486 EVOLUTION, GENETICS, AND EUGENICS 



In one sense, the controversy about the relative importance of 

 heredity and environment is rather silly and futile, for both heredity 

 and environment are absolutely essential for development to take place 

 at all. The real controversy hinges on this question: With respect 

 to any given character, do the actually existing differences in heredity have 

 a greater or a less effect in determining the differences found in the adidt 

 than do the actually existing differences in the environment? 



Now some characters, such as eye-color, hair-color, shape and ar- 

 rangement of teeth, finger and palm patterns, features, body build, 

 and a long list of others, are evidently not much altered by existing 

 differences in the environment and may therefore be called purely 

 hereditary — and by this we do not mean that the environment has 

 nothing to do with their determination. Other characters, such as 

 body weight, muscular development, skills of various kinds, extent of 

 tanning of the skin, certain types of eye defects, effects due to the 

 ravages of disease, and many others, seem to be very largely influenced 

 by existing differences in the environment, and may be called environ- 

 mental. Large numbers of characters fall in between these extremes, 

 characters that are surely predetermined, within limits, by heredity 

 but are also much modified by actually existing differences in the en- 

 vironment. Among the most significant types of human difference 

 that are both hereditary and environmental, in the sense just discussed, 

 are differences in mental capacity (as determined by intelligence tests) 

 and differences in temperamental and emotional make-up (as deter- 

 mined by psychological tests). 



Identical twins have furnished some highly significant data on 

 this point. As a preliminary step it was necessary to determine the 

 average differences in mental capacity and in temperament for fifty 

 pairs of identical twins reared together and for the same number of 

 same-sexed fraternal twins reared together. Since the heredity was 

 the same and also the environment essentially the same for identical 

 twins reared together, the observed differences must be due largely to 

 the workings of the asymmetry mechanism, the same mechanism that 

 makes the two halves of a single individual different. In order to test 

 the effect of differences in environment, therefore, we must compare 

 the differences found in identical twins reared together with those 

 of identical twins reared apart. In both those reared together and 

 those reared apart the asymmetry mechanism works in the same way; 

 therefore, greater differences in separated twins than in those reared 

 together may fairly be attributed to differences in environment. 



