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But with any such increase of the physiological variety the
species itself would inevitably suffer by the large proportion of
sterile unions in its midst, and would thus be at a great disadvantage
in competition with other species which were fertile throughout. Thus,
natural selection will always tend to weed out any species with too
great a tendency to sterility among its own members, and will therefore
prevent such sterility from becoming the general characteristic of
varying species, which this theory demands should be the case.
On the whole, then, it appears clear that no form of infertility or
sterility between the individuals of a species, can be increased by
natural selection unless correlated with some useful variation, while
all infertility not so correlated has a constant tendency to effect its
own elimination. But the opposite property, fertility, is of vital
importance to every species, and gives the offspring of the individuals
which possess it, in consequence of their superior numbers, a greater
chance of survival in the battle of life.


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