308 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



of gradual and progressive modifications, a sketch that might 

 readily be expanded into a volume, were all the details filled 

 in. While each set of organs, teeth, skull, neck, body, limbs 

 and feet, might appear to advance independently of the others, 



Fig. 159. — Skeleton of a Pampean horse {i Hippidion neogceum), National Museum, 

 Buenos Aires. For restoration, see Fig. 119, p. 214. Note the splint-like nasal bones 

 attached only at the hinder end. 



in reality there was no such independence, for at every stage of 

 the progression all the parts must have been so coordinated 

 into a harmonious whole, that the animal could thrive and 

 hold its own in the stress of competition. Could we but dis- 

 cover all the facts of environment, on the one hand, and or- 

 ganization, on the other, we should doubtless learn that the 

 little \Eohippus was as exquisitely fitted to its place in the 

 Wasatch world, as are the horses, asses and zebras of the 

 present day to theirs. It was the response to changing needs, 

 whether of food, climate, disease or competition, that was 

 the main factor of development. 



2. ]Titanotheriid(E. iTitanotheres 



This family, all of whose members vanished from the earth 

 ages ago, was a comparatively short-lived group and nearly 

 the whole of its recorded history was enacted in North America ; 



