322 



THE CONTINUITY OF THE RACE 



vidual's capacity for mental, physical, and moral development is deter- 

 mined by his nature are even more clear-cut and unmistakable. Evidence 

 from many sources indicates that men differ tremendously as to the kinds 

 of genes they receive at fertilization and that this gene complement 

 (nature) both limits and determines the extent to which they can utilize 

 and profit from whatever type of nurture they encounter. There is, 

 indeed, much evidence for the conclusion that the effectiveness of any 

 nurture that society can provide is dependent upon the maintenance of a 

 superior nature for at least a part of the population. 



Fig. 22.1. The inheritance of hair texture. Kinky hair dominates curly hair, which domi- 

 nates wavy hair, which dominates straight hair. 



Some of the kinds of evidence for the biological inheritance of human 

 qualities are as follows: 



1. Family pedigree, in which physical and mental traits can be traced 

 through many related individuals and the mode of inheritance correlated 

 with known types of inheritance in experimental animals. By this method 

 the inheritance of several hundred human characters has been clearly 

 worked out. The difficulty here is that the method works best for simple 

 cases of inheritance — unit characters and the less complex types of 

 modifying and complementary factors. It is difficult to use in cases where 

 there is complex interaction of genes or where environmental influences 

 affect the phenotypic expression of the genes. 



2. Comparisons of the degrees of resemblance and difference between 

 uniovular (identical) twins on the one hand and biovular (fraternal) twins 



