48 THE FOUR SEGREGATIVE PRINCIPLES. 
point is that the greatest difference existing between any form of 
organisms 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 
exist 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. Segregate Assoctation,* 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 codperate 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 of 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. 
