204 GENETICS 



identical, the criminal careers begin at about the same age, 

 and the behavior of both members in court and in prison 

 corresponds absolutely." 



But in the two-egg twins, that started life with different 

 parental materials, with different heredity, the situation was 

 different. As we have seen, 14 out of the 17 had totally dif- 

 ferent careers, one being a criminal, the other not. And 

 there was no such detailed parallelism in the records of the 

 two as was found in the one-egg twins. As before remarked, 

 the similarity of environment was as great in the two-egg 

 twins as it was in those that came from one egg. But this 

 similarity of environment did not result in identity of fate 

 nor in detailed parallelism in careers. It was the identity of 

 original nature, of the materials out of which they are 

 made, that gave the same careers and the same fate to the 

 one-egg twins. They were almost as much alike in their be- 

 havior, their mentality, their career and their fate, as they 

 were in their physical features — in which, like other one- 

 egg twins, they were almost indistinguishable. 



It is certain therefore that all the things that affect 

 character and conduct are deeply influenced by the heredi- 

 tary materials. There is no characteristic or quality of hu- 

 man beings that is exempted from its influence. This con- 

 clusion is confirmed by all the many studies that have been 

 made on the two types of twins. And it is in harmony too 

 with all that we know of the science of genetics. Experi- 

 mental work on the breeding of other organisms shows, as 

 we have seen, that they have no characteristics of any kind 

 that are not affected by the genes that they receive at the 

 beginning of their lives. 



Are characteristics influenced also by other conditions, by 

 the environment? The fact that all are affected by genes 

 does not at all imply that they may not be affected in other 

 ways. The influence of the environment is dealt with in 

 Chapter X. 



