390 LAND MAMMALS IN THE WESTERN HEMISPHERE 



vertebrae are proportionately long, which gives a somewhat 

 stiff and awkward motion to the neck. . . . The Giraffe 

 feeds almost exclusively on the foliage of trees . . . for brows- 

 ing on which its prehensile tongue and large free lips are specially 

 adapted." ^ 



In teeth and skeleton the phyla of the true camels and of 

 the llamas in the lower Pliocene did not differ very strongly 

 from the living forms ; the upper incisors were already reduced 

 to one, but the premolars were not so small ; the ulna and 

 radius had coalesced and of the fibula only the lower end re- 

 mained ; the cannon-bones were completely formed, and that 

 the pads of the feet had already been developed is shown by 

 the phalanges, especially the irregular, nodular unguals. 



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 reasonably be asked 

 why only one phylum migrated to Asia and only the other to 

 South America. Why did not each continent 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 solu- 

 tions of the problem suggest themselves. It is to be noted, in 

 the first place, that a migration 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 encountered 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 respec- 

 tively, 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 the llamas are cold-country animals, 

 but this is true only of the existing species, for fossil forms are 



1 Flower and Lydekker, Mammals Living and Extinct, p. 332. 



