CAUSES OF STERILITY IN THE MULE. 5 



observed it in the ass. This conservatism in color and freedom 

 from irregular markings, shown by the ass, is very remarkable; 

 considering how greatly the coat of the horse varies in this respect 

 and that the ass has, in all probability, been longer under the 

 influence of domestication than the horse. 



"n. The ass is higher over the croup, than at the withers 

 which is a peculiarity that tends to make his withers appear unduly 

 low (p. 241). The spines of the vertebrae at the withers are only 

 a little shorter in the ass than they are in the horse. As a rule 

 horses are higher at the withers than they are at the croup. 



"12. The horse's dock is thicker, stronger, and shorter than 

 that of the ass. 



"13. The horse, on each side of his croup and covering his 

 pelvis, has, underneath, and closely adhering to the skin of the 

 part, a thick and extremely dense layer of connective tissue, which 

 is so close and hard, that it looks like horn, when the skin has 

 been tanned and dried. These two patches of thickened skin 

 are separated from each other about four or five inches apart, so 

 that there is a strip of skin of ordinary thickness running down 

 the croup towards the tail. . . . The 'shell' is connected to the 

 skin so closely that the two form one piece; although their re- 

 spective consistencies are different. If a section be made through 

 the hide their line of union may be readily seen. In the ass, the 

 'shell' is not confined to the skin that covers the pelvis; but also 

 extends over the ribs, which are consequently not as sensitive 

 to the effects of blows as are those of the horse. . . . 



"14. The ass has no tufts of hair at the fetlocks (p. 290). 



"15. Messrs. Tegetmeier and Sutherland appear to have been 

 the first to note the difference between the respective periods of 

 gestation of asses and horses; the former period being twelve 

 months; the latter, eleven months. 



" 1 6. The foot of the horse is more highly specialized than that 

 of the ass. One of the best marked evolutionary changes in the 

 equine foot, was the gradual curtailment of its posterior bearing 

 surface (frog and sole), until, in the horse (Fig. 382), the length 

 of this bearing surface is not much greater than its width, and is 

 included between the heels and the 'toe' of the hoof. In the 

 ass, this bearing surface is relatively longer than in the horse, 



