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Excerpts
from the HTML
Electronic Text at the web site of the Wonderland (taken from the
original ascii text by the Eris Project, Virginia
Tech). [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! |