TWINS AND HEREDITY 485 



character, who influenced him to stop drinking and stealing, and helped 

 him to become a fairly useful member of society. This case tends to 

 show that there is an environmental element in crime; but the point 

 of the whole study is that there are certain hereditary types of mind 

 that yield more readily than others to the ordinary social conditions 

 that lead to crime. In this sense, and in this sense only, may it be 

 said that criminality is hereditary. 



IV. TWINS AND THE RELATIVE POTENCY OF HEREDITY 

 AND ENVIRONMENT 



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

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

 development, or evolution. One category of factors is intrinsic and 

 seems to depend upon the physical organization of the germinal proto- 

 plasm or upon the mechanisms that are involved in cell multiplication 

 and differentiation: all such factors are here 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 inequita- 

 ble distribution of environmental and educational advantages. This 

 has been until recently the prevailing opinion in educational, sociologi- 

 cal, and political circles. This view, ignoring hereditary differences 

 and overemphasizing of the potency of environment, has 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. 



