68 



GENETICS AND EUGENICS 



The second use of the correlation coefficient is still more 

 important, viz., to measure the strength of heredity. It 

 affords a means of comparing the strength of a character in 

 successive generations and of thus measuring its heredity. 

 Thus the amount of white on the body of piebald rats is a 

 variable character (Fig. 125) to some extent inherited. The 

 resemblance between parents and offspring in grade of white- 

 ness as shown in Table 4 is about 23 per cent, the correlation 

 coefficient in this case being 0.233. Pearson found, for his 

 human data, the height of father and son to have a correla- 

 tion of 0.514; between brother and brother he found the 

 correlation to be 0.511, figures which indicate the strong 

 inheritance of size differences in man. 



TABLE 3 



Correlation Table showing the Relation between Femur-length 



AND HUMERUS-LENGTH IN 370 RaBBITS. T = 0.857 



From MacDowell, Appendix, Table 16. 



Probable error is a measure of the reliability of a statistical 

 conclusion. The need of such a measure rests on the fact 

 that the number of observations on which the conclusion rests 

 is finite, that is the number of observations is smaller than 

 the class concerning which generalization is made. For ex- 



