RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 141 



without food and shelter. This principle was pointed out several 

 years ago by Scott (1913, p. 143) in the following statement. 



In the case of lands newly raised above the sea and connecting formerly 

 separated areas, it is necessary that they should first be taken possession 

 of by vegetation, before they can become passable by animals, for the 

 migration of mammals from continent to continent is an entirely distinct 

 phenomenon from the annual migration of birds. 



Not only the land bridge, but the abutments to it must possess 

 ecological conditions suitable for those kinds that are likely to make 

 the crossing (Simpson, 1947). Simpson {ibid., p. 685), no doubt 

 influenced by earlier workers such as Merriam (1894) and Scott 

 (1913), stressed the climate as being "Not the only, but probably 

 the most important, selective factor..." in the faunal interchanges 

 between Eurasia and North i\merica. Further, he stated that 

 "the migrants generally are those groups tolerant of relatively 

 cold climates," and thereby gave importance to the temperature 

 at the time of crossing. But, he did not rule out other ecological 

 factors. I am of the opinion that climatic conditions, as regards 

 movements of mammals over long periods of time, are important 

 in an indirect way, as they affect vegetation and soils, rather than 

 in a direct one. It is fairly well established that through Tertiary 

 times the climate was more moderate than at present (Emiliani, 

 1958). Even in the Pleistocene, the interglacial stages had fairly 

 moderate climates (Deevey, 1949). Temperature tolerance in 

 mammals is primarily physiological, and most of them can with- 

 stand great changes if supplied with food. It is true that those 

 mammals that live in cold-temperate climates, especially the large 

 mammals that live above the snow, are tolerant of greater fluctu- 

 ations in air temperature than are many of the tropical kinds. 

 Yet, there are several kinds that range through the tropics to the 

 colder regions today (mountain lion, Felis; river otter, Lutra; 

 weasel, Mustela; and others). Recent studies, on the bioclimate of 

 small mammals that live beneath the snow in winter in an Alaskan 

 taiga, show that the temperature at ground surface, where many of 

 the small mammals live, ranged through no more than 27°F from 

 summer to winter, whereas the air above the snow ranged through 

 152° F. The temperature where many of these small mammals 

 live rarely goes below + 20° F even in the most severe winter 



