INTRODUCTION 3 



invariably present differences in detail by which they can be 

 readily distinguished. Individual variations of this kind 

 are of universal occurrence. Moreover, it often happens 

 that the members of a species are divisible into groups 

 distinguishable by fairly constant characters : among 

 Domestic Cats, for instance, we find white, black, tabby, 

 gray, and tortoiseshell Cats, besides the large long-haired 

 Persian breed, and the tailless Manx Cat. All these are dis- 

 tinguished as varieties of the single species Felts domestica. 



It is often difficult to decide whether two kinds of 

 animals should be considered as distinct species or as 

 varieties of a single species, and no universal rule can be 

 given for determining this point. Among the higher 

 animals mutual fertility is a fair practical test, the varieties 

 of a species usually breeding freely with one another and 

 producing fertile offspring, while distinct species either do 

 not breed together or produce infertile hybrids or mules. 

 Compare, for instance, the fertile mongrels produced by the 

 union of the various breeds of Domestic Dog with the 

 infertile mule produced by the union of the Horse and Ass. 

 But this rule is not without exception, and in the case of 

 wild animals is, more often than not, impossible of 

 application : failing it, the only criterion of a " good 

 species " is usually the presence of constant differences from 

 allied species. Suppose, for instance, that a naturalist 

 receives for description a number of skins of wild Cats, and 

 finds, after an accurate examination, that in some specimens 

 the tail is two-thirds the length of the body and the skin of 

 a uniform reddish tint with a few markings on the head, 

 while in the rest the tail is nearly half as long as the body 

 and the skin tawny with black stripes. If there are no 

 intermediate gradations between these two sets of 

 individuals, they will be placed without hesitation in 



B 2 



