VARIATION AND HEREDITY IN TWINS 157 



Wilder has devised an elaborate method of classifying 

 the various types of pattern in both palm and sole, 

 and an equally elaborate system of condensed formulae 

 for denoting the pattern complex of any individual. 

 For a description of this method of formulating friction- 

 skin diversity the reader is referred to Wilder's various 

 papers on these subjects. It would lead us too far 

 afield to attempt to introduce any adequate key to the 

 study of human palms and soles in this place. Suffice 

 it to say that in his studies of fifteen pairs of same-sexed 

 twins and one set of opposite-sexed triplets he noted 

 that eleven of them were "identical" in palm and sole 

 patterns and five were unHke. The conclusion is reached 

 that those which are "identical" are monozygotic and 

 those which fall considerably short of identity are dizy- 

 gotic. Since there is no knowledge of the intra-uterine 

 conditions in any case, this reasoning backward from 

 identity to origin is in this case unjustified. That this 

 kind of reasoning may lead to grave error is shown in 

 connection with those armadillo quadruplets in which 

 there are quite pronounced differences between the 

 -members of a set; yet their origin from a single egg-cell 

 cannot be questioned. In certain monozygotic human 

 twins it might readily happen that by somatic segrega- 

 tion the paternal condition of friction ridges would go 

 to one individual and the maternal to the other; or 

 there might be a partial segregation of important 

 elements of the pattern so that they would be dis- 

 tributed differently in the two. 



It must not be lost sight of, however, that identity 

 in friction-ridge patterns of twins makes their mono- 

 zygotic origin very highly probable. Identity may 



