184 THE OX TRIBE. 



This being the case, it is quite clear that the fact of 

 two animals producing an intermediate race is no proof 

 whatever of their specific identity ; for it is well known, 

 and has been already alluded to, that several animals, 

 Birds as well as Mammalia, produce offspring, and are 

 nevertheless distinct, both as it regards anatomical 

 structure and external form. 



Neither does it constitute the species identical if 

 either or both the hybrids be even capable of fruitful 

 intercourse with the original or parent species. Hamilton 

 Smith goes so far as to say, that " if it even were proved 

 that a prolific intermediate race exist, produced by the 

 intermixture of both, it would not fully determine that 

 both form only one original species : what forms a 

 species, and what a variety, is as yet far from being well 

 understood." 



It is, however, pretty generally agreed, that animals 

 are of the same species, that is to say, have been derived 

 from one common stock, when their offspring have the 

 power, inter se, of indefinitely continuing their kind ; 

 and conversely, that animals of distinct species, or de- 

 scendants of stocks originally different, cannot produce 

 a mixed race which shall possess the capability of 

 perpetuating itself. 



To conclude, it must be obvious, that permanent 

 anatomical differences are the only true criteria of dis- 

 tinctions of species. 



