VARIATION AND HEREDITY IN TWINS i6i 



sequent doubling of the lines thus formed. Even the double 

 scale in the armadillo may have its counterpart in a twin sweat- 

 pore, which indicates the composite nature of the unit to which 

 they belong. 



Wilder believes, however, that the far greater 

 complexity of pattern in the human friction ridges 

 gives a better basis for detailed comparison than the 

 simpler condition seen in the armadillo. He also 

 admits that there is a great advantage on the side of 

 the armadillo in the possibiHty of studying the embryonic 

 conditions. 



It is remarkable that the same situations of exact 

 resemblance and of various grades of symmetry reversal 

 occur in both human and armadillo monozygotic twins. 

 The significance of these phenomena must, I believe, 

 be the same in both cases. In the armadillo these 

 manifestations are the result of polyembryonic develop- 

 ment; it is almost certainly the same for man. 



Coefficients of correlation between twins. — The human 

 friction ridges do not furnish as good material as do the 

 bands of' the armadillo for establishing an exact numer- 

 ical measure of the degree of resemblance between twins. 

 An enumeration of the individual friction ridges of the 

 numbers of sweat-pores in a given pattern would give 

 results comparable in availabihty to the enumeration 

 of scales in the banded region or in individual bands of the 

 armadillo, Doubtless these data could be obtained for 

 human twins, but as yet there is no such information 

 at hand. 



Attempts have been made, however, to work out 

 the percentage differences between twins on the basis 

 of a comparison of various physical measurements and 



