RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 145 



lighter, friable types, but arid conditions, with accompanying light 

 soils and a paucity of soil organisms are not suitable for them. 



Shrews (Family Soricidae). Present day shrews live in various 

 types of habitats and in various climatic zones (Fig. 3). With a 

 vegetative cover of any kind they would find easy passage. Both 

 early Tertiary and late passages are indicated by the fossil record 

 and present relationships. 



Man (Family Hominidae). There is no problem here, even 

 without a land bridge man could have made the crossing. 



Bears (Family Ursidae). There is no problem in getting bears 

 from one continent to the other. Polar bears, which are not strictly 

 terrestrial, are circumpolar in distribution today, and the other 

 bears on the two continents are closely related (the grizzlies are 

 considered to be of the same species by Erdbrink, 1953). Exchanges 

 probably continued through the period of the last connection. 



Weasel-like mammals (Family Mustelidae). Fossil records in- 

 dicate an early Tertiary passage and Recent relationships and 

 distributions indicate late passages, particularly of the fisher, 

 marten, otter, wolverine, mink, and weasels. The fisher and marten 

 would indicate a forested bridge, the others might pass over open 

 or forested country. Temperature would not be a factor with these 

 animals. However, for some of the skunks (Mephitinae), particularly 

 those of the genus Conepatus, climatic zoning could have served as a 

 selective agent in late Tertiary and Pleistocene times. Mephitis 

 and Spilogale now live under temperature conditions that are 

 probably more severe than they were during much of the time that 

 the two continents were connected. 



Dogs (Family Canidae). Because of the diversity of habitats 

 occupied by this group, we need not look for special conditions to 

 effect an interchange. Fossil records and present distributions and 

 relationships indicate exchanges from early Tertiary to the last 

 land connection. 



Cats (Family Felidae). Members of this family are nearly 

 worldwide in distribution and occupy various habitats. Indications 

 are that they passed from one continent to another many times 

 from early Tertiary to Recent. Although some kinds are now 

 confined to the tropics and some to cold areas, others, including 

 Felis concolor, range through the different climatic zones. 



