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Charles Darwin [1] Now let us turn to the other side of the argument. Under
domestication we see much variability. This seems to be mainly due to the
reproductive system being eminently susceptible to changes in the conditions
of life so that this system, when not rendered impotent, fails to
reproduce offspring exactly like the parent-form. Variability is
governed by many complex laws, -- by correlation of growth, by use and
disuse, and by the direct action of the physical conditions of life. There is
much difficulty in ascertaining how much modification our domestic
productions have undergone; but we may safely infer that the amount has been
large, and that modifications can be inherited for long periods. As long
as the conditions of life remain the same, we have reason to believe that
a modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations. On the
other hand we have evidence that variability, when it has once come into
play, does not wholly cease; for new varieties are still occasionally
produced by our most anciently domesticated productions. [2] Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts on the
organisation, and causes variability. But man
can and does select the variations given to him by nature, and thus
accumulate them in any desired manner. He thus adapts animals and
plants for his own benefit or pleasure. He may do this methodically,
or he may do it unconsciously by preserving the individuals most useful
to him at the time, without any thought of altering the breed. It is certain
that he can largely influence the character of a breed by selecting, in each
successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been the
great agency in the production of the most distinct and useful domestic
breeds. That many of the breeds produced by man have to a large extent the
character of natural species, is shown by the inextricable doubts whether
very many of them are varieties or aboriginal species. [3] There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In the
preservation of favoured individuals and races,
during the constantly-recurrent Struggle for Existence, we see the most
powerful and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is
common to all organic beings. This high rate of increase is proved by
calculation, by the effects of a succession of peculiar seasons, and by the
results of naturalisation, as explained in the
third chapter. More individuals are born than can possibly survive. A
grain in the balance will determine which individual shall live and which
shall die, -- which variety or species shall increase in number, and which
shall decrease, or finally become extinct. As the individuals of the same
species come in all respects into the closest competition with each other,
the struggle will generally be most severe between them; it will be almost
equally severe between the varieties of the same species, and next in
severity between the species of the same genus. But the struggle will often
be very severe between beings most remote in the scale of nature. The
slightest advantage in one being, at any age or during any season, over those
with which it comes into competition, or better adaptation in however slight
a degree to the surrounding physical conditions, will turn the balance. [4] With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most vigorous
individuals, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will often
depend on having special weapons or means of defence, or on the charms of the
males; and the slightest advantage will lead to victory. [5] As geology plainly proclaims that each land has undergone great
physical changes, we might have expected that organic beings would have
varied under nature, in the same way as they generally have varied under the
changed conditions of domestication. And if there be any variability under
nature, it would be an unaccountable fact if natural selection had not come
into play. It has often been asserted, but the assertion is quite incapable
of proof, that the amount of variation under nature is a strictly limited
quantity. Man, though acting on external characters alone and often
capriciously, can produce within a short period a great result by adding up
mere individual differences in his domestic productions; and every one admits
that there are at least individual differences in species under nature. But,
besides such differences, all naturalists have admitted the existence of
varieties, which they think sufficiently distinct to be worthy of record in
systematic works. No one can draw any clear distinction between individual
differences and slight varieties; or between more plainly marked varieties
and subspecies, and species. Let it be observed how naturalists differ in the
rank which they assign to the many representative forms in Europe and North
America. [6] If then we have under nature variability and a powerful agent
always ready to act and select, why should we doubt that variations in any
way useful to beings, under their excessively complex relations of life,
would be preserved, accumulated, and inherited? Why, if man can by patience
select variations most useful to himself, should nature fail in selecting
variations useful, under changing conditions of life, to her living products?
What limit can be put to this power, acting during long ages and rigidly scrutinising the whole constitution, structure, and
habits of each creature, — favouring the good and
rejecting the bad? I can see no limit to this power, in slowly and
beautifully adapting each form to the most complex relations of life. The
theory of natural selection, even if we looked no further than this, seems to
me to be in itself probable. I have already recapitulated, as fairly as I
could, the opposed difficulties and objections: now let us turn to the
special facts and arguments in favour of the
theory. [7] On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see why
it is that no line of demarcation can be drawn between species, commonly
supposed to have been produced by special acts of creation, and varieties
which are acknowledged to have been produced by secondary laws. On this same
view we can understand how it is that in each region where many species of a
genus have been produced, and where they now flourish, these same species
should present many varieties; for where the manufactory of species has been
active, we might expect, as a general rule, to find it still in action; and
this is the case if varieties be incipient species. Moreover, the species of
the large genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for they
differ from each other by a less amount of difference than do the species of
smaller genera. The closely allied species also of the larger genera
apparently have restricted ranges, and they are clustered in little groups
round other species -- in which respects they resemble varieties. These are
strange relations on the view of each species having been independently
created, but are intelligible if all species first existed as varieties. [8] As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of each
species will be enabled to increase by so much the more as they become more
diversified in habits and structure, so as to be enabled to seize on many and
widely different places in the economy of nature, there will be a constant
tendency in natural selection to preserve the most divergent offspring of any
one species. Hence during a long-continued course of modification, the slight
differences, characteristic of varieties of the same species, tend to be
augmented into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and exterminate
the older, less improved and intermediate varieties; and thus species are
rendered to a large extent defined and distinct objects. Dominant species
belonging to the larger groups tend to give birth to new and dominant forms;
so that each large group tends to become still larger, and at the same time
more divergent in character. But as all groups cannot thus succeed in
increasing in size, for the world would not hold them, the more dominant
groups beat the less dominant. This tendency in the large groups to go on
increasing in size and diverging in character, together with the almost
inevitable contingency of much extinction, explains the arrangement of all
the forms of life, in groups subordinate to groups, all within a few great
classes, which we now see everywhere around us, and which has prevailed
throughout all time. This grand fact of the grouping of all organic beings
seems to me utterly inexplicable on the theory of creation. [9] As natural selection acts solely by accumulating slight,
successive, favourable variations, it can produce
no great or sudden modification; it can act only by very short and slow
steps. Hence the canon of `Natura non facit saltum,' which every
fresh addition to our knowledge tends to make more strictly correct, is on
this theory simply intelligible. We can plainly see why nature is prodigal in
variety, though niggard in innovation. But why this should be a law of nature
if each species has been independently created, no man can explain. [10] Many other facts are, as it seems to me, explicable on this
theory. How strange it is that a bird, under the form of woodpecker, should
have been created to prey on insects on the ground; that upland geese, which
never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic insects; and
that a petrel should have been created with habits and structure fitting it
for the life of an auk or grebe! and so on in endless
other cases. But on the view of each species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill-occupied place in nature, these
facts cease to be strange, or perhaps might even have been anticipated. [11] As natural selection acts by competition, it adapts the
inhabitants of each country only in relation to the degree of perfection of
their associates; so that we need feel no surprise at the inhabitants of any
one country, although on the ordinary view supposed to have been specially
created and adapted for that country, being beaten and supplanted by the naturalised productions from another land. Nor ought we
to marvel if all the contrivances in nature be not, as far as we can judge,
absolutely perfect; and if some of them be abhorrent to our ideas of fitness.
We need not marvel at the sting of the bee causing the bee's own death; at
drones being produced in such vast numbers for one single act, and being then
slaughtered by their sterile sisters; at the astonishing waste of pollen by
our fir-trees; at the instinctive hatred of the queen bee for her own fertile
daughters; at ichneumonidae feeding within the live
bodies of caterpillars; and at other such cases. The wonder indeed is, on the
theory of natural selection, that more cases of the want of absolute
perfection have not been observed. [12] The complex and little known laws governing variation are the
same, as far as we can see, with the laws which have governed the production
of so-called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone, they
occasionally assume some of the characters of the species proper to that
zone. In both varieties and species, use and disuse seem to have produced
some effect; for it is difficult to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings incapable of flight, in
nearly the same condition as in the domestic duck; or when we look at the
burrowing tucutucu, which is occasionally blind,
and then at certain moles, which are habitually blind and have their eyes
covered with skin; or when we look at the blind animals inhabiting the dark
caves of America and Europe. In both varieties and species correction of
growth seems to have played a most important part, so that when one part has
been modified other parts are necessarily modified. In both varieties and
species reversions to long-lost characters occur. How inexplicable on the
theory of creation is the occasional appearance of stripes on the shoulder
and legs of the several species of the horse-genus and in their hybrids! How
simply is this fact explained if we believe that these species have descended
from a striped progenitor, in the same manner as the several domestic breeds
of pigeon have descended from the blue and barred rock-pigeon! |