520 



GENETICS IN RELATION TO AGRICULTURE 



however, there appears to be no reason for questioning the possibility of 

 the occasional production of functional sperm and egg-cells, although un- 

 questionably they would be very rare. In that case it is difficult to see 

 why mare mules when bred to stallions may not occasionally produce 

 foals which would either be very horse-like or very mule-like in appear- 

 ance. On this basis, however, we are justified in regarding with grave 

 doubt those cases of mare mules which have produced several foals, for 

 cases of fertility should be isolated phenomena which should rarely be 

 repeated in the same animal. The matter of fertile mules possesses 

 some theoretical interest, but little practical importance. 



FIG. 202. A zebroid, produced by mating a burro to the Grevy zebra. (After Rommel.) 



Hybrids between other species of Equus have been obtained from time 

 to time, and some attempts have been made, as yet unsuccessful, to find 

 a place for them in practical agriculture. The zebra crosses with the 

 horse and the ass, producing in both cases vigorous, growthy hybrids. 

 In Fig. 202 is shown a zebra-ass hybrid, in Fig. 222, a Grevy zebra, and 

 in Fig. 224, a zebra-horse hybrid. Other hybrids have also been pro- 

 duced but they are all simply zoological curiosities. They all appear to 

 be infertile, like the mule, although here again reports are not agreed. 

 Darwin mentions a zebra-ass hybrid which when bred to a mare got a 

 foal, very horse-like in appearance. Rommel reports the zebra-ass 

 hybrid to be infertile, although giving greater promise of a limited 

 fertility than the mule. 



In the genus Bos a num.ber of species hybrids are known. Detlefsen 

 has given a list of them taken from Nathusius. Thus the domestic 

 cow, Bos taunts, has been mated with the yak, Bibos grunniens; with the 



