CHAPTER XL 



TWINS AND THE RELATIVE POTENCY OF 



HEREDITY AND ENVIRONMENT 



IN DEVELOPMENT' 



Two sets of factors are involved in the development of an indi- 

 vidual, and doubtless the same two sets of factors are responsible for 

 racial development or revolution. One category of factors is intrinsic 

 and seems to depend upon the physical organization of the germinal 

 protoplasm or upon the mechanisms that are involved in cell multi- 

 plication and differentiation: all such factors are included under the 

 term heredity. The other category of factors is extrinsic and seems to 

 involve both environment and training: these factors are usually in- 

 cluded together under the term environment. The controversy as to 

 the relative importance of these two sets of factors is so old as to be 

 time-honored. In the case of man especially the question as to what 

 characters are due to nature and what to nurture has long been an 

 active issue. 



Before the rise and growth of democracy the opinion was very com- 

 monly held that man was born noble or base, with high qualities or 

 low, according as he came from good or bad stock. The various well- 

 defined strata of society were believed to have a basis in blood. Blue 

 blood was the criterion of nobility or of high character. With the 

 rise of democracy, however, the view has come to prevail that "all 

 men are created equal" and that inequalities arise only as the result of 

 inequitable distribution of environmental and educational advantages. 

 This has been until recently the prevailing opinion in educational, 

 sociological, and political circles. This ignoring of hereditary differ- 

 ences and overemphasizing of the potency of environment have 

 caused the pendulum to swing to the opposite extreme. 



The present century has seen such a surprising advance in our 

 knowledge of the laws and the mechanism of heredity that it is no 

 wonder that biologists have come to feel that heredity is far and away 

 the chief factor in human development and that environment and 

 training are only minor modifying factors. 



' Reprinted from the Publications of the American Sociological Society, Vol 

 XVII (1Q22). An address given before the American Sociological Society by 

 H. H. Newman. 



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