492 EVOLUTION, GENETICS, AND EUGENICS 



will be inspired by these facts to write historical novels and dramas; 

 another will find them helpful in shaping economic and social legisla- 

 tion ; while the third may remain almost wholly unaffected and devote 

 his life to keeping a country grocery or to inventing a new method of 

 making soap. They all had the same facts, the same teacher, the same 

 school-room. But they all reacted differently, each according to his 

 own ability and temperament — that is, his inborn make-up. 



So an unalterable fact of human nature is that men are different. 

 The whole question at issue in this world-old heredity-environment 

 debate is, what causes these differences? 



All modern biology .and psychology support the view that heredity 

 plays a great part, and probably a preponderant part, in shaping a 

 man's actions and reactions, his goings and comings, his health and 

 happiness in this world. We found in the chapters on twins and on the 

 royal families that this is true. But it may be of interest to present 

 here some of the main arguments on both sides of this question, with 

 these scientific discoveries as their background. 



It is commonly said that it is impossible to separate heredity from 

 environment. This is probably true when we consider merely one 

 individual. I doubt if we shall ever be able to determine whether a 

 particular act by a particular individual, say whether he takes a drink 

 of alcohol on some particular occasion or whether he commits a crime, 

 is due to his heredity or to his environment. The causes are so hope- 

 lessly intertwined that no one so far as I am aware has presented the 

 slightest hope of measuring the relative influence of the two forces 

 within the individual. As I have already pointed out in a foot-note to 

 a previous chapter, the Behaviorists — a school of psychologists founded 

 by Doctor John B. Watson — assert (at least some of the leading ex- 

 ponents of this school assert) that "ninety per cent, of a man's behavior 

 is due to his environment." 



This may be entirely true, but so far it seems to me it has not been 

 removed from the realm of assertion into the realm of exact measure- 

 ment. No one doubts that a man's early education is of great influ- 

 ence in determining his character and behavior in later life, but exactly 

 how great — whether ninety per cent, or twenty per cent. — has not as 

 yet so far as I am aware been measured. And I do not believe we can 

 speak of percentages until we have measurements. Environment is 

 important in determining behavior, but precisely how important I doubt 

 very much if we have any means at present for determining, when it 

 comes to one individual, whether we consider one particular act or the 



