The Descent of Man

continued use or disuse of parts will have done
much in the same direction with natural selection.
Modifications formerly of importance,
though no longer of any special use, are longinherited.
When one part is modified, other
parts change through the principle of correlation,
of which we have instances in many curious
cases of correlated monstrosities. Something
may be attributed to the direct and definite
action of the surrounding conditions of life,
such as abundant food, heat or moisture; and
lastly, many characters of slight physiological
importance, some indeed of considerable importance,
have been gained through sexual
selection.
No doubt man, as well as every other animal,
presents structures, which seem to our limited
knowledge, not to be now of any service to
him, nor to have been so formerly, either for the
general conditions of life, or in the relations of
one sex to the other. Such structures cannot be