BIOLOGY IN HUMAN AFFAIRS 



ditions, though the facts are not altogether harmonious 

 in the few cases of this type that have been adequately 

 studied. 



The intelligence tests add to this evidence. Intelligence 

 tests measure acquirements, it is true, and they have been 

 criticized severely on this score. But the critics should real- 

 ize that there is no more reason w^hy properly safeguarded 

 measures of intellectual acquirements should not be inter- 

 preted in terms of genes than that developed physical 

 characters should not be thus interpreted. Tv^o ten-year-old 

 children, for example, shov^ significant differences in their 

 intelligence quotients. They are given the same studies and 

 are tested again at the end of four years. They are now 

 further apart mentally than they were in the earlier test. 

 The more intelligent has advanced further than the less 

 intelligent. What conclusion can be drawn other than that 

 they differed in genetic constitution? 



From the sum total of all the evidence geneticists have 

 concluded that mentality is a complex trait inherited like 

 any other complex character. Some of the numerous genes 

 concerned presumably have varied into what one may call 

 plus and minus types; and it takes only 20 of such genie 

 differences, inherited independently, to give the possibility 

 of 1,000,000 different recombinations. "If the Thinker 

 requires 20 plus genes and the Simpleton 20 minus genes, 

 then the Average Man may be supposed to have about 10 

 plus and 10 minus genes. If a family stock, by selective 

 matings, gathers together a preponderant proportion of 

 plus genes, its average worth will rise; conversely, if a 

 family puts its efforts into accumulating minus genes, its 

 social value will drop. But even the mediocrities may 

 produce Thinkers— or Simpletons— if the constitutions of 

 the fusing germ cells are such as to bring together the 

 required genetic complex. Thus there is no difficulty in 

 accounting for emergent individuals like Carlyle and 



[186] 



