IIO2 



THE UNGULATES, OR HOOFED MAMMALS 



droves of these animals are kept solely for the sake of their milk. A peculiarity in 

 the disposition of the ass, is its reluctance to cross even the smallest stream of water; 

 this aversion being doubtless a direct inheritance from its desert-haunting wild 



ancestors. 



The term mule is strictly applicable only to the hybrid between the 

 Mule male ass and the mare; the product of the union of the opposite sexes 



of these two species being known as the hinny. Mules, although they frequently 

 display the stubbornness and obstinacy of the ass in an intensified degree, are for 

 some purposes more valuable than either of their parents, being very sure footed 

 and with great powers of endurance. Some of the finest mules are bred in Spain, 

 the United States, and Northwestern India, where they frequently attain the height 

 of sixteen hands. In Spain they are generally employed to carry burdens, and 

 march in long droves, following in single file a leader distinguished by a bell. 

 Among the dun-colored mules of the Punjab, dark stripes on the legs are very com- 

 mon. 



There appear to be no authenticated instances of mules breeding among them- 

 selves, although the female mule will occasionally produce offspring with the male 

 horse or ass. And it is somewhat remarkable that it does not appear that the 

 hybrids between any other members of the Equine family are mutually fertile. 



FOSSIL HORSES 



It has already been mentioned that remains, undistinguishable from the exist- 

 ing horses, occur in the superficial deposits of Europe and Arctic America; but that 

 those found in the corresponding formations of the United States and South America 

 appear to belong to extinct species, of the genus Equus. In the upper molar teeth 

 of all these species the front inner pillar marked^ in figure B on p. 1075 is much 

 elongated from front to back. In the figured tooth which belongs to an extinct species 

 (E. sivalensis} from the Siwalik hills of India, that pillar is, however, shorter; and 

 in Steno's horse (E. stenonis], from the Pliocene deposits of Europe, it is so much 

 shortened as to be almost cylindrical. The same is the case with certain extinct 

 species from the later deposits of the United States and Argentine, which, on ac- 

 count of the great length of the slit for the nose in the skull, are separated as a dis- 

 tinct genus, under the name of Hippidium. All the foregoing have but a single toe 

 to each foot, but we now come to certain other species in which there were three 

 distinct hoofs. One of these is the Protohippiis of the lower Pliocene strata of the 

 United States, in which the upper molar teeth approximate to the one represented in 

 figure B on p. 1075, but have shorter crowns. The other is the European and Asiatic 

 hipparion, or three-toed horse, of which an upper molar tooth is represented in figure 

 C of the page quoted. From that figure it will be seen that the front inner pillar p- 

 is completely separated from the portion pi. That the Protohippus was the ancestor 

 of the true extinct horses of America, there can be but little doubt; but, from the 

 separation of the inner pillar of the molars, it is not so certain that the hipparion 

 gave rise to the existing European members of the family. 



