\ 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; 
Fie. £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. Im the 
