Phenomena of Inheritance 75 



enced by the "Mutation Theory" of deVries, which has placed a 

 new emphasis upon the importance of sudden variations in the 

 process of evolution. At first deVries was inclined to emphasize 

 the degree of difference, that is the discontinuity, in these varia- 

 tions, but in later works this distinction is given a minor place as 

 compared with the question whether variations are inherited or 

 not. Inherited variations, whether large or small, are called by 

 deVries "mutations," whereas non-inherited variations are known 

 as "fluctuations." The former are caused by changes in germinal 

 constitution, the latter by alterations in environmental conditions ; 

 the former represent changes in heredity, the latter changes in 

 development. 



3. Mutations and Flucttuftionis. This clear cut distinction be- 

 tween mutations and fluctuations marks one of the most impor- 

 tant advances ever made in the study of development and evolu- 

 tion. Thousands of fluctuations occur which are purely somatic 

 in character and which do not affect the germ cells, for everv 

 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 environment ( fluctuations )^ 

 and those due to hereditary causes (mutations) was recognized 

 by Weismann and many of his followers, but the actual demon- 

 stration 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 organization of the germ cells, just as do in- 

 herited resemblances. Heredity is not to be contrasted with var- 

 iation, nor are hereditary likeness and unlikeness due to con- 

 flicting principles; both are the results of germinal organization 

 and both are phenomena of heredity. 



4. Every Individual Unique. As a result of the permutations 

 of ancestral characters, the appearance of mutations, and the 



