The simplest case to consider, will be that in which two forms or
varieties of a species, occupying an extensive area, are in process of
adaptation to somewhat different modes of life within the same area. If
these two forms freely intercross with each other, and produce mongrel
offspring which are quite fertile _inter se_, then the further
differentiation of the forms into two distinct species will be retarded,
or perhaps entirely prevented; for the offspring of the crossed unions
will be, perhaps, more vigorous on account of the cross, although less
perfectly adapted to the conditions of existence than either of the pure
breeds; and this would certainly establish a powerful antagonistic
influence to the further differentiation of the two forms.
Now, let us suppose that a partial sterility of the hybrids between the
two forms arises, in correlation with the different modes of life and
the slight external or internal peculiarities that exist between them,
both of which we have seen to be real causes of infertility. The result
will be that, even if the hybrids between the two forms are still freely
produced, these hybrids will not themselves increase so rapidly as the
two pure forms; and as these latter are, by the terms of the problem,
better suited to their conditions of life than are the hybrids between
them, they will not only increase more rapidly, but will also tend to
supplant the hybrids altogether whenever the struggle for existence
becomes exceptionally severe.
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