From the survey which has been given it will be realized that every part of the skeleton is modified in connexion with sex. Further it will be evident that there is a variation in the degree to which these characters are developed from individual to individual and from part to part. For instance, in the case just related the female characters of the pelvis were not apparent until it was contrasted with the thorax. It often happens, however, when we have to deal with individual skulls or limb bones that great difficulty is experienced in determining sex. Most anatomists agree that in every hundred skulls there are five at least, possibly ten, of which the sex cannot be postulated with any degree of certainty. This is particularly the case when the skulls are those of women who have passed the age of child bearing, for after that period growth changes often set in.
The problem of the origin of sexes does not concern us here ; it is sufficient to say that long before the very lowest forms of vertebrate animals had been evolved, individuals were already born either male or female. The questions one would like to answer are the following: Have man and woman always been as sharply marked off from each other as now ? Is the sexual difference really decreasing or increasing ? Is it possible that the day may come when to all outward appearances man and woman may seem the same ? We turn to the evidence which the anthropoids can afford us. The most primitive—the small anthropoids or gibbons—show us the sexes so alike that it is hard to tell the male from the female. The female is the heavier animal; her canine teeth are as long and sharp as in the male ; the muscular ridges in her bones are equally well marked. Yet there are minor differences between them, but they are not so decisive as to allow us to determine sex except by examining the essential organs of generation. Here, then, is a primitive condition showing an apparent equality in sexual characters. In the great anthropoids the sexual differences are very marked. These are most pronounced in the gorilla, the anthropoid which stands nearest to man if we take all the characters of the body into account. In mass of body, in muscular development, strength of limb, size of jaws and teeth, the male gorilla exceeds the female to a greater degree than man overshadows woman. The male is the fighter. Amongst chimpanzees the difference between the sexes is about the same in amount and in kind as in the human species. Amongst orangs the sex characters are more marked than among chimpanzees, but less than is seen in gorillas. The canine or eye teeth mark the sexes of the great anthropoids. They are especially large in the male gorilla and are most reduced and therefore most nearly resemble the human canine in the female chimpanzee. Among human races the canines have fallen to the level of the other teeth in both male and female, yet we must infer that at one time they were large in size with prominent conical points, just as in the great anthropoids, for their buds are formed deeper in the jaw than the other teeth. The buds of our canine teeth are formed in the same anomalous position as in the chimpanzee and gorilla.
There is also a sexual difference as regards the brain. The male anthropoid has much the heavier brain. Amongst gorillas the male brain is eighteen per cent, larger than that of the female, amongst orangs fourteen per cent., and amongst chimpanzees eight per cent. ; while in human races it is about twelve per cent. When we sum up the bearing of these facts on human history we must infer that the differences which we see between the bodies of man and woman are of old standing. The great anthropoids show about the same degree and the same kind of sexual differentiation. Man and anthropoid seem to have inherited their sexual characters from that common stock of primates which appeared many millions of years ago in an early stage of the Miocene period. Even in the most remote and dark period of jungle existence we see that the body of man is specialized in one direction, and that of woman in another. It is a standing law of Nature that a difference in the structure of the whole body signifies a different function in the whole body. We must, therefore, conclude that before the period of civilization dawned on the world the bodies of man and woman were already specialized for different sides of human life. Legislation can give the sexes equal opportunities of life, but it cannot blot out the structural differences between man and woman. These have taken geological epochs to produce. When, too, we see that the degree of sexual differentiation is just as marked in high as in low races of mankind we must infer that there is no evidence to support the idea that civilization will in time produce a structural equality in the two sexes.
We can obtain further light on the nature of our sexual characters by watching their development in the foetus and child. Although we have reason to believe that the sex of the individual is already determined when the human egg is fertilized, we cannot distinguish a male embryo from a female until the second month of development is nearly finished. When the anatomy of a human embryo of this stage is examined it is seen that there are four tubes passing side by side within the hinder part of the body to end in the common cloacal passage or exit. The two inner or middle tubes become the oviducts or channels for the two genital glands of the female ; the two outer or lateral ducts—the sperm or Wolffian ducts—become the passages for the two male glands. Before the end of the second month it becomes possible to tell whether the two genital glands are to be those of a male or of a female. The gland assumes either the structure of an ovary or of a testicle. As soon as the characters of the glands are determined we see changes taking place in the genital passages. If the sexual glands are those of the female the oviducts grow together, fuse in their lower parts to form the uterus, while the upper parts remain separate and form the Fallopian tubes or egg-ducts. On the other hand, the two lateral or Wolffian ducts are arrested in growth, and only vestiges of them persist. If the sexual glands of the embryo are of the male character —testicles—the lateral or sperm ducts develop, while the oviducts become reduced to vestiges. In the male foetus the cloaca becomes enclosed to form a male organ; in the female the cloaca remains open. We believe that the sexual glands, as soon as they are differentiated, throw a secretion into the circulation which acts on the sex tubes or channels; the secretion of the testis causes the male parts to grow and the female parts to atrophy ; that of the ovary stimulates the female parts and restrains the male.