METHODS GEOLOGICAL 3 



that there are many degrees of speciaUzation or adaptation to 

 particular modes of hfe. Thus, for example, among the marine 

 mammals, the Whales and their allies. Porpoises, etc., are so 

 completely adapted to a life in the seas that they cannot come 

 upon the land, and even stranding is fatal to them, while the 

 Seals frequently land and move about upon the shore. It 

 should further be observed that mammals of the most diverse 

 groups are adapted to similar modes of existence. Thus in one 

 natural group or order of related forms, occur terrestrial, burrow- 

 ing, arboreal and aquatic members, and the converse state- 

 ment is of course equally true, that animals of similar life- 

 habits are not necessarily related to one another, and very 

 frequently, in fact, are not so related. Among the typically 

 marine mammals, for example, there are at least three and 

 probably four distinct series, which have independently be- 

 come adapted to life in the sea. 



Before attempting to set forth an outline of what has 

 been learned regarding the history of mammalian life in the 

 western hemisphere, it is essential to give the reader some con- 

 ception of the manner in which that knowledge has been ob- 

 tained. Without such an understanding of the methods em- 

 ployed in the investigation the reader can only blindly accept 

 or as blindly reject what purports to be the logical inference 

 from well-established evidence. How is that evidence to be 

 discovered ? and how may trustworthy conclusions be derived 

 from it ? 



The first and most obvious step is to gather all possible in- 

 formation concerning the mammals of the present day, their 

 structure (comparative anatomy), functions (physiology), and 

 their geographical arrangement. This latter domain, of the 

 geographical distribution of mammals, is one of peculiar signif- 

 icance. Not only do the animals of North America differ radi- 

 cally from those of Central and South America, but within the 

 limits of each continent are more or less well-defined areas, 



