Thus, incipient species which were infertile together would
have an advantage over other incipient species which were fertile, and,
whenever the struggle for existence became severe, would prevail over
them and take their place. Such infertility, being correlated with
constitutional or structural differences, would probably, as already
suggested, go on increasing as these differences increased; and thus, by
the time the new species became fully differentiated from its parent
form (or brother variety) the infertility might have become as well
marked as we usually find it to be between distinct species.
This discussion has led us to some conclusions of the greatest
importance as bearing on the difficult problem of the cause of the
sterility of the hybrids between distinct species. Accepting, as highly
probable, the fact of variations in fertility occurring in correlation
with variations in habits, colour, or structure, we see, that so long as
such variations occurred only sporadically, and affected but a small
proportion of the individuals in any area, the infertility could not be
increased by natural selection, but would tend to die out almost as fast
as it was produced.
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