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 
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 
Digitized by Microsoft® 
