A CRITICAL EXAMINATION 15 



parent and offspring can be measured, and since 

 heredity is a cause of the resemblance between 

 parent and offspring, therefore the correlation 

 method measures the degree or intensity of in- 

 heritance. This argument seems sound but it is 

 really specious. For heredity is not the sole cause 

 which can lead statistically to a significant corre- 

 lation between parent and offspring. Anything 

 whatsoever which tends to bring about local or 

 group differentiation within the sample included 

 in the table will tend to produce the same result, 

 altogether independently of any genetic relation- 

 ship or the absence of it. Such possible differen- 

 tiating factors are so numerous and so difficult 

 of detection in many cases, as to make exceedingly 

 dangerous any assumption that, in a particular 

 case, the coefficient of correlation is a measure of 

 heredity. 



The essential difficulty here is just as real 

 whether one goes to the length of calculating cor- 

 relation coefficients or not. The logical fallacy 

 involved is particularly insidious in the case of at- 

 tempts to study by statistical methods alone the 

 problem of the effects of selecting fluctuating vari- 

 ations. Starting with heterogeneous material, as all 

 such studies except those on self -fertilized plants or 

 protozoa have started, if one does not keep an 

 exact pedigree record of every single individual 1 



1 Furthermore, it is not enough merely to keep the pedigrees. The 

 individual pedigree-line-of-descent must be the unit of analysis rather 



