412 CLASSIFICATION OF THE EQUIDA£. 



be produced by the inter-crossing of zebras and asses, 

 and of different kinds of zebras, and of different kinds 

 of asses. This further classification is therefore purely 

 theoretical, which is a remark that also applies to any 

 present distinction made between the domestic horse and 

 Prjevalsky's horse (p. 640). 



When some members of a species possess hereditary 

 characteristics which distinguish them in a well-marked 

 manner from the other members of their species, we are 

 justified in considering that they belong to a distinct 

 variety. Darwin rightly remarks that " I look at the 

 term species as one arbitrarily given for the sake of 

 convenience to a set of individuals closely resembling 

 each other, and that it does not essentially differ from 

 the term variety, which is given to less distinct and more 

 fluctuating forms." 



The distinctive characteristics which belong to the 

 members of a species or of a variety, are the natural 

 result of evolution, in which climate, soil and enemies are 

 the chief factors in the case of the Equidae. Among 

 domesticated animals, artificial selection and artificial 

 conditions produce new types, which are termed breeds. 



Differences Betiween Asses and Horses. — Some 

 of the following differences between asses and horses are 

 relative ; and others absolute. Most of these differences 

 also exist between zebras and horses. 



1. The ass has, practically speaking, chestnuts only on 

 the fore legs, which is a peculiarity that is met with in 

 certain breeds of horses (p. 319). In some cases, the ass 

 has vestiges of chestnuts on his hind legs. The chestnuts 

 and ergots (p. 319) of the ass are much thinner than those 

 of the horse. 



2. The ass has a tufted tail, somewhat like that of an 

 ox, an erect mane, and no forelock. The horse, with the 

 exception of the Prjevalsky wild horse (p. 640), has a bushy 

 tail, drooping mane, and a forelock, when they have been 



