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
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