THE LIFE CYCLE 89 



develop into a fish and the other into a salamander. This 

 certainty of an embryo to become an individual of a certain 

 kind is called the law of heredity. 



55. The significance of the facts of development. The 

 significance of the developmental phenomena is a matter 

 about which naturalists have yet very much to learn. It is 

 believed, however, by practically all naturalists that many 

 of the various stages in the development of an animal cor- 

 respond to or repeat the structural condition of the ani- 

 mal's ancestors. Naturalists believe that all backboned or 

 vertebrate animals are related to each other through being 

 descended from a common ancestor, the first or oldest 

 backboned animal. In fact, it is because all these back- 

 boned animals the fishes, the batrachians, the reptiles, the 

 birds, and the mammals have descended from a common 

 ancestor that they all have a backbone. It is believed that 

 the descendants of the first backboned animal have in the 

 course of many generations branched off little by little from 

 the original type until there have, come to exist very real and 

 obvious differences among the backboned animals differ- 

 ences which among the living backboned animals are familiar 

 to all of us. The course of development of an individual ani- 

 mal is believed by many naturalists to be a very rapid, and 

 evidently much condensed and changed, recapitulation of 

 the history which the species or kind of animal to which the' 

 developing individual belongs has passed through in the 

 course of its descent through a long series of gradually chang- 

 ing ancestors. If this is true, then we can readily under- 

 stand why the fish and the salamander and tortoise and 

 bird and rabbit are all so much alike in their earlier stages 

 of development, and gradually come to differ more and 

 more as they pass through later and later developmental 

 stages. 



Some naturalists believe that the ontogenetic stages are 

 not as significant in throwing light upon the evolutionary 

 history of the species as just indicated. Some think that 



