CAUSES OF STERILITY IN THE MULE. 3 



year ago (Wodsedalek, '14), has rendered several phases of this 

 difficult problem much more intelligible than would have been 

 possible otherwise. It might be added that certain phases of 

 this problem would have become even more perspicuous had the 

 spermatogenesis of the ass also been carefully worked out and 

 thoroughly understood; but unfortunately the jack material 

 has so far been unavailable. 



COMPARISON OF THE HORSE AND THE Ass. 



Hayes ('04) says: "Owing to the extreme want of uniformity 

 in the gaps left, during the process of evolution, between de- 

 scendants from similar ancestors, we are unable to lay down any 

 exact general rules for classification. The inclusion of horses, 

 asses, and zebras in the genus Equns admits of no controversy, 

 because they are the only possessors of the distinguishing char- 

 acteristic of having only one complete hoofed toe on each foot." 



While there seems to be no doubt that the horse, ass, and zebra 

 have descended from common ancestors, at the present time, there 

 is considerable difference between the horse and the other two 

 Equidse. The only distinctive difference between the asses and 

 zebras in general seems to lie in the tiger-like stripes which are 

 invariably lacking in the ass. The differences between asses 

 and horses are marked, as can be seen from the following quota- 

 tion of a detailed comparison by Hayes ('04). "Some of the 

 following differences between asses and horses are relative, and 

 others absolute. Most of these differences also exist between 

 zebras and horses. 



"i. The ass has, practically speaking, chestnuts only on the 

 forelegs, which is a peculiarity that is met with in certain breeds 

 of horses (p. 319). In some cases, the ass has vestiges of chest- 

 nuts on his hind legs. The chestnuts and ergots (p. 319) of the 

 ass are much thinner than those of the horse. 



"2. The ass has a tufted tail, somewhat like that of an ox, an 

 erect mane, and no forelock. The horse, with the exception of 

 Prjevalsky wild horse (p. 640), has a bushy tail, drooping mane, 

 and a forelock, when they have been allowed to grow. The dif- 

 ference in the mane is due to the length of the hairs of the part. 

 In almost all breeds of horses the hairs of the tail grow long from 



