26 EVOLUTION 



increasing aridity of climate, gave great im- 

 petus to the development of broad meadow 

 lands, and to the true prairie as well. Thus 

 there were three conditions woodland, mead- 

 ows and dry prairie, which seem to have 

 given rise to several parallel lines of evolution, 

 some of which terminated, being overcome 

 in the struggle for existence, while others 

 flourished and gave rise to the horses of the 

 Miocene." 



Of the Miocene types we may select Pro- 

 tohippus, with three toes on each foot, but 

 only one touching the ground. The short- 

 crowned teeth without cement are now re- 

 placed by long-crowned cement-covered teeth 

 like those of the modern horse. Protohippus 

 was about thirty-six inches high at the shoul- 

 der, and had a wide distribution from Texas 

 to Montana and Oregon. In a closely re- 

 lated genus, Merychippus, we find the first 

 instance of the completion of a bridge of bone 

 at the hinder border of the orbit, one of the 

 characteristic differences between the skull 

 of a horse and that of a carnivore, for in- 

 stance. Merychippus is of particular inter- 

 est, because it is almost certainly in the 

 direct line of ancestry to all subsequent 

 Equidse. The forest-horse, Hypohippus, 

 with spreading three-toed feet, suited, like 

 the reindeer's, for soft ground, is a good ex- 



