224 HEREDITY AND EUGENICS 



isolation. This evolutionary principle of isolation 

 is of enormous importance and has usually been 

 overlooked by anthropologists. 



The question is often discussed as to whether 

 modern man is a single species or more than one. 

 The fact that all the races of mankind are fertile with 

 each other is no longer a sufficient reason for class- 

 ing them as one species. The present generation of 

 naturalists is describing innumerable species of plants 

 and animals as distinct species, although they are 

 perfecth^ fertile with each other. Sterility as a 

 criterion of species has almost completely broken 

 dow^n. The origin, causes, and nature of inter- 

 specific sterility are still largely obscure. On the one 

 hand, species of Drosophila, so closely similar that 

 they are scarcely distinguishable even by experts, 

 may show complete sterility with each other or, at 

 least, provide onl}' sterile hybrids ;* while, on the other 

 hand, all the species of cattle {Bovidce) are inter- 

 fertile, although man}^ of them show striking 

 differences. In mankind the differences between the 

 five great colour varieties are not merely in the skin 

 colour, but also in such points as stature, hair colour 

 and shape, cranial conformation, facial peculiarities, 

 skin secretions, and intelligence. These differences 

 are quite as distinctive and varied as those betw^een 

 many described species of higher animals. Such 

 differences could only have grown up during long 

 periods of isolation, and can only be maintained by 

 isolation or an absence of crossing. The five colour 

 types of mankind also occupied, until modern trans- 

 portation began, more or less marked^ separated 



* Even more extreme are the various cases of self-sterility in 

 plants, in which many of the individuals are sterile with their 

 own pollen. In such cases as Primula, crosses between individuals 

 of the same species but having different types of flower are more 

 fertile than self-fertilised plants. 



