568 HUMAN BIOLOGY 



cause of any character is found in conditions outside of the 

 germ plasm, as for example in the production of bhndness 

 by drinking wood alcohol, the character is plainly environ- 

 mental. When such environmental factors act at an early 

 stage of development, and especially before birth, it is often 

 difficult to distinguish them from hereditary ones. But 

 whenever a particular trait appears in several individuals 

 of the same family or confraternity, it is generally safe to 

 say that its differential cause is inherited. On the other hand, 

 if it occurs but once or rarely its cause may be either heredi- 

 tary or environmental and in such a case only the study 

 of the progeny of such individuals, preferably under experi- 

 mental conditions, will reveal which is the differential 

 factor. 



Even the development of inherited characters may be 

 modified by environment, especially if it acts at an early stage 

 in development, and consequently the breeder must select 

 not only good seed or stock but he must also provide good 

 soil, food and care if he is to produce superior plants or 

 animals. Heredity and environment are not contestants 

 but cooperators in making or destroying breeds or races or 

 civihzations. Heredity furnishes the materials, environment 

 shapes and uses them; heredity is the mechanism, environ- 

 ment is the stimulus which sets it in action; heredity fixes 

 the possibihties of development, environment determines 

 which of these possibihties shall become reahties. 



As apphed to man these principles of good breeding 

 are known as eugenics and euthenics. They are never in 

 conflict, though this cannot be said of all eugenicists and 

 euthenicists. Both principles are indispensable in all develop- 

 ment whether it be that of the body or of the mind, of 

 the individual or of society, and where two factors are in- 

 dispensable it is useless to debate which is the more 

 important. 



But while we cannot match these factors against each 

 other we can point out some of their contrasts. In general 

 heredity is more constant, environment more variable; 

 heredity more specific, environment more general; heredity 

 less easily controlled, environment more easily controlled. 

 Therefore in efforts for human betterment more attention has 



