturalized in new countries. More in-
dividuals are born than can possibly survive. A grain in the balance may


determine which individuals shall live, and which shall die — ^which variety
or species shall increase in number, and which shall decrease, or finally be-
come extinct. As the individuals of the same species come in all respects into
the closest competition with each other, the struggle will generally be most
severe between them; it will be almost equally severe between the varieties
of the same species, and next in severity between the species of the same
genus. On the other hand the struggle will often be severe between beings
remote in the scale of nature. The slightest advantage in certain individuals,
at any age or during any season, over those with which they come into com-
petition, or better adaptation in however slight a degree to the surrounding
physical conditions, will, in the long-run, turn the balance.

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

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under nature, in the same way as they have varied under domestication. And
if there has been any variability under nature, it would be an unaccountable
fact if natural selection had not come into play. It has often been asserted,
but the assertion is incapable of proof, that the amount of variation under
nature is a strictly limited quantity. Man, though acting on external charac-
ters alone and often capriciously, can produce within a short period a great
result by adding up mere individual differences in his domestic productions;
and every one admits that species present individual differences. But, beside
such differences, all naturalists admit that natural varieties exist, which are
considered sufficiently distinct to be worthy of record in systematic works.
No one has drawn any clear distinction between individual differences and
slight varieties ; or between more plainly marked varieties and sub-species and i
species. On separate continents, and on different parts of the same continent,
when divided by barriers of any kind, and on out-lying islands, what a mul-
titude of forms exist, which some experienced naturalists rank as varieties,
others as geographical races or sub-species, and others as distinct though
closely allied species!

If, then, animals and plants do vary, let it be ever so slightly or slowly,
why should not variations or individual differences, which are in any way
beneficial, be preserved and accumulated through natural selection, or the
survival of the fittest? If man can by patience select variations useful to him,
why, under changing and complex conditions of life, should not variations
useful to nature's living products often arise, and be preserved or selected?
What limit can be put to this power, acting during long ages and rigidly]
scrutinizing the whole constitution, structure, and habits of each creature,]
favoring the good and rejecting the bad? I can see no limit to this power, in
slowly and beautifully adapting each form to the most complex relations of


life. The theory of natural selection, even if we look no further than this,
seems to be in the highest degree probable. I have already recapitulated, as
fairly as I could, the opposed difficulties and objections: now let us turn to
the special facts and arguments in favor of the theory.

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

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

As natural selection acts solely by accumulating slight, successive, favorable


variations, it can produce no great or sudden modifications; it can act only
by short and slow steps. Hence, the canon of "Natura non facit saltum,"
which every fresh addition to our knowledge tends to confirm, is on this
theory intelligible. We can see why throughout nature the same general end
is gained by an almost infinite diversity of means, for every peculiarity when
once acquired is long inherited, and structures already modified in many dif-
ferent ways have to be adapted for the same general purpose. We can, in
short, see why nature is prodigal in variety, though niggard in innovation.
But why this should be a law of nature if each species has been independ-
ently created, no man can explain.

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

We can to a certain extent understand how it is that there is so much
beauty throughout nature; for this may be largely attributed to the agency
of selection. That beauty, according to our sense of it, is not universal, must
be admitted by every one who will look at some venomous snakes, at some
fishes, and at certain hideous bats with a distorted resemblance to the human

Online LibraryCharles DarwinThe origin of species → online text (page 46 of 50)