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Mr. Darwin tells us that, at one time,
this appeared to him probable, but he found the problem to be one of
extreme complexity; and he was also influenced against the view by many
considerations which seemed to render such an origin of the sterility or
infertility of species when intercrossed very improbable. The fact that
species which occupy distinct areas, and which nowhere come in contact
with each other, are often sterile when crossed, is one of the
difficulties; but this may perhaps be overcome by the consideration
that, though now isolated, they may, and often must, have been in
contact at their origination. More important is the objection that
natural selection could not possibly have produced the difference that
often occurs between reciprocal crosses, one of these being sometimes
fertile, while the other is sterile. The extremely different amounts of
infertility or sterility between different species of the same genus,
the infertility often bearing no proportion to the difference between
the species crossed, is also an important objection.


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