es the young of both sexes resemble each
other; and the female somewhat resembles her young offspring throughout
life. In almost every great class a few anomalous cases occur, where
there has been an almost complete transposition of the characters proper
to the two sexes; the females assuming characters which properly belong
to the males. This surprisingly uniformity in the laws regulating the
differences between the sexes in so many and such widely separated
classes is intelligible if we admit the action throughout all the higher
divisions of the animal kingdom of one common cause; namely, sexual
selection.

Sexual selection depends on the success of certain individuals over
others of the same sex, in relation to the propagation of the species;
while natural selection depends on the success of both sexes, at all
ages, in relation to the general conditions of life. The sexual
struggle is of two kinds; in the one it is between the individuals of
the same sex, generally the males, in order to drive away or kill their
rivals, the females remaining passive; while in the other, the struggle
is likewise between the individuals of the same sex, in order to excite
or charm those of the opposite sex, generally the females, which no
longer remain passive, but select the more agreeable partners. This
latter kind of selection is closely analogous to that which man
unintentionally, yet effectually, brings to bear on his domesticated
productions, when he preserves during a long period the most pleasing or
useful individuals, without any wish to modify the breed.

The laws of inheritance determine whether characters gained through
sexual selection by either sex shall be transmitted to the same sex, or
to both; as well as the age at which they shall be developed. It appears
that variations arising late in life are commonly transmitted to one and
the same sex. Variability is the necessary basis for the action of
selection and is wholly independent of it. It follows from this that
variations of the same general nature have often been taken advantage of
and accumulated through sexual selection in relation to the propagation
of the species, as well as through natural selection in relation to the
general purposes of life. Hence secondary sexual characters, when
equally transmitted to both sexes, can be distinguished from ordinary
specific characters only by the light of analogy. The modifications
acquired through sexual selection are often