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 
