CHAPTER XVII 



THE EVOLUTION OF ANIMALS (Continued) 



There remains for consideration in this chapter the possible 

 origin of mammals from simpler types. This inquiry narrows 

 itself essentially to the batrachian (amphibian) origin that 

 Huxley advocated, or to the reptilian origin that most recent 

 naturalists have favored. The writer can not refrain from 

 expressing the opinion that in the past discussion of this ques- 

 tion too much emphasis seems frequently to have been laid 

 on minor bones of the osseous system, that often appear and 

 disappear in mammalian groups. Further the question of 

 dentition has been so emphasized as a safe guide that other 

 important evidence is often ignored. Again the fragmentary 

 parts of fossil remains have at times had large generalizations 

 tacked on to them — in some cases very appropriately, in other 

 cases with doubtful results. Finally the lack of soft parts 

 in fossil forms often cuts off from us important means of reach- 

 ing valid conclusions. 



A highly valuable and necessary matter to determine here 

 is the en^dronnlent that obtained when primitive mammals 

 were evolving into those of recent times, and the environal 

 relation of these animals to that environment. The prepon- 

 derating number of living batrachians and Amnionta consists 

 of land or fresh-water animals, while those that are marine 

 have evidently become adapted from a land life. Thus the 

 sea turtles, the seals, whales, etc., are conceded to have sprung 

 from fresh- water and more primitively from land ancestors. 

 Now our knowledge of land forms is largely confined to tliree 

 geologic periods, the carboniferous, the liassic, and the eocene, 

 we know much less as to the perm ian and the cretaceous. 



But, owing to constant and extensive denudation changes, 

 chances for the preservation in fossil state of land animals 



475 



