264 GENETICS 



categories (VIII), and cases in which they are alike in two 

 categories and diverse in the third, or ahke in one category 

 and diverse in the other two. 



On the whole, it is clear that other things, beside genetic 

 constitution, play important roles in determining mentality, 

 temperament and emotional traits. The principal difficulty 

 in Interpreting the results lies In the uncertainty as to how 

 much Is the result of the manner in which division of the 

 egg occurred, and how much Is due to later environmental 

 differences. Both certainly play a role. For these types of 

 characteristics, it appears probable that of the two, the 

 later environmental differences play the greater role. It 

 Is further clear that genetic resemblances and differences 

 play a very large role In mentality, temperament and emo- 

 tional traits. 



It Is to be noted that in all this study, the general en- 

 vironment of the twins was much alike in all cases. None of 

 them lived in different civilizations, or at different epochs in 

 cultural development. How great a diversity In the charac- 

 teristics studied could be made by such differences remains 

 uncertain. 



It Is further to be noted that the tests to which the twins 

 were subjected were designed to bring out rather perma- 

 nent personal traits, as distinguished from mental content 

 and habitudes. The behavior of human beings depends very 

 largely upon acquired mental content, upon knowledge, and 

 upon habitudes, and these things are in a high degree de- 

 pendent on the conditions under which the individuals live 

 and the experiences to which they are subjected. 



In general. It is clear that In a human population there 

 are great numbers of different types of Individuals that 

 are diverse because their genetic constitutions are diverse. 

 In consequence of this they differ in their capabilities, in 

 their tastes, in their tendencies toward any particular line 

 of action. But all these classes, so far as they do not fall 



