esticated 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 any
intention 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 inappreciable
except by an educated eye. This unconscious process of selection has
been the great 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 the rapid
increase of many animals and plants during a succession of peculiar
seasons, and when naturalised in new countries. More individuals are
born than can possibly survive. A grain in the balance may determine
which individuals 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. On the other hand the struggle will often
be severe between beings remote in the scale of nature. The slightest
advantage in certain individuals, at any age or during any season, over
those with which they come into competition, or better adaptatio