Under these
circumstances the incipient sterility between the two forms would
rapidly be eliminated, and could never rise much above the numbers which
were produced by sporadic variation each year.
It was, probably, by a consideration of some such case as this that Mr.
Darwin came to the conclusion that infertility arising between incipient
species could not be increased by natural selection; and this is the
more likely, as he was always disposed to minimise both the frequency
and the amount even of structural variations.
We have yet to notice another mode of action of natural selection in
favouring and perpetuating any infertility that may arise between two
incipient species. If several distinct species are undergoing
modification at the same time and in the same area, to adapt them to
some new conditions that have arisen there, then any species in which
the structural or colour differences that have arisen between it and its
varieties or close allies were correlated with infertility of the
crosses between them, would have an advantage over the corresponding
varieties of other species in which there was no such physiological
peculiarity.
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