OUR NATURAL INHERITANCE 79 



Professor Karl Pearson gives a vivid concrete illustra- 

 tion in his Grammar of Science : — " The father with 

 a great excess of the character contributes sons with 

 an excess, but a less excess of it ; the father with a great 

 defect of the character contributes sons with a defect, 

 but less of it." If we take a group of fathers 6 feet 

 in height the mean height of their sons will be 70''8 ; 

 if we take a group of fathers of 5 feet six inches the 

 mean height of their sons will be 68''3. 



Regression is a technical term which has nothing to 

 do with retrogression or with reversion ; it refers to a 

 movement towards the mean of the stock, whether the 

 movement be in the direction of progress or the re- 

 verse. The reason for the regression is tersely stated 

 by Pearson : — " A man is not only the product of his 

 father, but of all his past ancestry, and unless very 

 careful selection has taken place, the mean of that 

 ancestry is probably not far from that of the general 

 population. In the tenth generation a man has (theo- 

 retically) 1,024 tenth great-grandparents. He is even- 

 tually the product of a population of this size, and their 

 mean can hardly differ from that of the general popula- 

 tion. It is the heavy weight of this mediocre ancestry 

 that causes the son of an exceptional father to regress 

 towards the general population mean ; it is the balance 

 of this sturdy commonplaceness which enables the son 

 of a degenerate father to escape the whole burden of the 

 parental ill." 



These statistical conclusions must be pondered over. 

 {a) They are average generalisations for bodies of people, 



