THE ORIGIN AND HISTORY OF ANIMAL LIFE 



619 



was correlated with an elongation of the 

 head and neck, which was necessary in or- 

 der to reach the ground. The front teeth 

 were modified as chisel-like cropping struc- 

 tures; and the back teeth evolved from sim- 

 ple molars into wonderfully effective grind- 

 ing organs with tortuous ridges of enamel 

 and with supporting and protecting layers 

 of dentine and cement. During the later pe- 

 riods, the molars elongated, and thus be- 

 came adapted for grinding the dry grasses 

 which caused them to wear down more rap- 

 idly than did the softer vegetation. During 

 this evolution, the body gradually increased 

 in size from that of the earliest known form, 

 which was about as large as a domestic cat, 

 to that of the horse of today. 



Eohippus (dawn horse) 



This tiny "dawn horse," also called Hyra- 

 cotherium (Fig. 437), was only 11 inches 

 high at the shoulder and about the size of 

 a fox terrier. It lived during the Lower 

 Eocene. Eohippus lived in North America 

 and Europe and was a browsing forest dwel- 

 ler. Its forefeet have 4 complete toes each, 

 but no trace of the first * (thumb); and the 

 hindfeet have three complete toes each and 

 the rudiments (splints) of the first and the 

 "little finger" (fifth) which are shown in 

 Fig. 437. 



Mesohippus {intermediate horse) 



This horse belongs to the Oligocene and 

 reached the size of a sheep. Its forefeet 

 possess three complete toes each, and the 

 fifth digit is represented by a splint; the 

 hindfeet also possess three complete toes 

 each, but no splint. All three toes touched 

 the ground, but the middle toe is larger and 

 bore most of the weight of the body. 



Merychippns (ruminating horse) 



This horse lived in the Miocene, but be- 

 came extinct in the Pliocene. It marks the 

 transition from the primitive horse to the 



* It was formerly claimed that the first toe was 

 represented by a splint, but such a claim is not 

 supported by any known specimen. 



modern horse. The milk teeth are short- 

 crowned and uncemented, like those of the 

 primitive horse, but the permanent teeth are 

 long-crowned and fully cemented grinders, 

 suited to the harsh vegetation of the plains. 

 Both its forefeet and its hindfeet possess 3 

 toes each. 



Pliohippus (Pliocene horse) 



This horse from the Upper Miocene and 

 Pliocene is the first one-toed horse. Both 

 fore- and hindfeet are one-toed, and the 

 second and fourth toes are represented by 

 splints. The crowns of the upper molars 

 are similar to those of the modern horse, but 

 they do not possess as complex a pattern 

 of ridges on the surface. Pliohippus had a 

 shoulder height of some 40 inches, about 

 the size of a modern pony. 



Equns (horse) 



The modern horses of the Pleistocene and 

 Recent epochs have lost the first and fifth 

 digits entirely, and the second and fourth 

 digits are represented by splints. The third 

 toe alone sustains the weight of the body. 

 The crowns of the molar teeth are much 

 elongated with complex enameled ridges 

 well adapted for grinding dry harsh vegeta- 

 tion. The lengthened skull is accompanied 

 by a larger and more complex brain. This 

 horse is about 60 inches tall, being consider- 

 ably larger than any of its ancestors. The 

 evolution of the horse has resulted in the 

 development of an intelligent, long-legged, 

 swift-running animal that is suited to live 

 and feed on the open grasslands. 



At the present time, true wild represent- 

 atives of the genus Equus occur only in 

 Asia (the Asiatic wild ass, Equus hemionus, 

 and Przewalsky's horse, E. przewalskii) and 

 in Africa (the African wild ass, E. asinus, 

 and the zebras, E. zebra, and E. burchelli). 

 The mustangs and broncos of our western 

 plains and South America are not true wild 

 horses, but are descendants of domesticated 

 horses brought over from Europe. 



The evolution of the elephant (see head- 

 piece, p. 601), camel, dog, and many othei 



