THE EVIDENCE FOR ORGANIC EVOLUTION 



505 



development is viewed in the light of an evolutionary interpreta- 

 tion, it becomes reasonable. In fact, anything else might be 

 impossible. If the ancestors of vertebrates were originally fish- 

 Uke animals, as the fossil record indicates (c/. Fig. 259), it is seen 

 that the Amphibia represent a transition from aquatic to terrestrial 

 life and that their development has been modified accordingly. 

 The reptiles, birds and mammals are true terrestrial animals in 

 their mode of development and in their adult condition. The 

 later stages have been modified, but the early stages still resemble 

 those of fishes. It is not correct to say that the embryo of a reptile, 



Fig. 268. — An ancient reptile, Ichthyosaurus, adapted for aquatic life like the 

 whales among mammals at the present time. 



This is an unusual fossil since the outline of the body is shown by a darkening of the 

 rock due to carbon from the flesh. (Courtesy, American Museum of Natural History.) 



bird, or mammal is a '' fish " at one stage of its development, but 

 it can be said that such embryos resemble the embryos of fishes 

 at corresponding stages. The statement that the animal " climbs 

 the ancestral tree " in the course of its development is like- 

 wise unfortunate, although it can be said that an animal has 

 certain structures in its development that were present in its 

 ancestors, because it " has never lost them." Thus, the fish 

 embryo becomes an adult that is not unlike the embryo, while the 

 embryo of a terrestrial vertebrate develops the organization of the 

 reptile, bird, or mammal. This is what might be expected if the 

 ancestors of these higher forms Avere fish-like organisms. The gill- 

 slit stages may, therefore, be regarded as occurring in reptiles, birds, 

 and mammals, because they were -present in the ancestors and have 

 never been co7?ipletely obliterated despite evolutionary changes that have 

 produced a very different organization in the adult animal. Indi- 



