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. 
Fria. 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 zodlogical 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 number of species hybrids are known. Detlefsen 
has given a list of them taken from Nathusius. Thus the domestic 
cow, Bos taurus, has been mated with the yak, Bibos grunniens; with the 
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