272 INTRODUCTION TO EVOLUTION 



ica until the Pleistocene. Again we note the effect of the isolation of the 

 latter continent during most of the Tertiary. 



Rather surprisingly, perhaps, for an animal we do not usually associate 

 with our own country, the major portion of camel evolution occurred in 

 North America. Here the fossil record dates back to the Eocene, to a little 

 animal, Protylopus, which was in a stage of evolution comparable to that of 

 Hyracotherium in the horse line. The subsequent evolutionary history of 

 camels is almost as completely documented by North American fossils as is 

 the evolution of the horse. 



Evidently, therefore, North America was the center of camel evolution 

 and dispersal. Camels reached the Old World across the Bering Strait 

 land bridge, and South America across the Isthmus of Panama, when the 

 latter was elevated above the sea in Pleistocene times (see arrows on the 

 map. Fig. 12.5). Subsequently, camels became extinct in their center of 

 dispersal. North America. As for the disappearance of the horses (p. 206), 

 the reason is not known, although the same factor or factors may well 

 have operated in both instances. 



Barriers 



The examples cited above reveal a common pattern underlying much of 

 evolution and dispersal. An animal arises by evolution in a certain region. 

 Typically it then attempts to expand its range, under pressure of factors 

 such as overpopulation in the original center. Whether or not it succeeds in 

 expanding its range, and the extent of expansion, depends upon many 

 factors. 



First among such factors we may mention physical barriers. Bodies of 

 water are barriers to land-dwelling animals, the effectiveness of the barrier 

 depending upon the attributes of the particular animal in question. Con- 

 versely, dry land is a barrier to the dispersal of aquatic animals. High 

 mountain ranges, deserts, open plains (to forest dwellers), forest (to 

 dwellers on open plains), as well as such climatic factors as intolerable ex- 

 tremes of temperature and many other physical factors serve as barriers 

 to the dispersal of one animal or another. 



No less effective are biological barriers. Among examples of these we 

 may mention absence from a given region of food suitable to the species in 

 question, presence in a given region of competitors for the same food sup- 

 ply or nesting sites, presence in a region of predatory animals, and the like. 



The action of barriers may be nearly or quite complete, as in the case of 

 the barrier to amphibian dispersal presented by sea water, or the barrier 

 may be only partially effective. The degree of effectiveness depends not 



