Friedrich Ratzel, in the introduction to his great work on “The Races of Man,” also alludes to this primitive obscuration of sexual contrasts in earlier stages of civilization, and draws therefrom interesting conclusions regarding the existence of a primordial gynecocracy, a “regiment of women.” I have myself discussed this question in the second volume of my book, “Contributions to the Etiology of Psychopathia Sexualis,” and shall return to the subject when dealing with masochism. W. H. Riehl, and after him Heinrich Schurtz, have laid stress on the dangers to civilization involved in the obliteration of sexual differences. Sexual differentiation stands and falls with civilization. The former is the indispensable preliminary of the latter. Destroy it, and the whole course of development will be reversed. Sexual differences comprise for the most part the diverse development of the so-called “secondary sexual characters”--that is to say, all the differential characteristics which distinguish man from woman, over and above those strictly related to the work of sex--for instance, stature, skeleton, muscles, skin, voice, etc. The masculine body has evolved to a greater extent than the feminine body as a force-producing machine, for in man the bones and the muscles have a larger development, whereas in woman we observe a greater development of fat, whereby the plasticity of the body is enhanced, but its mechanical utility and energy are impaired. According to the most recent scientific representation of sexual differences, as we find them enumerated in the monograph of Oskar Schultze, based upon his own observations, and also on the earlier works of Vierordt, Quetelet, Topinard, Pfitzner, Waldeyer, C. H. Stratz, J. Ranke, E. von Lange, Havelock Ellis, Merkel, Bischoff, Rebentisch, Welcker, Schwalbe, Marchand, and others, the most important physical differentiæ between man and woman are the following: