FAMILIES OF PERISSODACTYLA. 



739 



mentary, and soon lost in both sexes, and the canines are rarely 



present in the mare. The orbit is complete. 



The modern horses are connected by a very complete series of forms 

 with ancestral Eocene types. The progress shows an increase of size, 

 a diminution in the number of digits, an increased folding of the back 

 teeth, and other differentiations. The Eocene Phenacodus is regarded 

 by some as near the origin of the stock, it had five complete digits on 

 each foot ; Hyracotheriuni and Systemodon had only four functional 

 digits in the manus ; Anchitherittm from the Miocene, an animal about 

 the size of a sheep, had three digits, or three and a rudiment ; Hippo- 

 therhtin and Protohippns from the Pliocene, were as large as donkeys, 



FIG. 365. Feet of horse and its progenitors. 

 From Neumayr. 



i. PaUeotherium ; 2. Anchitherium ; 3. Hippotherium ; 4. Equus. 



and show a marked diminution of the second and fourth digits ; finally, 

 in the Pleistocene, the modern forms appeared. 



The living species are the horses (Eqinis caballus], apparently 

 originating in Asia, domesticated in prehistoric times, artificially selected 

 into many breeds, sometimes reverting to wildness, as in the case of 

 those imported into America and Australia by European settlers ; the 

 wild horse of Central Asia (E. przevalskii] ; the donkey (E. asimis) of 

 African origin ; the wild asses of Africa and Asia ; the striped African 

 species the zebras and the (exterminated) quagga. 



Family Rhinocerotidre. There is now but one genus Rhinoceros, 

 species of which occur in Africa and in some parts of India and 

 Indo-Malaya. They are large, heavy Ungulates, shy and noc- 

 turnal, fond of wallowing in water or mud, feeding on herbage, 

 shoots, and leaves. The skin is very thick, with scanty hair. 



