Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the
reproductive system being eminently susceptible to changes in the
conditions of life; so that this system, when not rendered impotent,
fails to reproduce offspring exactly like the parent-form. Variability
is governed by many complex laws, by correlation of growth, by use and
disuse, and by the direct action of the physical conditions of life.
There is much difficulty in ascertaining how much modification our
domestic productions have undergone; but we may safely infer that the
amount has been large, and that modifications can be inherited for long
periods. As long as the conditions of life remain the same, we have
reason to believe that a modification, which has already been inherited
for many generations, may continue to be inherited for an almost
infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not wholly cease;
for new varieties are still occasionally produced by our most anciently
domesticated productions.
Man does not actually produce variability; he only
unintentionally exposes organic beings to new conditions of life, and
then nature acts on the organisation, and causes variability. But man
can and does select the variations given to him by nature, and thus
accumulate them in any desired manner. He thus adapts animals and
plants for his own benefit or pleasure. He may do this methodically, or
he may do it unconsciously by preserving the individuals most useful to
him at the time, without any thought of altering the breed. It is
certain that he can largely influence the character of a breed by
selecting, in each successive generation, individual differences so
slight as to be quite inappreciable by an uneducated eye. This process
of selection has been the great agency in the production of the most
distinct and useful domestic breeds. That many of the breeds produced
by man have to a large extent the character of natural species, is
shown by the inextricable doubts whether very many of them are
varieties or aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
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