THE MULTIPLICATION OF ANIMALS AND SEX 59 



ily tell the peacock, with its splendidly ornamental tail 

 feathers, from the unadorned peafowl, or the horned ram 

 from the bleating ewe. There is here, plainly, a dimor- 

 phism the existence of two kinds of individuals belonging 

 to a single species. This dimorphism is due to sex, and 

 the condition may be called sex dimorphism. Among some 

 animals this sex dimorphism, or difference between the 

 sexes, is carried to extraordinary extremes. This is espe- 

 cially true among polygamous animals, or those in which 

 the males mate with many females, and are forced to fight 

 for their possession. The male bird of paradise, with its 

 gorgeous display of brilliantly colored and fantastically 

 shaped feathers (Fig. 27), seems a wholly different kind of 

 bird from the modest brown female. The male golden and 

 silver pheasants, and allied species with their elaborate 

 plumage, are very unlike the dull-colored females. The 

 great, rough, warlike male fur seal, roaring like a lion, is 

 three times as large as the dainty, soft-furred female, which 

 bleats like a sheep. 



Among some of the lower animals the differences be- 

 tween male and female are even greater. The males of 

 the common cankerworm moth (Fig. 28) have four wings ; 



FIG. 8. Cankerworm moth ; the winged male and wingless female. 



the females are wingless, and several other insect species 

 show this same difference. Among certain species of white 

 ants the females grow to be five or six inches long, while 

 the males do not exceed half an inch in length. In the 



