If, for example, five per cent of each form
thus varied so as to be infertile with the other form, the result would
be hardly perceptible, because the individuals which formed cross-unions
and produced hybrids would constitute a very small portion of the whole
species; and the hybrid offspring, being at a disadvantage in the
struggle for existence and being themselves infertile, would soon die
out, while the much more numerous fertile portion of the two forms would
increase rapidly, and furnish a sufficient number of pure-bred offspring
of each form to take the place of the somewhat inferior hybrids between
them whenever the struggle for existence became severe. We must suppose
that the normal fertile forms would transmit their fertility to their
progeny, and the few infertile forms their infertility; but the latter
would necessarily lose half their proper increase by the sterility of
their hybrid offspring whenever they crossed with the other form, and
when they bred with their own form the tendency to sterility would die
out except in the very minute proportion of the five per cent
(one-twentieth) that chance would lead to pair together.
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