played a most important part, and a useful modification
of one part will often have entailed on other parts diversified changes 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 {200} 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 nipper 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 nipper, 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, &c. 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 tak