78 HEREDITY AND SOCIAL FITNESS 



XIII. HERITABILITY OF SOCIALLY EFFECTIVE TRAITS. 



In tracing the history of such traits as the ability to calculate, 

 aggressiveness, and perseverance in the foregoing, the lesson seems 

 clear that through selective matings the grade of these traits may be 

 noticeabh' increased or diminished. For example, in Line C, a com- 

 paratively low ability to calculate has, through selective matings with 

 good or average ability, been built up into an ability well above the 

 average. This holds true also of aggressiveness. In Line D, on the 

 contrary, ability to calculate and aggressiveness, through matings 

 with a very low grade of these traits, has persisted at the low level 

 shown by one of the original founders. In the same way the trait- 

 complex of perseverance may be followed and found to move up or 

 down according to the type of mating made. 



Let us now see how far the behavior of these traits in inheritance 

 conforms to Mendel's law. At the outset of this discussion we wish to 

 concede that our data, since they appertain to mental and tempera- 

 mental traits, do not permit of the accuracy of measurentent possible 

 in handling physical traits. Our conclusions, accordingly, must at 

 best be only approximations to the truth. Our only justification for 

 attempting to carry Mendel's principles into this domain is the value 

 which even tentative and approximate conclusions may have for 

 fruitful effort along similar lines in the future. 



1. Calculating Ability. 



Considering the wide range of calculating ability in the general 

 population, it would seem unreasonable to suppose its manifestation 

 to be due to a single unit character. Unfortunately, too, we know too 

 little of the grade of this ability in the stocks into which the earlier 

 descendants of Aaron and Mary Rufer married to fit the distribution 

 in the various fraternities to a mathematical formula. But here the 

 faculty is also present in such variety of degree as to forbid the con- 

 clusion that it is due to a single unit character, unless we ascribe this 

 variation in grade to a variation in potency of the unit character. 

 Such variation in potency would be comparable to that invoked to 

 explain the phenomena of inheritance of many physical traits. In 

 these cases, while there may not be a regular Mendelian proportion, 

 dominance and segregation seem nevertheless to be taking place. 

 Here, too, the degree of development of the somatic character is often 

 an index of its strength in the gamete, and selection of the individual, 

 though of secondary importance to the selection of the strain to which 

 he belongs, still appears to play a part in the grade of development 

 attained in the offspring; that is, an individual, possessing a charac- 

 teristic in high degree, will with a certain mating produce a higher 

 degree of that characteristic in his offspring than would a member of 



