FILIAL REGRESSION 321 



Now, the amount of the regression affords a useful measure 

 of the intensity of the inheritance. If the regression is slight, 

 it means that the intensity of the inheritance is high ; if the 

 regression is considerable, it means that the intensity of the in- 

 heritance is low. The ratio between the deviation of sons in 

 general and the deviation of their fathers in general in respect 

 to a given character gives a measure of the intensity of inheritance 

 for that character, and is called the " coefficient of correlation." 

 A simple and very clear account of the way of obtaining a "co- 

 efficient of correlation " will be found in Doncaster's Heredity, 

 1910, chap. iv. 



The correlations worked out by Pearson and others for a 

 number of characters in plants, animals, and man, vary between 

 0*42 and 0-52, which means that on the average the offspring 

 deviate from the mean of the general population about half as 

 much as the parent. 



" It seems likely that in cases where the mating of parents 

 is not determined to any serious extent by their likeness or 

 unlikeness in the character discussed, the regression of children 

 on parents has a value very nearly the same, and very nearly 

 equal to |-, for a large series of characters, mental as well as 

 physical, in human beings, and for a large series of characters 

 in the higher animals, at all events, if not in animals generally " 

 (Weldon, 1906, p. 108). 



Summary. Many individual organisms differ markedly from 

 .the mean of the stock or race to which they belong. In some 

 character or characters they are extraordinary individuals. 

 What is the chief conclusion in regard to the offspring of these 

 individuals ? It is that they are, on an average, more mediocre 

 than their parents. 



As Mr. Yule puts it, " This phenomenon of the relapse of the 

 offspring from the parental type towards mediocrity is termed 

 regression. Regression and not constancy of type is, for the 

 statistician, the fundamental phenomenon of heredity and the 



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