MORE ABOUT SPECIES 103 



many kinds of fossil extinct horses known, some of which 

 clearly must be placed in the genus Equus with the 

 living kinds, others which have to be separated into 

 special genera (Hippidium, Onohippidium, etc.). Now, 

 as to the living forms or form-kinds of the genus Equus 

 which are we to regard as true species, and which are 

 only varieties and races of lower significance than species ? 

 The answer is clear enough in regard to several of them. 

 The wild Mongolian horse and all the domesticated horses 

 are varieties, races, or breeds of one species, judged not 

 only by such marks as the possession of callosities on 

 both the hind and the fore legs, but also by the test of 

 breeding. They breed together and produce persisting 

 races. But the asses and the zebras, though they will 

 form mules with the horse, do not in a state of nature 

 freely breed with it. When an ass or zebra is mated by 

 man with the horse it will produce hybrids, called " Mules," 

 but will not in " a state of nature " establish a hybrid race. 

 The asses and the zebras are distinct from the horse, not 

 only in markings and certain details of shape and hair, 

 but in the fact that they cannot be fused into one race 

 with him. There are no sufficient experiments on the 

 aloofness of zebras and asses from one another in regard 

 to breeding, although it seems that they cannot establish 

 a mixed race, and are, therefore, distinct species judged 

 by that test as well as by their form and marking. It 

 is not known whether the so-called species of wild ass 

 the Asiatic and the African would prove to produce 

 fertile or infertile mules if intercrossed, nor has the test 

 been applied to the very differently-marked local races 

 of the African zebras Grevy's zebra, Burchell's zebra, 

 and the mountain zebra. It is likely enough that the 

 three or more " species " distinguished among zebras on 

 account of their being differently striped, and existing 

 in different localities, would be found to breed freely 



