All plants and animals exhibit the tendency to vary, the causes of which
have yet to be ascertained; it is the tendency of the conditions of
life, at any given time, while favouring the existence of the variations
best adapted to them, to oppose that of the rest and thus to exercise
selection; and all living things tend to multiply without limit, while
the means of support are limited; the obvious cause of which is the
production of offspring more numerous than their progenitors, but with
actual expectation of life in the actuarial sense. Without tne first
tendency there could be no evolution. Without the second, there would be
no good reason why one variation should disappear and another take its
place; that is to say, there would be no selection. Without the third,
the struggle for existence, the agent of the selective process in the
state of nature, would vanish.