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THE AMERICAN NATURALIST 



[Vol. L 



by the accompanying table in which actual values have been re- 

 duced to per mille frequencies. Pigs with 12 nipples or fewer 

 are preponderantly females; pigs with 13 nipples or more are 

 preponderantly males. 



The correlation between the number of nipples on the two 

 sides are : 



.6359 ± .0073 

 .5419 ± .0088 

 .0940 ± .0114 

 .6063 ± .0055 



The correlations are fairly high. Those for males seem to be 

 slightly larger than those for females. 



Correlation between the Number op the Young in the 

 Litter and the Number of Mammae in the Dam 



The relationship between the number of young per litter and the 

 number of mammae in the female has at various times aroused 

 considerable interest. As Pearl 2 has pointed out, two kinds of 

 correlation are to be recognized. First, interracial correlation, 

 that between the mean size of the litters and the mean number 

 of mammae in the females of a series of races or species. Second, 

 intraracial correlation, that between the number of mammse in an 

 individual mother and the number of young that she bears. 



It is the rather obvious interracial correlation that has given 

 rise to such statements as that of Gegenbaur: "Die Zahl der 

 Zitzen stent in inniger Beziehung zur Menge der Jungen." It 



2 Pearl, R., "On the Correlation between the Number of Mamma? of the 

 Dam and Size of Litter in Mammals. I. Interracial Correlation," Proc. 

 Boc. Exp. Biol. Med., 11: 27-30, 1913. 



