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.
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