72 SEXUAL DIMORPHISM 



defence, or for producing a sound, or was merely an expres- 

 sion of the emotions, it is evident, I think, that the importance 

 of the bladder depends upon the inflation. The habit of 

 inflation would tend in the individual to distend the wall 

 of the nasal cavities, and, therefore, in this case also the 

 unisexual modification corresponds to a mechanical strain 

 which affects only the mature males under the influence 

 of sexual excitement, or at least under the influence of 

 rage and ferocity originally associated with the sexual 

 instinct. 



The habits of the fur seal of Behring Sea, Callorhinus 

 ursinus, are perhaps more accurately known than those of 

 any other species, on account of the political and commercial 

 importance attached to questions concerning them. The 

 male is polygamous, and the difference in size between the 

 sexes is extraordinarily great, the adult male being six times 

 as large as the female. At a certain time of the year these 

 seals land in large numbers at particular places on the 

 Pribylov and other islands in order to breed. The males 

 wait for the females as they land, and each keeps as large a 

 number as possible in his harem, guarding and fighting for 

 his wives with all his might. The young males are not 

 allowed among the adults, but are compelled to herd by 

 themselves. The extraordinary activity and exertion of the 

 male in the breeding habits, and the comparatively passive 

 and submissive part played by the females, is evidently the 

 only cause of the great difference between them in size and 

 strength, but there are no special habits in fighting and 

 courtship, and accordingly no special organs or characters. 



The tusks of the Walrus are enlarged canines continually 

 growing at the roots. They are larger in the males than in 

 the females, but not exclusively confined to the former. 

 Occasionally they are absent in the female, and the males 

 fight ferociously with these weapons. 



