704 



THE AMERICAN NATURALIST [Vol.L 



where the n and x without subscripts denote number of individ- 

 uals per litter and number of nipples per individual, without 

 reference to sex. Numerically 



r*u* = .305 ± .019. 



This is lower than the relationship for either of the sexes in- 

 dividually considered, just as one might have predicted on a 

 priori grounds from the low value of the cross correlation and 

 from the differentiation in the number of mammae in male and 

 female pigs. 18 



The correlation coefficients here given show that there is a 

 very material degree of resemblance with respect of nipple num- 

 ber in pigs from the same litter. 19 Indeed the correlation is 

 about one third of the maximum value. Such correlation can 

 be due only to differences in intra-uterine environment or to a 

 strong inheritance of nipple number. The latter seems by far 

 the more probable explanation. 



J. Arthur Harris 



is Harris, J. Arthur, "On Spurious Values of Intra-class Correlation 

 Coefficients Arising from Disorderly Differentiation within the Classes," 

 Biometrika, 10: 412-416, 1914. 



io These values of the fraternal correlation will be but slightly influenced 

 by the weighting of the individuals in the determination of the correlations, 



