134 THE WHITE RIVER BADLANDS 



"Under modern conditions, no mammals could seem 

 more completely foreign to North America than those of the 

 camel family, which, now restricted to two well-defined 

 genera, inhabit central Asia and the colder parts of South 

 America. Yet, as a matter of fact, this family passed 

 through nearly the whole of its development in North 

 America and did not emigrate to the other continents be- 

 fore the late Miocene or early Pliocene, and it is this North 

 American origin of the family which explains its otherwise 

 inexplicable distribution at the present time. To all appear- 

 ances, the whole family had completely disappeared from 

 this continent in the later Pleistocene, but in the middle and 

 earlier portions of that epoch both true camels and large 

 llama-like animals were very abundant. * * * 



"The most ancient known camels of the Old World are 

 found in the Pliocene of India, and the first llamas recorded 

 in South America are also Pliocene. Since both camels and 

 llamas existed together in North America, it may be reason- 

 ably asked why only one phylum migrated to Asia and 

 only the other to South America. Why did not each con- 

 tinent receive migrants of both kinds? Without knowing 

 more than we are ever likely to learn about the details of 

 these migrations, it will not be possible to answer these 

 questions, though plausible solutions of the problem suggest 

 themselves. It is to be noted, in the first place, that a mi- 

 gration from the central portion of North America to Asia 

 was by way of the far north and thus involved very different 

 climatic conditions from those which must have been en- 

 countered in passing through the tropics to South America. 

 It is perfectly possible that animals which lived together in 

 temperate North America should have had very different 

 powers of adaptation to heat and cold respectively, and the 

 northern route may have been impassable to one and the 

 southern route to the other. To this it might perhaps be 

 objected that llamas are cold-country animals, but this is 

 true only of the existing species, for fossil forms are found 

 abundantly in the Pleistocene of Ecuador, Brazil and Ar- 

 gentina. Another possibility is that both phyla did actually 

 migrate to both continents and that only the camels suc- 

 ceeded in permanently establishing themselves in Asia and 

 only the llamas in South America, though for this solution 

 the fossils afford no evidence." 



