150 W. H. BURT 



ancestral forms can first be recognized as raccoons (subfamily 

 Procyoninae). Raccoons proper probably had their early evolution 

 in tropical America. Climatic zoning might have been a selective 

 factor with these animals. However, they do range into southern 

 Canada today and it is difficult to see why they did not reach 

 Asia in late Tertiary or Pleistocene times. From fossil evidence we 

 may assume that the early ancestors passed over in Early Tertiary 

 times. The coati {Nasua) is a southern form that likely never did 

 get very far north. Climatic factors could have been important in 

 limiting the dispersal of this mammal. 



Ringtails (Family Bassariscidae). The ringtail occupies the 

 same kind of situation as the coati (Nasua) discussed above. 



Prairie dog (Family Sciuridae). The prairie dog was mentioned 

 earlier, but should be included in this section. It is an inhabitant 

 of short-grass areas, and ecological factors probably prevented it 

 from making the crossing. Although it now inhabits areas where 

 winter temperatures are low, it avoids the extreme cold by going 

 into hibernation. 



Pocket gophers (Family Geomyidae). The present and what we 

 know of the past distribution of pocket gophers is primarily western 

 North American. A segment inhabits southeastern United States. 

 Their latitudinal range is from southern Canada to tropical America. 

 They are excellent diggers and can occupy the regions of heavy 

 soils as well as sandy loams. They apparently require soil moisture 

 sufficient to grow a good cover of vegetation. Normally, non- 

 forested areas are preferred. I suspect that a continuous, dense 

 forest would be a barrier to their dispersal — shallow, rocky soils 

 might serve the same purpose. Temperature probably would not 

 have prevented them from reaching the land bridge — there must 

 have been a barrier of forests or soil types, or both. 



Kangaroo rats and pocket mice (Family Heteromyidae). Here 

 again, the fossil record indicates a strictly North American evolu- 

 tionary sequence. Soil type is probably the most important limiting 

 factor in the distribution of these rodents. Although partially 

 fossorial, they are weak diggers and, therefore, they require friable 

 soil. This kind of soil is to be found primarily in the arid and semi- 

 arid western part of North America (Fig. 4). Some of these animals, 

 particularly along the eastern border of their ranges, penetrate 

 areas of fairly heavy clay soils, but they prefer the lighter types. 



