54 



SEXUAL DIMORPHISM 



appear in its development, seems undoubtedly on the whole 

 to be intimately associated with the wearing of hats or head- 

 gear, and to be developed more quickly by heavy head 

 coverings which prevent ventilation. Baldness is much more 

 characteristic in civilised nations of men than of women, 

 and those who lead sedentary lives in ill-ventilated rooms 

 seem more prone to it. As woman's head-gear is usually 

 light and open, and often more of the nature of ornament than 

 covering, it would naturally not tend to produce baldness to 

 the same degree. 



The retention of hair on the head in man would therefore 

 agree with the view that the loss of hair on the body was 

 due to the wearing of clothes. But the length of the hair on 

 the head, especially in women, is not a universal character in 

 Primates. In some degree this increased length is perhaps due 

 to what may be called the cultivation of the hair. In many 

 races, as among women in our own society, the hair of the 

 head is regularly dressed and tended, and such manipulation, 

 involving as it does a frequent mechanical irritation of the 

 hair follicles, has very probably caused a more vigorous 

 proliferation of the epidermic cells which form the hair. 



Another unisexual character in man is the lower pitch of 

 the voice, which is the result of a rapid alteration of the 

 larynx occurring at the period of puberty. That this change 

 and the growth of the beard are secondary sexual characters 

 of the typical kind is shown by the fact that they do not 

 develop in castrated individuals. The change in the voice 

 was probably originally due to the shouting of the male under 

 the influence of sexual excitement in contests with other 

 males. 



Among the Primates the difference between the sexes is in 

 no case more marked than in the Mandrill, which is a species 

 of baboon (Cynocephalus maimon). In the adult male the 

 brow ridges are very prominent, and the small and closely 



