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Hence, as fixed differences of form and
colour, slowly gained by natural selection in adaptation to changed
conditions, are what essentially characterise distinct species, some
amount of infertility between species is the usual result.
Here the problem was left by Mr. Darwin; but we have shown that its
solution may be carried a step further. If we accept the association of
some degree of infertility, however slight, as a not unfrequent
accompaniment of the external differences which always arise in a state
of nature between varieties and incipient species, it has been shown
that natural selection _has_ power to increase that infertility just as
it has power to increase other favourable variations. Such an increase
of infertility will be beneficial, whenever new species arise in the
same area with the parent form; and we thus see how, out of the
fluctuating and very unequal amounts of infertility correlated with
physical variations, there may have arisen that larger and more constant
amount which appears usually to characterise well-marked species.


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