The Descent of Man

have not frequently been accumulated through
natural selection, in relation to the ordinary
habits of life. If this had occurred, the two sexes
would often have been differently modified, for
the sake, for instance, of capturing prey or of
escaping from danger. Differences of this kind
between the two sexes do occasionally occur,
especially in the lower classes. But this implies
that the two sexes follow different habits in
their struggles for existence, which is a rare
circumstance with the higher animals. The case,
however, is widely different with the reproductive
functions, in which respect the sexes necessarily
differ. For variations in structure which
are related to these functions, have often proved
of value to one sex, and from having arisen
at a late period of life, have been transmitted to
one sex alone; and such variations, thus preserved
and transmitted, have given rise to secondary
sexual characters.