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 domestication for a very long
period; nor do we know that it ever ceases, for new varieties are still
occasionally produced by our oldest domesticated productions.

Variability is not actually caused by man; he only unintentionally
exposes organic beings to new conditions of life, and then Nature acts
on the organisation 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 may do it unconsciously by
preserving the individuals most useful or pleasing to him without an
intention of altering the breed.

It is certain that he can influence the character of a breed by
selecting, in each successive generation, individual differences so
slight as to be inappreciable except by an educated eye. This
unconscious process of selection has been the 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 favoured individuals and races, during the constantly recurrent
struggle for existence, we see a powerful and ever-acting form 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