HEREDITY AND ENVIRONMENT IN DEVELOPMENT 445 



For the human species, the interaction of these three factors 

 may be expressed in tabular form by saying, as one American 

 zoologist has done, that there are twenty-seven different kinds of 

 men (Fig. 233). The case is more complex, because good, medium, 

 and poor do not represent all the innumerable gradations that 

 may occur in the three factors represented by the triangle. Never- 

 theless, the table indicates how environment and heredity make 





A 



-S ■ 



» 



I 

 I 

 t 



\ 



! / 



« '/ 



-^ 



Herziiiy 



Fig. 232. — Diagram to illustrate the influence of hereditj', environment and 

 training in the development of an individual (c/. Fig. 233). 



The heredity is assumed to be a constant in the particular individual, but heredity is 

 expressed in diflferent ways according to the environment and training. Thus various types 

 of individuals, as represented by the several triangles, may be produced from zygotes 

 having the same hereditary potentialities. (After Conklin, "Heredity and Environment," 

 copyright, 1916, by Princeton University Press, reprinted by permission.) 



an individual what he finally becomes. Environmental factors 

 are important at every step in development, whether of men or 

 sponges; yet the biologist can hardly escape the conviction that 

 the ultimate possibilities of the individual are hmited by its 

 heredity even though certain environmental conditions are neces- 

 sary if a given hereditary constitution is to reach its fullest develop- 

 ment. The manner of sex determination, as described in the 

 section that follows, is an example of such interaction with a 

 preponderance of hereditary over environmental factors in 

 development. 



