218 HEREDITY AND ENVIRONMENT 



and evolution. Thousands of fluctuations oc- 

 cur which are purely somatic in character and 

 which do not affect the germ cells, for every 

 single mutation or change in the hereditary 

 constitution; and yet only the latter are of 

 significance in heredity and evolution. This 

 distinction between variations due to environ- 

 ment (fluctuations) and those due to hered- 

 itary causes (mutations) was recognized by 

 Weismann and many of his followers, but the 

 actual demonstration on a large scale of the 

 importance of this distinction is due mainly to 

 deVries. 



All hereditary variations, whether due to 

 new combinations of old characters or to 

 the appearance of actually new characters, 

 whether small and continuous or large and 

 discontinuous, have their causes in the organi- 

 zation of the germ cells, just as do inherited 

 resemblances. Heredity is not to be con- 

 trasted with variation, nor are hereditary like- 

 ness and unlikeness due to conflicting 

 principles; both are the results of germinal 

 organization and both are phenomena of 

 heredity. 



4. Every Individual Unique. — As a result 



