III. Vertebrates (_continued_)--_Man and woman_.-- Among primates sexual dimorphism is but little accentuated, especially when the male and female live the same life in the open air and share the same labours. The male gorilla, very strong and very pig-headed, flees from no enemy; the female on the contrary is almost timid: when surprised in company with the male, she cries out, gives the alarm and escapes. But attacked when alone with her offspring, she resists. One can easily distinguish the male and female orang-outang, the male is larger with longer more bristling hair, he alone has a Horace Greeley beard; in the female the patches of bald skin are much less callous. But the great difference between the sexes in gorilla and orang-outang is in the males having vocal sacks descending over the chest to the arm-pits. Thanks to these air-reservoirs, these bag-pipe bags, inflatable at will, the male can howl for a very long time and with great violence; the females' sacks are very small. Other monkeys, notably howling apes, are provided with these air-chambers, as are also certain other mammifers well known for the extravagance of their cries: polecats and pigs. Birds and batrachians have analogous organs. Dimorphism of men and women varies according to race or rather according to species. Very feeble in most blacks and reds it is accentuated among Semites, Aryans, and Finns. But in man as in all animals of separate sexes one must differentiate between the primary dimorphism, which is necessary and produced by the specialization of sexual organs, and the secondary dimorphism with which the relation of sex is less evident or wholly uncertain. Limited to the non-sexual elements, human dimorphism is very feeble. Almost null in infancy, it develops with approaching puberty, is maintained during the genital period, and diminishes, sometimes almost to vanishing point, in old age. It varies individually, even during the years of greatest reproductivity, in males feebly sexed and in women heavily sexed: that is to say there are men and women whose type closely approaches the type-ideal formed by the fusion of sexes; neither one nor the other escapes the radical dimorphism imposed by the difference of sexual organs.