MENTAL CHARACTERS IN MAN 159 



fact that " one feebleminded man left at large five 

 generations ago is responsible for seventy-five feeble- 

 minded persons living at the present time." The 

 normal population of the country, apart from immi- 

 gration, can hardly have increased to anything like 

 the same extent during this time. 



But even if all feebleminded individuals were 

 prevented b}- segregation from multiplying, the most 

 difficult part of the process of eliminating feeble- 

 mindedness from the germ plasm of the population 

 would scarcety have begun. For, as East (191 7) has 

 pointed out, while the number of feebleminded in the 

 whole population of the United vStates may be 

 estimated at 3 per 1,000, the number which are 

 carrying feeblemindedness as a recessive defect must 

 be nearer i in 14. Punnett (191 7) puts the number 

 even higher, and emphasises the impossibihty of 

 greatly reducing feeblemindedness in the population, 

 except by segregating also those who are carriers of 

 the disease as a recessive quality. He shows that if 

 the proportion of the feebleminded in the United States 

 is now 3 per 1,000, then it would require 250 genera- 

 tions, or about 8,000 years, to reduce the proportion 

 to I in 100,000 b}- the method of merely segregating 

 or sterilising those who show the character. It is 

 evident, then, that the elimination from reproduction 

 of those showing any recessive racial defect merely 

 prevents the problem from becoming more serious, 

 while marked improvement in the germ plasm of the 

 population can only be effected by selection against 

 the heterozygotes, who are carrying the defect in half 

 their germ cells. More accurate mental tests ma>' 

 make it possible to distinguish such heterozygous 

 individuals from the fully normal members of the 

 population. This has happened in the history of the 

 study of many Mendelian characters. From this 

 point of view, any tendency for those transmitting 



