MORPHOLOGY OF ANIMALS. 335 



female of the same species are greatly different in their 

 appearance. An example of this is familiar in the can- 

 kerworm-moths, the male of 

 which is winged, the female 

 wingless. Again, we have to 

 recognize a seasonal dimor- 

 phism. Thus certain butter- 

 flies produce several broods in 

 a year. Those of the summer 

 broods are so different from 



FIG. 148. Male (m) and female (/) 



those Which COme from CO- of one of the isopod Crustacea, an 

 n . , , .. extreme example of sexual di- 



coons which have passed morphism. 



through the winter, that without following through the 



whole history the relationships would not be suspected. 



Closely connected with this polymorphism is the phenom- 

 enon of alternation of generations, of which instances are 

 abundant in some groups of the animal kingdom (p. 307). 

 Thus in the butterflies just mentioned, from the eggs of the 

 winter-brood individuals are produced the summer brood 

 presenting far different appearances from the parents, while 

 the eggs of the summer brood produce in turn the winter 

 brood. Again, in certain gall-wasps the difference between 

 two generations is so great both in appearance and in 

 habits that they would never be regarded as belonging to 

 the same species, or even to the same genus, were it not 

 that the whole history had been followed, so that it was 

 ascertained that each generation resembles, not its parents, 

 but its grandparents. 



Many animals in the course of their development pass 

 through a metamorphosis, which is not to be confused with 

 polymorphism. In forms where a metamorphosis occurs 

 the young, as it hatches from the egg, is greatly different 

 from the parent, but by successive changes of form it at 



