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 were being simultaneously modified, those
between whose varieties infertility arose would have an advantage over
those whose varieties remained fertile _inter se_, and would ultimately
supplant them.
The preceding argument, it will be seen, depends entirely upon the
assumption that some amount of infertility characterises the distinct
varieties which are in process of differentiation into species; and it
may be objected that of such infertility there is no proof.
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