VARIATION AND HEREDITY IN TWINS 155 



which it exercises progressively greater dominance as 

 development proceeds. 



B. VARIATION AND HEREDITY IN HUMAN TWINS 



No .studies of value have appeared dealing with 

 variation and heredity in dizygotic or fraternal twins; 

 consequently our attention must be directed solely 

 to these phenomena in duplicate (presumably mono- 

 zygotic) twins. Perhaps the strongest evidence in 

 favor of the idea that certain human twins are mono- 

 zygotic comes from a study of the variability or lack 

 of variabiKty between these twins. So pronounced is 

 the lack of variability in some cases that such twins are 

 called "identical." A treatment of human twins 

 paralleling that of armadillo quadruplets will show 

 many points in common. 



THE DEGREE OF IDENTITY IN HUMAN DUPLICATE TWINS 



The first seripus attempt to determine the closeness 

 of resemblance between twins was made by Galton. As 

 early as 1875 he showed his appreciation of the value 

 of such a determination; this comes out clearly in his 

 paper entitled "The History of Twins, as a Criterion 

 of the Relative Power of Nature and Nurture." Gal- 

 ton conceived the idea that the degree of resemblance 

 between duphcate twins is an index of the strength 

 of heredity as opposed to environment. In attempting 

 a minute comparison between twins he found them so 

 much alike that he had to resort to such details as the 

 patterns the friction ridges in the palms and soles. 



It remained for Wilder, however, actually to carry 

 out this study which Galton formulated, and some 



