HUMAN GENETICS 353 



(correlation 0-21) and of her grandmother (correlation 0-1065). Fisher 

 concludes that about 40 per cent of the variance in human fertility is 

 due to hereditary causes. Secondly, he shows, again by data from 

 noble famiUes, that marriage with an heiress very frequently leads to 

 the extinction of the family, heiresses being presumably highly infertile 

 since they come from families in which no sons were produced. Fisher 

 beUeves that this principle is responsible for the fall of past civilizations 

 and is Hkely to bring down our own unless we learn to control it. 



The argument has been criticized on two main grounds.^ Firstly, it 

 may be denied that the apparent differences in intelligence between the 

 classes really represent genetic differences. We have discussed this 

 question earUer in this chapter. It has been urged in this connection 

 that the upper classes are primarily recruited from the infertile, intelli- 

 gence having much less to do with social advancement than Fisher 

 suggests, and in this case the differential infertiUty does not actually 

 sift out and remove the inteUigent. Secondly, it has been claimed 

 that recent investigations, mainly in Germany, show that the differen- 

 tial fertiUty is disappearing, and that the birth-rate of the lower 

 classes is sinking to the level of that of the upper classes. This has 

 certainly occurred in some regions, but we require much more exten- 

 sive data before we can judge how widespread it is. Germany in the 

 post-war years is scarcely typical of European civilization as a whole. 

 It is probable, however, that the first statistics on the matter, which 

 dealt with a period when birth control was just coming into use among 

 the upper classes, gave an exaggerated view of the differential fertility. 

 Possibly, with the general adoption of contraception in wider circles of 

 society, the birth-rates of different classes will converge to some extent; 

 whether they will attain equality is open to question. 



A second differential of the greatest social importance exists between 

 rural ccmmvmities with a typically high birth-rate and urban or indus- 

 trial communities with a low one. The genetic effects, if any, of this are 

 completely unknown. 



The effects of modem warfare on population composition are diffi- 

 cult to assess. Some authors, carried away by the doctrine of the 

 struggle for existence, have urged that warfare is the agency through 

 which natural selection in man has its most important effects and that 

 the aboUtion of war would be followed by the decline of human society 

 into a state of sybaritic incapacity. This, however, turns mainly on a 

 misinterpretation of the word struggle. The struggle for existence in 

 ^ Cf. Charles 1934. See also Haldane 1938. 



