48 HEREDITY [ch. 



course applies especially to inheritance in Man, 

 where experimental evidence is not available. 



By collecting family histories of distinguished 

 men, Galton showed long ago [15] that exceptional 

 mental qualities were inherited; and this work 

 has recently been much extended and made more 

 definite by Professor Pearson and his school. It 

 is commonly believed that exceptionally gifted men 

 do not have distinguished sons, but this like many 

 other popular beliefs is only partly true. It 

 has been seen that if an individual deviates a 

 certain amount from the general mean, his children 

 will on the average deviate less, because when the 

 whole ancestry is taken into account, the effect of 

 previous generations is to cause regression on the 

 mean of the population. And since the theory of 

 regression depends on the assumption that variation 

 is due to the existence of a large number of inde- 

 pendent causes acting concurrently, it is unlikely 

 that among the limited number of offspring of one 

 exceptional man any one child will unite in himself 

 the same combination of factors as went to make up 

 the father's character. Further, it is improbable that 

 an unusually gifted man will marry a wife equally 

 gifted in the same manner, and the mother's influence 

 on the children is nearly similar to that of the father. 

 It cannot therefore be expected that all great men 

 should have equally great sons, but they are far more 



