260 



tance that a knowledge of the subject 

 has in other fields. 



One consequence of the difficulty of 

 the material is that the exact mode of 

 inheritance is known for very few of 

 the differences among individuals. It 

 is important that suspected cases be 

 recorded, in order that other workers 

 may check them; but there is an un- 

 fortunate tendency to accept such 

 records as demonstrations rather than 

 as suggestions. After examining some 

 of the available published evidence, I 

 am convinced that, even for some of 

 the standard textbook examples, the 

 evidence for the accepted mode of in- 

 heritance is far from conclusive— and 

 that it would be recognized as at best 

 suggestive, if any organism other than 

 man were concerned. 



There are enough unambiguous ex- 

 amples known to make it clear that the 

 same principles are at work in man as 

 in all other higher animals and plants— 

 and even without such evidence, 

 enough is known about the cytology 

 of human tissues to give us confidence 

 that no peculiar kind of inheritance is 

 to be expected in man. In fact, much of 

 the argument concerning the practical 

 aspects of the genetics of man is best 

 based on experimental evidence from 

 other organisms rather than on what is 

 known directly from study of human 

 populations. 



The position is especially unsatisfac- 

 tory with respect to the heritability of 

 the most important of all human dif- 

 ferences—namely, mental ones. It 

 would be possible to quote recent au- 

 thorities for rather extreme positions 

 on each side of this question. To some 

 there appears to be no clear evidence 

 for genetic differences in mental ca- 

 pacities among most individuals or 

 among races, the observed mental di- 

 versity being attributed to environ- 

 mental effects; to others the position is 

 reversed— the environment accounts 



STURTEVANT 



for little, genetic differences for nearly 

 all the observed diversity. In these cir- 

 cumstances it is necessary to examine 

 what direct evidence we have. 



At the sensory level there is good 

 evidence for inherited differences. 

 There can be no question that such 

 things as color-blindness, night-blind- 

 ness, or sensitivity to the bitter taste 

 of phenylthiourea are simply inherited; 

 and one may confidently suppose that 

 other such inherited sensory differ- 

 ences remain to be discovered. As has 

 been pointed out by Blakeslee, we all 

 live in different worlds by virtue of 

 inherited differences in our sensory 

 reactions to external stimuli. It should 

 further be pointed out that these dif- 

 ferences have effects at the highest 

 mental levels. About 8 percent of 

 white males are at least partially red- 

 green blind; and when such a man 

 looks at a painting he does not see what 

 the artist put there or what other peo- 

 ple see. It is clear that this simple and 

 rather frequent genetic property has 

 inevitable effects on the esthetic life 

 of the individual. 



These remain rather trivial sorts of 

 differences; but there is another large 

 class of inherited mental differences 

 that is far from trivial. Certain types of 

 severe mental derangement, such as 

 Huntington's chorea or phenylketo- 

 nuria, clearly have at least a large in- 

 herited element in their causation, al- 

 though for most of them the exact 

 method of inheritance may be re- 

 garded as somewhat uncertain. 



However, what we are really^ most 

 interested in is the vast array of differ- 

 ences lying between these extremes; 

 and it is just here that the difficulty of 

 the human material becomes most seri- 

 ous. When one is dealing with complex 

 characters that vary more or less con- 

 tinuously in diverse respects, a genetic 

 analysis is difficult in any material; in 

 the case of man, a direct attack on the 



