* See "Evolution in Biology," Essays, vol. ii. p. 187
With none of these have I anything to do, at present, except with that
exhibited by the forms of life which tenant the earth. 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
equal expectation of life in the actuarial sense. Without the 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 [8]
third, the struggle for existence, the agent of the selective process
in the state of nature, would vanish.*
* Collected Essays, vol. ii. passim.
Granting the existence of these tendencies, all the known facts of the
history of plants and of animals may be brought into rational
correlation. And this is more than can be said for any other
hypothesis that I know of.
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