It is this inferiority of the hybrid
offspring that is the essential point; and as the number of these
hybrids will be permanently less where the infertility is greatest,
therefore those portions of the two forms in which infertility is
greatest will have the advantage, and will ultimately survive in the
struggle for existence.
The differentiation of the two forms into distinct species, with the
increase of infertility between them, would be greatly assisted by two
other important factors in the problem. It has already been shown that,
with each modification of form and habits, and especially with
modifications of colour, there arises a disinclination of the two forms
to pair together; and this would produce an amount of isolation which
would greatly assist the specialisation of the forms in adaptation to
their different conditions of life. Again, evidence has been adduced
that change of conditions or of mode of life is a potent cause of
disturbance of the reproductive system, and, consequently, of
infertility. We may therefore assume that, as the two forms adopted more
and more different modes of life, and perhaps acquired also decided
peculiarities of form and coloration, the infertility between them would
increase or become more general; and as we have seen that every such
increase of infertility would give that portion of the species in which
it arose an advantage over the remaining portions in which the two
varieties were more fertile together, all this induced infertility would
maintain itself, and still further increase the general infertility
between the two forms of the species.
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