No. 599] SHORTER ARTICLES AND DISCUSSION 701 



milk, cannot be of frequent occurrence." The development of 

 just such a "factor of safety" and not the origination of an intra- 

 racial correlation is, as emphasized above, just what one would 

 expect of natural selection. 



Natural selection, if operative, should, however, bring about an 

 interracial correlation, and this is exactly what observant biol- 

 ogists have always noted and Pearl has expressed statistically by 

 the value r=>. 594 ±.046, with non-linear regression— a value 

 distinctively higher than that for the intraracial relationship. 

 Thus, as far as they go, these observations instead of evidencing 

 against natural selection, actually show the very conditions to 

 exist which might be expected as the result of the action of this 

 factor of organic evolution. 



Inheritance of Number and Arrangement op Nipples in 

 Swine 



Attempts at the Mendelian analysis of inheritance of number 

 and arrangement of mammse in swine have been made by Went- 

 worth, 8 who has suggested that the presence of rudimentary 

 nipples is a sex-limited, 9 sex-linked, 10 or sex-limited 11 character, 

 His final stand is that the pair of rudimentaries posterior to the 

 inguinal pair behave as a Mendelian unit character in heredity, 

 but that somatically it develops in males, which are BB or Br, 

 but in the females only when they are BB, where B indicates the 

 presence and r the absence of the factor for rudimentaries. 



It is interesting to return to the sexual dimorphism with re- 

 spect to number of mamma? demonstrated above on the basis of 

 Parker's and Bullard's splendid series of data and to consider 

 it in connection with the hypothesis advanced by Wentworth. 



Pearson many years ago showed 12 that with continued random 

 mating the distribution in any generation subsequent to an orig- 

 inal random pairing of BB and rr individuals is 



iRR + iRr + trr. 



s Wentworth, E. N., "Inheritance of Number of Mammse in Swine," Rep. 

 Am. Breed. Ass., 8, 1912. 



» Wentworth, E. N., "Another Sex-limited Character," Science, N. S., 

 35: 986, 1912. 



"Wentworth, E. N., "Sex-linked Factors in the Inheritance of Rudi- 

 mentary Mammse in Swine," Proc. Iowa Acad. Sci., 21: 265-268, 1914. 



« Wentworth, E. K, "Rudimentary Mammae in Swine a Sex-limited Char- 

 acter," Science, N. S., 43: 648, 1916. 



"Pearson, K., Phil Trans. Boy. Soc. Lond., A, 203: 59-60, 1904. 



