534 EVOLUTION, GENETICS, AND EUGENICS 



both had measles, whooping cough, and scarlatina at the same time, 

 and neither has had any other serious illness. Both are and have been 

 exceedingly healthy, and have good abilities, yet they differ as much 

 from each other in mental cast as any one of my family differs from 

 another." 



Gal ton finally concludes as follows: "There is no escape from the 

 conclusion that nature prevails enormously over nurture when the 

 differences of nurture do not exceed what is commonly to be found 

 among persons of the same rank in society and in the same country." 



This was a strong statement and needed to be confirmed and made 

 more exact. Two American specialists set out to put this conclusion 

 on a firmer basis. 



Professor Thorndike used biometrical methods, especially the co- 

 efficient of correlation. He studied fifty pairs of twins in the New 

 York City schools with reference to their closeness of resemblance in 

 eight physical and six mental characters. He found that on the aver- 

 age twins, not distinguishing between fraternals and identicals, were 

 from two to three times as similar as were ordinary children of the 

 same ages brought up under similar environment; that twins from 

 twelve to fourteen years old were no more alike than twins nine to 

 eleven years old, although they had been several years longer under 

 similar environment during an extremely plastic period; that twins 

 were no more alike in traits subject to much training than those sub- 

 ject to little or no training. Thorndike concludes: 



"The facts are easily, simply and completely explained by one 

 simple hypothesis; namely, that the nature of the germ-cells — the 

 conditions of conception — cause whatever similarities and differences 

 exist in the original natures of men, that these conditions influence 

 mind and body equally, and that in life the differences in modification 

 of mind and body produced by such differences as obtain between the 

 environments of present-day New York City public school children 

 are slight." ■ 



It will be noted that Thorndike, although he used the data of twin 

 resemblance to test the relative powers of nature and nurture, makes 

 no clear distinction between fraternal and duplicate twins. In ex- 

 perience he discovered that in many cases the twins were obviously of 

 one category or the other, but that there were many marginal cases 

 that he was unable to classify. 



About the same time, or possibly a little earlier, Piofessor Wilder 

 was making a very elaborate comparison of duplicate twins with refer- 



