384 



SYSTEMATIC ZOOLOGY. 



as in artiodactyl forms, the pig for example, but one (third) 

 is enlarged and bears most of the weight of the body. 



The rhinoceroses have three toes on each foot; the skin 

 is extremely thick ; * the snout bears one or two well- 

 developed horns, in which there is no bony core; and 

 canine teeth are not developed even in the young. There 

 are six species known, those occurring in Africa having 

 two horns, while in the East Indies are both one- and two- 

 horned forms. 



In the horses the reduction of toes has gone still farther, 

 there being but one (the middle or third) in each foot. In 

 the skeleton, however, traces of two more can 

 be found in the ' splint-bones/ two small 

 bones occurring alongside the large 'cannon- 

 bone ' (fig. 170). All of the existing horse- 

 like forms have the teeth i-|, cf, pf, w-f, 

 and all are natives of the Old World, none 

 existing in America at the time of its dis-- 

 covery. All evidence goes to show that 

 the home of the domestic horse was in cen- 

 tral Asia, and indeed four different species 

 of horse run wild there to-day. The asses 

 have their centre around the eastern end 

 of the Mediterranean, while the zebras or 

 striped horses are all African. In geological 

 time, however, America had horses, and 

 the fossils in our western states give the 

 history of the race from small forms about 

 the size of a fox, and with three toes behind 

 and four in front; later, those as large as a 

 sheep, with three functional toes in each foot; and still 



* The elephants and rhinoceroses were formerly united as a group 

 called Pachydermata on account of the very thick skin. 



s-\ 



FIG. 170. Foot 

 of horse, 

 showing the 

 splint - bones 

 (second and 

 fourth toes) 

 at s; 3, third 

 toe. 



