FERTILITY IN ANIMALS 



557 



ility as the mule. Of other hybrids within the genus Equus the evidence 

 is even less satisfactory. Apparently the cross between the horse and 

 zebra is sterile, like the mule. But the zebra and ass appear to be more 

 closely related, and the possibilities of securing offspring from such 



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FIG. 218. Illustrating inheritance of barrenness through the female line of the Duchess 

 family of shorthorns; barren cows represented by solid black circles. 



hybrids appears to be somewhat greater. There is in fact one reference 

 in the literature to a fertile male hybrid between the zebra and the ass. 



In the genus Bos, taken in the wider sense to include the subgenera 

 Bison and Bibos, there are various degrees of sterility consequent upon 

 hybridization. The domestic cow, Bos taurus, gives fertile male and 



