MULES AND OTHER HYBRIDS 229 



of distinction between races and species ; the 

 former being practically species in the making. 

 On the contrary, fertility or sterility in the hybrid 

 depends on whether the two species (or even races) 

 severally represented by the parents are near or 

 distant relations of one another. P^or instance, the 

 Arab horse, whether it be regarded as a distinct 

 species or merely as a race of Equus caballus, as 

 exemplified by the wild tarpan, is clearly not far 

 removed from the latter, and the two therefore 

 interbreed freely. Again, as already mentioned, 

 the horse and the ass are two of the most widely 

 sundered members of the equine family, and their 

 hybrids are therefore sterile. 



On the other hand if hybrids between the 

 African wild ass and the true zebra, which as we 

 have seen are nearly related, were to prove fertile, 

 as they well might, this would be no argument 

 for regarding those animals as races of a single 

 species, and the same would be the case if fertile 

 hybrids — which is improbable — were the result of 

 a union between the bontequagga and Gravy's 

 zebra or between the latter and the true zebra. 



Practically all that can be inferred from the 

 interbreeding of the various members of the horse 

 family, so far as systematic zoology is concerned, is 

 that the whole of them are rightly included in the 

 single genus Equus. 



Although no complete list of all the hybrids 



