184 EVOLUTION AND GENETICS 



altogether probal^le that some of these numerous species were 

 intruders." 



Whatever may have been their source, all species living in the 

 Americas became extinct during the Pleistocene, and from the 

 Palearctic stock developed the zebras, asses, wild horses and 

 domesticated races of the present. Conditions are now so favor- 

 able for their existence that feral horses are found in the new 

 world. On the western ranges of our own country they have 

 even multiplied to such an extent that they are becoming a prob- 

 lem to stock raisers. What may have been the cause of their 

 ancient extinction becomes a difficult problem. Lull offers as a 

 theory the introduction of some virulent insect-borne disease, but 

 of course no such theory can be proved with evidence now avail- 

 able. All that we can know is that they died, and in some places 

 immense numbers are preserved as fossil skeletons. 



Camels. In still another group, the camels, an extraordinarily 

 complete series of fossil forms are available. These animals are 

 now represented only by domestic and feral individuals, and do 

 not occur in North America. They are Artiodactyla, or even-toed 

 ungulates and represent the course of adaptation for speed and for 

 subsistence on harsh and scanty herbage. The modifications of the 

 legs, however, show clearly the animal's fitness for progress over a soft 

 substratum such as the loose sand of many deserts, and in existing 

 forms the nostrils and eyes are equally eloquent of ability to resist 

 desert conditions. The ability of camels to store water beyond 

 their immediate needs, and the storage of fat in the hump, are 

 probably the best known adaptations. 



The evolution of the camels and their relatives, the llama and 

 alpaca of South America, is so similar to that of the horses that 

 Figure 110 will be an ample presentation for the purposes of this 

 work. It is significant that their development occurred at the 

 same time as that of the horses and in response, apparently, to the 

 same conditions of gradually increasing aridity. Although they no 

 longer occur in North America the remains of ancestral species 

 show that they were once common over the greater part of the 

 western United States and it was here that the true camels were 

 evolved. The camels of the Old World were apparently derived 

 from ancestors which migrated from North America over the 

 Bering Isthmus, leaving the main stock to perish. 



Such cases as these are the last word in evidence for evolution. 



