e is the difference in size between
the male and female cocoons of the silk-moth that in France they are
separated by a particular mode of weighing.[24] The same superiority
of the female is found among fishes and reptiles; and this relation,
wherever it occurs, may be associated with a habit of life in which
food conditions are simple and stimuli mandatory. As we rise in the
scale toward backboned and warm-blooded animals, the males become
larger in size; and this reversal of relation, like the development
of offensive and defensive weapons, is due to the superior variational
tendency of the male, resulting in characters which persist in the
species wherever they prove of life-saving advantage.[25]

The superior activity and variability of the male among lower forms
has been pointed out in great detail by Darwin and confirmed by
others.

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 a