408 



GENERAL ZOOLOGY. 



FIG. 176. Male (TO) and female (/) 

 of one of the isopod Crustacea, an 

 extreme example of sexual di- 

 morphism. 



Again, we have to recognize a seasonal dimorphism. Thus 



certain butterflies produce 

 several broods in a year. 

 Those of the summer broods 

 are so different from those 

 which come from cocoons 

 which have passed through 

 the winter, that without 

 following through the whole 

 history the relationships 

 would not be suspected. 

 Closely connected with this polymorphism is the phe- 

 nomenon of alternation of generations, of which instances 

 are abundant in some groups of the animal kingdom 

 (p. 167). 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 his- 

 tory had been followed, so that it was ascertained that 

 each generation resembles, not its parents, but its grand- 

 parents. Another and a more complicated example is 

 furnished by the liver-fluke (p. 180). 



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 (the larva), as it hatches from the egg, is greatly 

 different from the parent, but by successive changes of 

 form it at last reaches the adult condition in which it 



