INTRODUCTION 3 



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 distin- 

 guishable by fairly constant characters : among domestic 

 cats, for instance, we find white, black, tabby, gray, and tor- 

 toiseshell cats, besides the large long-haired Persian breed, 

 and the tailless Manx cat. All these are distinguished as 

 varieties of the single species, Felis domestica. 



It is often difficult to decide whether two kinds of ani- 

 mals 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 fer- 

 tile 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 pres- 

 ence 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 grada- 

 tions between these two sets of individuals, they will be 

 placed without hesitation in distinct species : if, on the 



