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. 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. If, however, it was so closely correlated with
physical variations or diverse modes of life as to affect, even in a
small degree, a considerable proportion of the individuals of the two
forms in definite areas, it would be preserved by natural selection, and
the portion of the varying species thus affected would increase at the
expense of those portions which were more fertile when crossed. Each
further variation towards infertility between the two forms would be
again preserved, and thus the incipient infertility of the hybrid
offspring might be increased till it became so great as almost to amount
to sterility. Yet further, we have seen that if several competing
species in the same area w