240 



EVOLUTION AND ANIMAL LIFE 



of the skull to the upper side. In some related forms, called 

 soles, the small eye passes through the head and not around 

 it, appearing finall)' in the same socket with the other eye. 



Thus in almost all the great groups of animals we find 

 certain kinds which show metamorphosis in their postem- 

 bryonic development. But metamorphosis is simply develop- 

 ment; its striking and extraordinary features are usually due 

 to the fact that the orderly, gradual course of the development 

 is revealed to us only occasionally, with the result of giving 



Fig. 143. — Young stages of a flounder, Platophrys podas. The eyes in the young 

 flounder are arranged normally; that is, one on each side of the head. (After Emery.) 



the impression that the development is proceeding by leaps and 

 bounds from one strange stage to another. If metamorphosis 

 is carefully studied it loses its aspect of marvel, although never 

 its great interest. 



After an animal has completed its development it has but 

 one thing to do to complete its life cycle, and that is the pro- 

 duction of offspring. When it has laid eggs or given birth to 

 young, it has insured the beginning of a new life cycle. Does 

 it now die? Is the business of its life accomplished? There are 

 many animals which die immediately or very soon after laying 

 eggs. Some of the May flies — ephemeral insects which issue 

 as winged adults from ponds or lakes in which they have spent 

 from one to three years as aquatic crawling or swimming larvae, 

 flutter about for an evening, mate, drop their packets of fertil- 

 ized eggs into the water, and die before the sunrise — are ex- 

 treme examples of the numerous kinds of animals whose adult 

 life lasts only long enough for mating and egg4aying. But 

 elephants live for two hundred years. Whales probably live 

 longer. A horse lives about thirty years, and so may a cat or 

 toad. A sea anemone, which was kept in an aquarium, lived 

 sixty-six years. Crayfishes may live twenty years. A queen 

 bee was kept in captivity for fifteen years. Most birds have 



