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- 

 f pws__and jirv 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 thp lyTinppflp types, we may select Pro-., 

 t.ohippus. 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 t^iirt^-six inchg^ 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_ th^^inder^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- 



