264 THE STATUS OF MAN. 



How much inter-marriage is possible between two species 

 without loss of identity? How effective must be the barriers 

 between them ? We see that this depends upon the power of the 

 species automatically to restrict their variability. Hybrids will 

 mate with pures, and their offspring with pures, until no 

 trace of the cross is left. So long as the heightening of the varia- 

 bility by crossing does not exceed the possible reduction of 

 variability through any or all of the causes which bring this 

 about, a species will conserve its identity. 



Whenever we compare specific distinction in man to specific 

 distinction in animals and plants, we must remember that, in 

 nature and under cultivation, both genetic differences and 

 circumstances keep species distinct. We saw how in nature two 

 species, mutually fertile, can live in approximately the same 

 environment, if their difference in germinal constitution makes 

 matings between members of one group more common than 

 matings between individuals of different groups. The very 

 existence of a multitude of individuals of one species guaran- 

 tees the future of this species. And in those cases where a great 

 difference is caused by a hereditary difference of genotype, the 

 process is obvious. 



Geographic barriers are the most striking examples of non- 

 genetic causes for specific distinctness. 



Under cultivation we again meet both causes. Here we find 

 instances where groups of animals and plants constitute spe- 

 cies in every essential sense, because the massing of a multitude 

 of like individuals is a guarantee for the future of the type, 

 even if no genotypic difference is responsible for the massing. 

 Even obligatory allogamous plants like beets, maintain spe- 

 cific distinctiveness even where some cross-breeding between 

 types takes place, when plants of one species are cultivated in 

 masses. In man, we find all sorts of causes responsible for spe- 

 cific difference, specific distinctiveness. 



Difference is a necessary result of distinctness. If two groups 

 are effectively separated, each will tend to become pure for its 

 own type, and chance will greatly determine what the type 



