Waldeyer, in his notable address on the somatic differences between the sexes, delivered in 1895 at the Anthropological Congress in Kassel, drew attention to the fact that the higher development of any particular species is notably characterized by the increasing differentiation of the sexes. The further we advance in the animal and vegetable world from the lower to the higher forms, the more markedly are the male and the female individuals distinguished one from another. In the human species also, in the course of phylogenetic development, this sexual differentiation increases in extent. In the development of these sexual differences, the antagonism first shown by Herbert Spencer to exist between reproduction and the higher evolutionary tendency plays an important part. Among the higher species of animals the males exhibit a stronger evolutionary tendency than the females, owing to the fact that their share in the work of reproduction has become less important. The more extensive organic expenditure demanded by the reproductive functions limits the feminine development to a notably greater extent than the masculine. In the human species this retardation of growth in the female is especially increased in consequence of menstruation, and this affords a striking example of the truth of Spencer’s law. I quote also in this connexion the remarks of the Würzburg anatomist Oskar Schultze, in his recently published valuable monograph on “Woman from an Anthropological Point of View,” pp. 55, 56 (Würzburg, 1906); “The undulatory periodicity of the principal functions of the feminine organism, which depends on the processes of ovulation and menstruation, and is invariable in the females of the human species, does not occur in the other mammalia (with the exception of apes). In these latter, as far as we have been able to observe, the secondary sexual characters, in the matter of differences in muscular development and in strength, are not so developed, or sometimes are not so developed, as in the human species. We must in this connexion exclude the differences which appear in domestic animals as a result of domestication (for example, the difference between the cow and the bull). In the human female, the periodicity, which begins to act even on the youthful, still undeveloped body, has during thousands of years increased the secondary sexual differences. Periodicity is, in my opinion, an important cause of the fact that woman is inferior to man, more especially in the development of the muscular system and in strength, and that her organs, for the most part, are more closely approximated to the infantile type.