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:
The supporting framework of the body, the osseous skeleton, exhibits important differences in man and woman. The bones of women are on the whole smaller and weaker. Especially extensive sexual differences are noticeable in the pelvis. Wiedersheim regards these sexual differences of the woman’s pelvis as a specific characteristic of the human species. In all the anthropoid apes they are far less strongly marked than in man. Moreover, these differences exhibit a progressive development, which is to an important extent dependent upon advancing civilization. For this reason, as G. Fritsch, Alsberg, and others, point out, among the majority of savage races the differences between the male and the female pelvis are far less extensive than among civilized nations. The characteristic peculiarities of the pelvis of the European woman, which can be distinguished from the male pelvis at a glance—namely, its greater extent in transverse diameter, the greater depression and the wider opening of the anterior osseous arch—are far less marked among women of the South African races and among the South Sea Islanders.
The enlargement of the female pelvis in the course of human evolution is dependent upon the most important of all the factors of civilization, the brain. Even in the human fœtus the great size of the brain gives rise to a far greater proportionate development of the skull than we find in the fœtus of any other mammal. This influences the pelvic inlet and the sacrum, but also the large pelvis, since, in consequence of the adoption by man of the upright posture, the pregnant uterus expands more laterally, and thus opens out the iliac fossæ. In the lower races of man, it is precisely this plate-like expansion of the iliac fossæ which is so much less developed than in the case of civilized races.
Another physical difference between the sexes concerns stature and body-weight.
The mean stature of woman is somewhat less than that of man. Among Europeans it is about 1·60 metres (5 feet 3 inches), as compared with 1·72 metres (5 feet 73⁄4 inches) for the average stature of the male. According to Vierordt, the new-born boy is already on the average from 1⁄2 to 1 centimetre (1⁄5 to 2⁄5 inch) longer than the new-born girl. Johannes Ranke characterizes the individual factors which give rise to these differences in the following manner:
“The typical bodily development of the human male is characterized by a trunk relatively shorter in relation to the whole stature; but in relation to the length of the trunk, the upper and the lower extremities are longer, the thighs and the legs longer, the hand and the foot also longer; relatively to the long upper arm and to the long thigh respectively, the forearm and the leg are still longer; and relatively to the entire upper extremity, the entire lower extremity is also longer.
“On the other hand, the feminine proportions, remaining more approximate to those of the youthful state, as compared with those of the fully developed male, are distinguished by the following characteristics: comparatively greater length of the trunk; relatively to the length of the trunk, comparatively shorter arms and lower extremities, shorter upper arm and forearm, shorter thigh and leg, shorter hands and feet; relatively to the shorter upper arm, still shorter forearm, and relatively to the shorter thigh, still shorter leg; finally, relatively to the entire upper extremity, shorter lower extremities.”
This difference in the stature is found also in primitive peoples. Among the savage races of Brazil, who are still living in the stone age, Karl von den Steinen found that the average height of the men was 162 centimetres (5 feet 3·8 inches), whilst that of the women was 10·5 centimetres (4·14 inches) less. This difference corresponds exactly with that given in Topinard’s figures as corresponding to the average male height of 162 centimetres (5 feet 3·8 inches).