ANCESTORS AND INTERRELATIONS OF VERTEBRATES 701 



complete toes and a splint of the fifth digit, and the hind feet 

 also possess three complete toes, but no splint. All three toes 

 touched the ground, but the middle toe is larger and bore most 

 of the weight of the body. Anchitherium from the Lower Mio- 

 cene is larger than Mesohippus ; Parahippus and Hypohippus 

 from the Middle Miocene were as large as a Shetland pony. 



(4) Protohippus and Pliohippus. In these animals from the 

 Middle and Upper Miocene there are three toes on each foot, but 

 the middle one is large, and the side toes are smaller and do not 

 touch the ground. The crowns of the upper molars are long and 

 provided with an effective grinding surface of ridges of cement. 

 Hipparion which lived during the Pliocene Period is larger than 

 Protohippus and has a more complicated tooth pattern. 



(5) Equus. The modern horses of the Pleistocene and Recent 

 periods 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, the skull has lengthened, and the body 

 is considerably larger than that of any of its ancestors. 



At the present time true wild horses occur only in Asia (the 

 Asiatic Wild Ass, Equus hemionus, and Przewalsky's Horse, E. 

 pryzewalskii) and in Africa (the African Wild Ass, E. asinus, and 

 the Zebras, E. zebra, E. burchelli, and E. quagga). The mus- 

 tangs and broncos of our Western Plains and South America are 

 not true wild horse, but are descendants of domesticated horses 

 brought over from Europe. 



The evolution of the elephant, dog, and many other animals 

 has been carefully worked out by paleontologists, but none quite 

 so much in detail as that of the horse. Nevertheless, they show 

 how much is possible toward a knowledge of the ancestors of 

 vertebrates from a study of fossil forms. 



