Heredity and Conduct 99 



largely from the unhealthy, the mentally 

 defective, the stupid and unconscientious 

 members of a society, your society will de- 

 generate, and that more rapidly than is 

 generally recognised. Thus 25 per cent, of 

 married couples produce 50 per cent, of the 

 next generation. In other words, owing to 

 difference in the size of families, probably 

 about one-sixth of the adult men and women 

 in the community produce more than half the 

 next generation. If that one-sixth chances 

 to be the less thrifty, less conscientious, less 

 sane and less able portion of the community, 

 a very few generations will suffice to totally 

 change the character of a nation. The inex- 

 plicable decline and fall of nations following 

 from no apparent external cause receives 

 instant light from the relative fertility of the 

 fitter and unfitter elements combined with 

 what we now know of the laws of inheritance. 

 The one safeguard against this multipli- 

 cation of the unfit lies in a selective death- 

 rate. And man as much as any other form 

 of life is subject to natural selection. 



[This was illustrated by diagrams of the < Inheritance 

 of Duration of Life,' and of the relation between 

 ' Number of Offspring and Length of Life '; also by 

 tables giving determinations of the selective death-rate 

 in man.] 



The effects of natural selection are in man, 

 however, interfered with by other causes, the 

 chief of which is the relative fertility of class. 



72 



