THE LIFE CYCLE 89 



is sure to develop into a fish and the other into a sala- 

 mander. This certainty of an embryo to become an indi- 

 vidual of a certain kind is called the law of heredity. 

 Viewed in the light of development, there must be as great 

 a difference between one egg and another as between one 

 animal and another, for the greater difference is included 

 in the less. 



55. The significance of the facts of development. The sig- 

 nificance 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 came to exist very real 

 and obvious differences among the backboned animals dif- 

 ferences which among the living backboned animals are 

 familiar to all of us. The course of development of an 

 individual animal is believed to be a very rapid and evi- 

 dently 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 alike in their earlier stages of devel- 

 opment, and gradually come to differ more and more as 

 they pass through later and later developmental stages. 



