IDENTICAL TWINS IN CATTLE. 3 



The comparison of the three frequency distributions is made 

 under P. The data of Cole and Lillie clearly do not correspond, 

 the male class in Cole's frequency distribution being proportion- 

 ately small, whereas the male class in Lillie's data is proportion- 

 ately large. The data of Lillie and this paper correspond rather 

 well. Here, again, the chief trouble is in the proportional differ- 

 ences of the male classes. The data of Cole and this paper are not 

 so very far different, the great discrepancy being in the male class. 

 The data of this paper are clearly about half way between those of 

 Cole and Lillie in the proportion of sex. Cole's data were ob- 

 tained from breeders. It is well known that the breeders register 

 only about one half as many males as females in their herd books. 

 Without question Cole would not knowingly have taken data so 

 loaded. It is conceivable, however, that the breeders from whom 

 he obtained his data might have been influenced by this proportion. 



If identical twins are lacking in cattle, the proportion of births 

 should be 1:2:1, providing no disturbances like lethal factors 

 exist in the stock. The comparison of the frequency distributions 

 to this proportion are given in the second column of P for Table I. 

 No excessive deviations from this proportion exist. The poorest 

 approach to expectation is the distribution presented by Cole. 

 Here, judging from the male and female twins and the female 

 twins, the male twins should be more frequent. This evidence is 

 opposed to that of expectation on the basis of the production of 

 identical twins. 1 Thus in human twin births, where identical 

 twins are produced, the proportions of sex are: males 234,497, 

 male and female 264,098, females 219.312, according to Nichols. 2 

 This proportion corresponds to a ratio of 1.07 : 1.20 : i. In species 

 where identical twins are produced it is clear that the like sex 

 should be produced more frequently than the unlike sex of twins, 

 whereas in Cole's data the like sex, the males in particular, are in 

 defect. The proportion of twins as given by Lillie's data is more 

 nearly in accord with the expectation of some identical twins. 

 However, it will be noted that the male class is in excess and the 



1 The explanation of Cole for this difference will be taken up later. 



2 Nichols, J. B., Memoirs of the American Anthropological Ass., I (1907). 

 also in Newman, H. H., " The Biology of Twins," Univ. Chicago Science 

 Series. 



