RELATIVE POTENCY OF HEREDITY AND ENVTRONMENT 533 



therefore, just the type of material we need: two indrviduals with 

 identical heredity. 



Thus we have twins with identical heredity and twins with quite 

 different heredity. If we could always distinguish the two types, we 

 should be able to determine the effects of diversified environment in 

 the former and similar environment in the latter. Galton assmned 

 that if the environment has the power to modify the inborn character 

 of individuals, identical twins that arise from one germ cell and have 

 identical heredity ought to become progressively more and more unlike 

 if separated and reared under different conditions. Fraternal twins, 

 on the other hand, though quite different in heredity, should, if reared 

 under the same environmental and educational conditions, tend to be- 

 come progressively more and more alike. If, however, the identical 

 twins remain alike and fraternal twins remain different, it would ap- 

 pear that environment has little if any power to modify inborn con- 

 stitution, 



galton's use or data from two kinds or twins 



In order to put this matter to the test, Galton collected data on one 

 hundred pairs of twins, eighty of which he classed as identical and 

 twenty as fraternal. 



After a considerable amount of comment on the extraordinary close- 

 ness of resemblance between identical twins, some instances of which are 

 rather comical, he comes to a study of a number of cases where the 

 twins were separated early in life and reared under diverse environ- 

 mental conditions. The resemblances in physical appearance, in ideas, 

 in habits underwent no divergence, but the twins remained as much 

 alike as ever up to old age. Only serious illness or accident to one 

 twin seemed to have any effect upon their inborn resemblance. 



With reference to the twenty pairs of fraternal twins who were 

 unlike from birth, the conclusion was equally striking. In no case 

 was there any tendency for unlike twins to grow alike when reared to- 

 gether under the same environmental conditions. The comments of 

 the two parents are typical: 



o) "They have had exactly the same nurture from their birth up to 

 the present time; they are both perfectly healthy and strong, yet they 

 are otherwise as dissimilar as two boys could be physically, mentally, 

 and in their emotional nature." 



b) "They had never been separated, never the least differently 

 treated in food, '•Nothing, education; both teethed at the same time, 



