AND PLANTS 107 



In his statements about regression, Mr. Galton always 

 dealt with the phenomena of inheritance by a beautiful 

 application of the mathematical theory of chance, which 

 he was the first to use in treating biological problems : 

 but very few of those biologists who have tried to use his 

 methods have taken the trouble to understand the process 

 by which he was led to adopt them, and we constantly 

 find regression spoken of as a peculiar property of living 

 things, by virtue of which variations are diminished in 

 intensity during their transmission from parent to child, 

 and the species is kept true to its type. This view may 

 seem plausible to those who simply consider that the 

 mean deviation of children is less than that of their 

 fathers : but if such persons would remember the equally 

 obvious fact that there is also a regression of fathers on 

 children, so that the fathers of abnormal children are on 

 the whole less abnormal than their children, they would 

 either have to attribute this feature of regression to a 

 vital property by which children are able to reduce the 

 abnormality of their parents, or else to recognize the real 

 nature of the phenomenon they are trying to discuss. 



Our business to-day is with methods rather than with 

 results. I will therefore only show you one example of 

 regression between parent and child, and I choose the 

 relation between the breadth of span in mothers and in 

 daughters, which has lately been determined by Professor 

 Pearson. Without giving you any more detailed numbers, 

 the circles on the diagram (fig. 2) will show you how nearly 

 the relation between the mean span of daughters and the 

 known span of mothers is given by a regression line with 

 the same slope as that giving the relation between the 

 first and second results in our experiment with dice. The 

 regression of second throws on first throws of dice is 

 very nearly exactly f . Professor Pearson's value for the 

 regression of daughter's span upon mother's span is 0.46. 



