Throughout the animal kingdom, when the sexes differ in
external appearance, it is, with rare exceptions, the male
which has been more modified; for, generally, the female
retains a closer resemblance to the young of her own species,
and to other adult members of the same group. The cause of
this seems to lie in the males of almost all animals having
stronger passions than the females.[26]

Darwin explains the greater variability of the males--as shown in
more brilliant colors, ornamental feathers, scent-pouches, the power
of music, spurs, larger canines and claws, horns, antlers, tusks,
dewlaps, manes, crests, beards, etc.--as due to the operation of
sexual selection, meaning by this "the advantage which certain
individuals have over others of the same sex and species solely in
respect of reproduction,"[27] the female choosing to pair with the
more attractive male, or the stronger male prevailing in a contest for
the female. Wallace[28] advanced the opposite view, that the female
owes her soberness to the fact that only inconspicuous females have
in the struggle for existence escaped destruction during the breeding
season. There are fatal objections to both these theories; and, taking
his cue from Tylor,[29] Wallace himself, in a later work, suggested
what is probably the true explanation, namely, that the superior
variability of the male is constitutional, and due to general laws
of growth and development. "If ornament," he says, "is the natural
product and direct outcome of superabundant health and vigor, then no
other mode of selection is needed to account for the presence of such
ornament."[30] That a tendency to spend energy more rapidly should
result in more striking morphological variation is to be expected;
or, put otherwise, the fact of a greater variational tendency in the
male is the outcome of a constitutional inclination to destructive