CENOZOIC ERA: AGE OF MAMMALS 



609 



and a rudimentary first toe on the fore foot, while the hind foot 

 had three toes and rudimentary first and fifth toes. The foot was 

 a spreading one, enabling the animal to walk on fairly soft ground. 

 From this earliest known, simple form, the course of evolution 

 consisted largely in such modifications of the skeleton as rendered 

 the animal better fitted to secure food and masticate it and to 

 escape its enemies. This, as will be seen, resulted in the production 

 of a very perfect grinding apparatus. The necessity for speed in 

 seeking safety and in going long distances «for food and water, 

 resulted in the remarkably perfect locomotive apparatus. 



THE EVOLUTION OF THE HORSE. 



Formations in Western United States and Characteristic Type of Horse in Each Fore Foot Hind Foot Teeth 



One Toe 



Splinb of 



Lon$> 



Crowned^ 



Th ree To es 



Side !0» 

 nolloiicliiiiffhe. grand 



$•' 



Th ree To es 

 Side tots 



touching thefo™ 

 jotint et S'i 4ifl 



touching ft* frount 



Sborl 

 Crowned 



F our Toe s 



Splinlof I'dijif 



T hree To es 



Splint of 5idi{it 



Hypothetical Ancestors with Five Toes on Each Foot 

 and Teeth like IhosTof Monkeys etc 



Fig. 547. — Table showing the evolution of the horse. (After W. D. Matthew.) 



The next horse in the line of descent (Protorohippus, Fig. 547) 

 appeared in the Upper Eocene and was four or five inches higher 

 than the early Eocene horse (Eohippus), with longer limbs, which 

 indicate ability for increased speed. The fore foot had four toes, 

 but lacked the rudimentary toe, or splint, of the Eohippus, while 

 a shortening of the outermost toe (the fifth) gave promise of a three- 

 toed foot in its descendants. The hind foot had three toes but no 

 splint. 



The Oligocene horse (Mesohippus, Fig. 547) differs from the 

 Eocene one (Proterohippus) in having but three toes on the fore foot 

 and a splint which represents the outermost (or fifth) toe of the 



