_Isolation to prevent Intercrossing._
Most writers on the subject consider the isolation of a portion of a
species a very important factor in the formation of new species, while
others maintain it to be absolutely essential. This latter view has
arisen from an exaggerated opinion as to the power of intercrossing to
keep down any variety or incipient species, and merge it in the parent
stock. But it is evident that this can only occur with varieties which
are not useful, or which, if useful, occur in very small numbers; and
from this kind of variations it is clear that new species do not arise.
Complete isolation, as in an oceanic island, will no doubt enable
natural selection to act more rapidly, for several reasons. In the first
place, the absence of competition will for some time allow the new
immigrants to increase rapidly till they reach the limits of
subsistence. They will then struggle among themselves, and by survival
of the fittest will quickly become adapted to the new conditions of
their environment. Organs which they formerly needed, to defend
themselves against, or to escape from, enemies, being no longer
required, would be encumbrances to be got rid of, while the power of
appropriating and digesting new and varied food would rise in
importance.
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