MAN AND MANKIND 



two or the periodic change from one to tlic other. And vvliere 

 one has failed it is time to choose the other. 



Our genetic principles, therefore, favour neither the extreme 

 advocate of racial purity nor the equally extreme anthropologists, 

 philosophers and historians (whether liberal, Marxist or Catholic) 

 who dogmatically assert what they desire to believe, namely, that 

 genetic and racial differences in man are trivial or temporary products 

 of an all-powerful environment, "not a cause but a consequence," 

 and so on. Genetic analysis resolves the conflict between "race" and 

 "environment." And it does so by assuming an entire determinacy 

 of individual differences, physical and mental; a determinacy in the 

 reaction of genotype and environment; a determinacy which is 

 likely to prove more important in explaining the past and present 

 structure of our species than of any other. 



Breeding Systems 



Ever since, in 1865, McLemian first used the terms endogamy 

 and exogamy the importance of the breeding system has been 

 understood in man. Risley, for example, in 191 5 pointed out its 

 genetic significance as follows (p. 154) : "Amongst the various causes 

 which contribute to the growth of a race or the making of a nation 

 by far the most effective and persistent is the jH5 cowiuhii—the body 

 of rules and conventions governing intermarriage. The influence 

 of these rules penetrates every family; it abides from generation 

 to generation, and gathers force as time goes on. The more eccentric 

 the system the more marked are the consequences it tends to 



produce." 



The first principle of importance in measuring the effect of 

 breeding systems is that the heterogeneity, the frequency of gene- 

 differences, within any group is reduced with the rise of inbreeding 

 and is reduced the more rapidly the smaller the group which the 

 inbreeding delimits. 



In man the two restrictions of breeding which are characteristic 

 of crossbreeding organisms are both maintained. Extreme out- 

 breeding and extreme inbreeding are both discouraged, but the 

 direct genetic control is, as usual in man, replaced by social and 



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