to
arise, tend to increase that infertility. It must particularly be noted
that this effect would result, not by the preservation of the infertile
variations on account of their infertility, but by the inferiority of
the hybrid offspring, both as being fewer in numbers, less able to
continue their race, and less adapted to the conditions of existence
than either of the pure forms. 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.

It follows, then, that specialisation to separate conditions of life,
differentiation of external characters, disinclination to cross-unions,
and the infertility of the hybrid produce of these unions, would all
proceed _pari passu_, and would ultimately lead to the production of two
distinct forms having all the characteristics, physiological as well as
structural, of true species.

In the case now discussed it has been supposed, that some amount of
gene