1 62 THE BIOLOGY OF TWINS 



weights. Vernon, for example, gives the data for two 

 pairs of identical twins, one of which, aged twenty- 

 three years, showed an average percentage difference 

 for a number of characters of 0.28; while the other, 

 aged twelve, showed a percentage difference of 0.71. 

 Weismann presented the data on one pair of twin 

 brothers, aged seventeen years, which showed a per- 

 centage difference of 2.2, nearly ten times that of 

 Vernon's first pair. 



Wilder obtained numerous measurements of three 

 pairs of twins, aged respectively 21.10, 17.10, and 

 17. II years. They showed an average percentage 

 of difference of. 2.03. Although Wilder realizes that 

 dimensional and other physical measurements are 

 quite unsafe criteria of the genetic resemblance between 

 twins, he is unable to furnish any substitute less objec- 

 tionable. It is well known that nutritional and other 

 environmental differences greatly affect the size and 

 weight of individuals. In spite of these facts, however, 

 the percentage differences brought out for twins are 

 quite comparable, in so far as they show close resem- 

 blance between monozygotic individuals, with the 

 coefficient of correlation brought out for the number of 

 scutes in the bands of the armadillo. 



It should be pointed out, however, that fairly 

 marked dimensional differences, even at birth, could 

 not be used as evidence against the monozygotic origin 

 of any particular pair of twins; in the armadillo, where 

 the monozygotic origin is specific and unequivocal, 

 there is frequently a striking size difference among the 

 quadruplets of a given set. On the whole, then, it 

 would seem inadvisable to use dimensional measure- 



