MULES AND OTHER HYBRIDS 227 



the kiang and the onager ; the African wild ass ; 

 and two or three kinds of zebra. There are, 

 besides, many kinds of domesticated horses, ranging 

 from the Shetland pony to the Flemish dray-horse, 

 and from the shire horse to the Arab. Then there 

 are 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 i^Hippidium, 

 Onohippidiuni, Sic). 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 siofnificance 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 per- 

 sisting races. But the asses and the zebras, though 

 they will form mules with the horse, do not freely 

 breed with it, nor establish a hybrid race. They 

 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 



