RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 153 



those kinds that made the crossing from one continent to the other. 

 The various groups of mammals of western North America are 

 analyzed for habitat selection, and general ecological conditions 

 are postulated for those that apparently made the crossing on the 

 Bering land bridge. Ecological conditions, soil and vegetation, 

 are considered to have been more important than climate, especially 

 temperature, in preventing certain kinds from making the crossing. 

 From the analysis of the mammalian fauna it is evident that 

 generalizations are dangerous. Each kind must be judged by itself. 

 What is applicable to one may not be to another. Still more hazard- 

 ous would be the application of rules derived from the study of 

 mammals to other classes of vertebrates or to invertebrates without 

 first understanding those groups. 



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