4 ; THE POUR SEGREGATIVE PRINCIPLES. 



point is ih it th( greatesl difference existing between any form of 

 org tnisms and any other form that is capable of crossing with it, is 

 very small compared with the differences presented by the vast 

 world of organisms that are incapable of crossing with it. Cross- 

 fertilization secures the blending of elements from individuals that 

 are more or less divergent; but the degree of divergence that may 

 it without resulting in sexual incompatibility, either physiological, 

 morphological, or psychological, is very small compared with the 

 divergences that lie beyond the limits of compatibility. Again, we 

 find that in some cases species incapable of crossing are so similar in 

 visible characters that the naturalist finds difficulty in distinguishing 

 them. 



3. S -1 isociation* a Fundamental Law on which the Social Structure of 



Each Species that is not wholly guided by Instinctive Aptitudes must rest. 



The necessity of a common language for a social group will be rec- 

 ognized by all. There must be a system of signs or signals by which 

 the members of one group may call to each other when they wish to 

 come together, may warn each other of approaching danger, and may 

 in other ways cooperate in securing the sustenance, protection, and 

 propagation of the group. If these signs are not instinctively made 

 and instinctively understood, they must depend on suggestion, train- 

 ing, and imitation; and this suggestion, training, and imitation is 

 made possible by the association maintained by the social group. It 

 is a universal fact that the social characteristics of individuals of the 

 same associating group are gradually unified by association. Again, 

 it is certain that a group of freely associating and freely intergenerat- 

 ing individuals i >f different races of mankind will in time become assimi- 

 lated in language, manners, and customs, and finally in race, however 

 different they may have been when first brought together. The 

 blending of two strongly marked species is usually prevented either 

 by instinctive aversion, by unfruitful crossing, or by the sterility of 

 the hybrid offspring, as in the case of the mule; while the blending of 

 two civilizations is liable to be prevented by the superior power of one 

 completely overshadowing the influence of the other. Moreover as 

 long as racial barriers are not broken down, distinctions of social 

 types are not often wholly obliterated. This is naturally the case as 

 long as the training of the young, and so the transmission of tradition, 

 remains chiefly with the parents. 



Segregate association, that is, the association of like with like, is 

 the fundamental factor in the production of habitudinal segregations; 



* That is, the association with each other of individuals with like acquired char- 

 acters. 



