THE INHERITANCE OF MENTAL ABILITY 105 



ment is correlated with superior performance of the student, 

 it does not follow that the former may not be the result of superior 

 heredity on the part of the parents. As Pearson remarks: "The 

 average home environment, the average parental influence is in 

 itself a part of the stock and not an external and additional factor 

 emphasizing the resemblance between children of the same 

 home." Doubtless this consideration which is not sufficiently 

 appreciated by those who would make environmental differences 

 all important, is of much weight. We are still lacking, however, 

 an adequate measure of the extent to which similarity of condi- 

 tions may produce similarities in mental characteristics. The 

 most reasonable position in the face of such evidence as we have 

 just considered is that as regards the traits in question, differences 

 in heredity are much more important than differences in environ- 

 ment. No other position seems to be easily reconciled with the 

 remarkable similarity in the degree of resemblance between 

 correlations for physical and mental characteristics. 



How often do we find among children of the same family 

 exposed to very similar conditions and having practically the 

 same training, but manifesting the greatest differences in tastes, 

 temperament, vivacity, ability, and other mental traits! Nor is 

 it a matter of common experience that these differences become 

 notably lessened with longer association and subjection to the 

 same environmental influences. The measurements of Thorn- 

 dike on the performance of school children who have been asso- 

 ciated for several years in the school, showed that the children 

 were quite as much unlike at 12 to 14 as between 9 and 10. Stu- 

 dents differing in their ability to perform certain tasks such as 

 addition were given precisely the same training, and then tested 

 again at a later period. Those who performed the task best at 

 the beginning of the experiment performed the task best at the 

 end, and they stood relatively further ahead of the poorer ones 

 than at first. Equalizing opportunity does not tend to make 

 people equal. If the opportunities for development are good 

 those with the best inheritance will profit so much more than 

 those with poor inheritance that the original differences between 



