EVIDENCES OF EVOLUTION 27 



gocene, Dr. Lull writes : " The drying up of 

 streams and lakes, due to the increasing aridity 

 of climate, gave great impetus to the develop- 

 ment of broad meadow lands, and to the true 

 prairie as well. Thus there were three con- 

 ditions woodland, meadows 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 Proto- 

 hippus, with three toes on each foot, but 

 only one touching the ground. The short- 

 crowned teeth without cement are now 

 replaced by long-crowned cement-covered 

 teeth like those of the modern horse. Proto- 

 hippus was about thirty-six inches high at 

 the shoulder, and had a wide distribution from 

 Texas to Montana and Oregon. In a closely 

 related 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 instance. 

 Merychippus is of particular interest, because 

 it is almost certainly in the direct line of 

 ancestry to all subsequent Equidae. The 

 forest-horse, Hypohippus, with spreading 



