ges of no direct use. So again
characters which formerly were useful, or which formerly had arisen from
correlation of growth, or from other unknown cause, may reappear from
the law of reversion, though now of no direct use. The effects of sexual
selection, when displayed in beauty to charm the females, can be called
useful only in rather a forced sense. But by far the most important
consideration is that the chief part of the organisation of every
being is simply due to inheritance; and consequently, though each being
assuredly is well fitted for its place in nature, many structures now
have no direct relation to the habits of life of each species. Thus, we
can hardly believe that the webbed feet of the upland goose or of the
frigate-bird are of special use to these birds; we cannot believe that
the same bones in the arm of the monkey, in the fore leg of the horse,
in the wing of the bat, and in the flipper of the seal, are of special
use to these animals. 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 th