We may safely attribute
these structures to inheritance. But to the progenitor of the upland
goose and of the frigate-bird, webbed feet no doubt were as useful as
they now are to the most aquatic of existing birds. So we may believe
that the progenitor of the seal had not a flipper, but a foot with
five toes fitted for walking or grasping; and we may further venture
to believe that the several bones in the limbs of the monkey, horse,
and bat, which have been inherited from a common progenitor, were
formerly of more special use to that progenitor, or its progenitors,
than they now are to these animals having such widely diversified
habits. Therefore we may infer that these several bones might have
been acquired through natural selection, subjected formerly, as now,
to the several laws of inheritance, reversion, correlation of growth,
etc. Hence every detail of structure in every living creature (making
some little allowance for the direct action of physical conditions)
may be viewed, either as having been of special use to some ancestral
form, or as being now of special use to the descendants of this
form--either directly, or indirectly through the complex laws of
growth.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the
fang of the adder, and in the ovipositor of the ichneumon, by which
its eggs are deposited in the living bodies of other insects.
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