VARIATION AND HEREDITY IN TWINS 175 



hereditary control in twins by concluding that the 

 general configurations of friction-skin patterns is pre- 

 determined and only the minutiae are beyond the limits 

 of hereditary control — is a question which will have 

 suggested itseK to the reader before this. This con- 

 clusion was based on those cases of dupUcates which 

 were alike in the general configuration of the patterns; 

 but if, as seems certain, monozygotic twins sometimes 

 differ not only in minutiae but in the general con- 

 figuration of friction ridges, this conclusion as to the 

 Umits of hereditary control fails to hold. 



It certainly fails to hold for armadillo quadruplets, 

 which are uniformly monozygotic. 



HEREDITARY CONTROL AND SOMATIC SEGREGATION 



To find a failure of bilaterality in certain friction- 

 ridge patterns in a single individual is not at all unusual. 

 For example, when I examine the friction patterns of 

 my own hands, I find a pronounced dissimilarity in the 

 two. The right hand has a very conspicuous hypothenar 

 whorl not even suggested in the left. In the left hand 

 there occurs a well-defined triradius, absent in the right. 

 In other respects the two hands are similar but not 

 identical. If hereditary control is to be tested by a 

 comparison of dupUcate twins, which are believed to be 

 monozygotic, why would it not be much simpler to 

 test it by a comparison of the antimeric halves of a 

 single individual who is known to be monozygotic? 

 Unquestionably, if by hereditary control is meant 

 that identity of two or more homologous or bilateral 

 products of a single zygote is predetermined, even the 

 main patterns of friction ridges cannot be said to be 



