Though nature grants long
periods of time for the work of natural selection, she does not grant an
indefinite period; for as all organic beings are striving to seize on each
place in the economy of nature, if any one species does not become modified
and improved in a corresponding degree with its competitors it will be
exterminated. Unless favourable variations be inherited by some at least
of the offspring, nothing can be effected by natural selection. The
tendency to reversion may often check or prevent the work; but as this
tendency has not prevented man from forming by selection numerous domestic
races, why should it prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter the
breed, have a nearly common standard of perfection, and all try to procure
and breed from the best animals, improvement surely but slowly follows from
this unconscious process of selection, notwithstanding that there is no
separation of selected individuals. Thus it will be under nature; for
within a confined area, with some place in the natural polity not perfectly
occupied, all the individuals varying in the right direction, though in
different degrees, will tend to be preserved. But if the area be large,
its several districts will almost certainly present different conditions of
life; and then, if the same species undergoes modification in different
districts, the newly formed varieties will intercross on the confines of
each.
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