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
general infertility might arise in correlation with the different modes
of life of two varieties or incipient species. A considerable body of
facts already adduced renders it probable that this _is_ the mode in
which any widespread infertility would arise; and, if so, it has been
shown that, by the influence of natural selection and the known laws
which affect varieties, the infertility would be gradually increased.
But, if we suppose the infertility to arise sporadically within the two
forms, and to affect only a small proportion of the individuals in any
area, it will be difficult, if not impossible, to show that such
infertility would have any tendency to increase, or would produce any
but a prejudicial effect.
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