Specifically different sexes come later, when mortality is a universal fate, as a means of rebirth and escape from death. Then the sexes develop their latest function, most prominent among the younger vertebrates, of acting as nature's most potent method of variation and differentiation. In the pursuit of the different, nature has exalted sex, and the intensity of the sex life. As far as the preservation of a species is concerned, and the reproduction of the individual, the asexual methods, budding, for example, would have done well enough. But when it comes to enacting a different individual apart from the effects of environment, sex stands out as the favored method of Life. The development of the sexes and the sexual life brought a new element of conflict into the living world. Before the advent of the sexes the conflict was essentially for the means of existence, food alone. But with the sexual life came a conflict for sex pleasure, a competition among members of the same species for the same individual as their sex partners. The result was the introduction of a factor in evolution which Darwin examined so closely in the "Descent of Man." The sex conflict has been the cause for the origin and the survival of certain physical and mental traits, helpful in sex attraction, sex combat, the growth of the embryo, and the nutrition and safety of the young of a species,--in short, the whole process of sexual selection. The proportions of the skeleton, the distribution of hair and fat, the construction of organs of attack and defense, the color of the skin, the cyclic processes of preparation for impregnation, the oestrus or heat period in animals, the menstrual period in the human being, the psychic reactions to danger and combat have all been thus determined. That man is bearded while woman is not,--that woman has potentially functional breasts while man has not,--the aggressive pugnacity of man contrasted with the more passive timidity of woman, have all been evolved in the sex struggle, surviving because most effective in that struggle. These so-called secondary sexual characteristics are an expression of the influence of the internal secretion of the gonads, or the interstitial glands. Some call them puberty glands, because their ripening initiates puberty. We know that these interstitial glands, to stick to that name, (rather than to the name of the puberty glands, since they serve not only to induce puberty but to maintain maturity) are the actual primary dictators of the process by which male and female are distinguished, if not created. Castration was probably the first surgical operation carried out for experimental purposes, suggested no doubt by a curiosity concerning its effects. Trepanning of the skull, the geologic record indicates, was done even by the cave man. But as an experimental operation, castration seems to hold the primary position in the annals of surgery.