RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 133 



ation) of the group. This may or may not be so. Here, it is important 

 to know what time interval is being considered and, also, the 

 taxonomic level of the group — species, genus, family, etc. We 

 know that faunas have shifted geographically in the past, and, 

 unless we have the fossil evidence we cannot prove that the faunal 

 element involved did not move in from some other locality. A good 

 example in the mammals, in the area under consideration, where 

 we do have a fossil record from the same general area now occupied 

 by the group, is the assemblage of pocket gophers, pocket mice, 

 and kangaroo rats, involving two closely related families as now 

 recognized by specialists, the Geomyidae and Heteromyidae. 

 These mammals are, and have been, primarily western North 

 American in their distribution, as indicated by the fossil record 

 and knowledge of Recent kinds (Fig. 4). This is true also of the 

 pronghorn. Family Antilocapridae. If we base our interpretation 

 solely on Recent kinds and their present geographic distribution 

 and use the criterion just set forth, we would be correct in saying 

 that the area of differentiation was in western North America. 

 But, if we were to use the same criterion for the camels, Family 

 Camelidae, we would be far wrong. None of the latter are now 

 found in North America where they had their great development 

 in Tertiary times and continued into the Quaternary. 



(b) The periphery of the present range has been considered by 

 some as being the most likely place for differentiation of a group. 

 This might hold for some of the lesser categories, such as species, 

 but for the higher categories, camels, for example, we find the same 

 objections as we did in the preceding discussion. As a matter of 

 fact, any segment of a population of a species might have the 

 potential of evolving in a different direction from the parent stock 

 if a barrier is established to isolate it. This barrier need but stop or 

 sufficiently dilute the genetic interchange between populations. 

 Even in a continuous population over a relatively large area, the 

 segments at the extremes may be sufficiently isolated by distance 

 that they will evolve in different directions {Peromyscus maniculatus 

 is an example). 



2. The Area of Present Greatest Abundance Is Likely the Place 

 Where Group Evolved. This theory assumes that the area where 

 optimum conditions prevail today was always so. It ignores the 



