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 metabolism. It is a general law in the courtship of the sexes that the male seeks the female. The secondary sexual characters of the male are developed with puberty, and in some cases these sexual distinctions come and go with the breeding season. What we know as physiological energy is the result of the dissociation of atoms in the organism; expressions of energy are the accompaniment of the katabolic or breaking-up process, and the brighter color of the male, especially at the breeding season, results from the fact that the waste products of the katabolism are deposited as pigments. When we compare the sexes of mankind morphologically, we find a greater tendency to variation in man:[31]