PHENOMENA OF INHERITANCE 321 



tween discontinuous and continuous variations, but in later works this 

 distinction is given a minor place as compared with the distinction be- 

 tween inherited and non-inherited variations. 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 environ- 

 mental conditions; the former represents changes in heredity, the latter 

 changes in development. 



3. Mutations and Fluctuations. — This clear cut distinction between 

 mutations and fluctuations marks one of the most important advances 

 ever made in the study of development and evolution. Thousands of 

 fluctuations occur 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 en- 

 vironment (fluctuations) and those due to hereditary 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 

 clue largely 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 or- 

 ganization of the germ cells, just as do inherited resemblances. Hered- 

 ity is not to be contrasted with variation, nor are hereditary likeness 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 of the permutations of an- 

 cestral characters, the appearance of mutations, and the fluctuations of 

 organisms due to environmental changes, it happens that in all cases off- 

 spring differ more or less from their parents and from one another. No 

 two children of the same family are ever exactly alike (except in the 

 case of identical twins which have come from the same oosperm) . Every 

 living being appears on careful examination to be the first and last of its 

 identical kind. This is one of the most remarkable peculiarities of liv- 

 ing things. The elements of chemistry are constant, and even the com- 

 pounds fall into definite categories which have constant characteristics. 

 But the individuals of biology are apparently never twice the same. This 

 may be clue to the immense complexity of living units as contrasted 

 with chemical ones, indeed lack of constancy is evident in itself of lack 

 of analysis into real elements or of lack of uniform conditions, but 

 whatever its cause the extraordinary fact remains that every living being 

 appears to be unique. 



There seems to be no reason to doubt that all the extraordinary dif- 



Beproduction is the generation of "unique beings that are, on the average, 

 more like their kind than like anything else (Brooks). 



