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 
toall 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 
