222 EVOLUTION AND ANIMAL LIFE 



The male and female individuals often show marked differences, 

 especially in external structural characters. We can readily 

 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 dimorphism 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 ex- 

 traordinary extremes. This is especially true among polyga- 



FIG. 129. Cankerworm moth: the winged male and wingless female. 



mous 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. 128), 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 between 

 male and female are even greater. The males of the common 

 cankerworm moth (Fig. 129) have four wings; 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 ex- 

 ceed half an inch in length. In the case of some of the para- 

 sitic worms which live in the bodies of other animals the male 

 has an extraordinarily degraded, simple body, much smaller 



