DIFFERENTIAL FERTILITY 73 



attached significance to the effects on a population 

 of such differential controls on fertility, and some 

 have suggested that certain nations are committing 

 intellectual suicide, inasmuch as the more intellectual 

 classes are proportionately less fertile than the lower 

 classes. The subject is extremely complex. It is 

 plain that many other factors come into operation, 

 notably the circumstance that those classes in which 

 fertility is limited from prudential considerations, at 

 least secure that their children, although numerically 

 fewer, are better protected against the accidents of 

 early life, better educated and more successfully 

 launched on their individual careers. I am very far 

 from confident as to the direction in which such 

 forms of internal selection may mould a stock, and 

 still less confident as to the amount of their effect, but 

 it is plain that the orientation and the quantity of 

 the results must differ from nation to nation and so 

 tend to the production of divergent modification. 

 It is much more clear that the relative absence of 

 intermarriage between persons of different nation- 

 ality must act as a powerful factor of isolation, for 

 so far as the stocks are concerned, isolation is no 

 more than relative infrequency of intermarriage. 



Whatever be the value of the various processes of 

 internal selection, these must work differently in 

 different nations. Galton, many years ago, sug- 

 gested that the mental darkness of the Middle Ages 

 might be due to the beliefs and customs that imposed 

 celibacy on the finer and more intellectual spirits, 

 and we may at least be certain that in each nation 

 those variations that are in harmony with the national 

 system of " Kultur " will be at no disadvantage. 



