254 GENETICS 



between individuals resulting from the different conditions 

 under which they have lived are common. Such is the case 

 with the habit of growth in higher plants; trees and other 

 plants grown under different conditions are very different in 

 form. In some of the lower animals, such as the hydroids, a 

 similar situation is found. 



It is obvious that the details of behavior are in a high 

 degree under the influence of environmental conditions. 

 The particular acts performed at given moments in any 

 given organism are determined almost entirely by the con- 

 ditions under which it finds itself. Yet organisms of different 

 genetic constitution behave very differently under effectively 

 the same conditions, and the general pattern of the behav- 

 ior of organisms may be largely determined by the genetic 

 constitution. 



Genes and Environment in Relation to Mental Character- 

 istics. Evidence from Twins 



The relative roles of heredity and environment are of 

 special interest in man, particularly in relation to the char- 

 acteristics that influence behavior. For the study of these 

 matters man is the most favorable organism that exists, 

 although for most other relations in genetics he is a singu- 

 larly unfavorable object of study. In man it is possible to 

 make detailed studies of temperament, mentality, charac- 

 ter, a group of characteristics hardly open to examination 

 in other organisms. Furthermore, there occur in man "iden- 

 tical" or one-egg twins, individuals having the same set of 

 genes, the same genetic constitution throughout.^ And for 

 comparison with these there occur fraternal twins: two 

 individuals of the same parentage, the same age and living 

 under the same environment, but developed from two sep- 

 arately formed eggs and thus having to some extent differ- 

 ent sets of genes. By comparing these two kinds of twins 



