6 HEREDITY AND EUGENICS 



is more important in the ontogeny of any character, 

 but whether a difference (variation) which appears 

 in an organism is due primarily to a difference in the 

 environment or a difference in heredity {cf. Sumner, 

 1922). 



This leads us to emphasise a point w^hich is not 

 always recognised — namely, that the relation between 

 the organism and its environment is not the simple 

 and direct relation between two reacting chemical 

 substances — it is rather one of stimulus and re- 

 sponse. It is, moreover, clear that not all elements 

 of the environment are equally effective in modifying 

 the organism. For example, a change in the light 

 may have a striking effect on the development of 

 one organism and no appreciable effect on another. 

 The relations of an organism to its environment are 

 therefore extremely complex, and can only be under- 

 stood after elaborate analysis. But the higher 

 organisms, and particularly man, have many regula- 

 tory mechanisms which enable him to triumph over 

 extreme variations in the environment without being 

 vitally affected b}^ them. This, with his weapons 

 and his intelligence, has enabled mankind to people 

 the four corners of the earth in almost everv extreme 

 of climatic conditions w^here organisms can live at all. 

 From a eugenic point of view it is to be remembered 

 that while hereditary differences of all kinds are per- 

 petuated in all conditions, yet optimum conditions 

 are desirable for the full expression of the characters 

 inherited by the organism. From this it follows 

 that those who insist upon the importance of heredity 

 in perpetuating good stocks should, at the same time, 

 realise the desirability^ of creating an environment 

 in which the best ph3^sical, mental, and moral qualities 

 of the individuals can find free expression. 



Before proceeding further it may be well to point 

 out that whereas heredity w^as formerly defined or 



