204 EVOLUTION AND GENETICS 



differences that fall within what would be called the 

 "normal" is more important from the point of view 

 of human evolution. Here also we are on very du- 

 bious grounds when we discuss the inheritance of 

 individual mental peculiarities, and in still greater 

 danger of error if we attempt to discriminate be- 

 tween racial complexes. The similarity in behavior or 

 in "temperament" or mental qualities of identical 

 twins might be expected to furnish important infor- 

 mation as to how much is acquired and how much 

 inherited. The very close physical similarity of twins 

 of this kind might make such material favorable for 

 study. There are, however, even here two serious 

 drawbacks that complicate the results. In the first 

 place unless the twins had been separated in very early 

 childhood it would be difficult to decide how much 

 is due to similarity of nature and how much to nur- 

 ture. A comparison with other children in the same 

 family may be helpful but is not decisive, for the 

 ex23erience of each child from successive births is 

 affected by older and younger children in the family. 

 In the second place the so-called standard tests may 

 measure training rather than constitutional factors. 

 Until these difficidties can be overcome, the manv 

 anecdotes of the close similarity in temperaments, or 

 abilities, of identical twins do not supply the needed 

 evidence. 



We can, by artificial selection, eliminate struc- 

 tural defects from a race of animals or plants and 



