74 GENETICS AND EUGENICS 



calls mutation. Mutation, he believes, involves a change in 

 the nature of the germ-cells, whereas fluctuation involves 

 only effects due to environment. These latter may indeed 

 modify the soma, and also the germ-plasm temporarily, but 

 not permanently. Weismann, as we have seen, admits for 

 certain cases a direct modification of the germ-cells by the 

 environment, and believes that such modifications when once 

 produced are permanent. De Vries on the other hand is 

 much more ready to admit modification of the germ-plasm 

 by the environment, but maintains that these modifications 

 are not permanent. Permanent changes in the genn-plasm, 

 according to De Vries, have no relation to the action of the 

 environment. They arise spontaneously out of internal 

 conditions and are not necessarily adaptive in nature. Most 

 of them perish because not adaptive {i. e., beneficial) in 

 character; only those mutations survive in a state of nature 

 which chance to be adaptive. The environment does not 

 cause mutations, according to De Vries, but only determines 

 what ones may survive. Evolution is thus due primarily to 

 internal causes; but its course is guided by the environment, 

 which selects those mutations which are capable of survival. 



The Evidences of Mutation 



Two lines of evidences in favor of mutation may be cited, 

 one general, the other special. 



1. The occurrence of elementary species. Among many wild 

 species of plants there occur varieties quite distinct and 

 breeding true, but differing from each other by such minor 

 characteristics as ordinarily escape notice. Thus in the 

 common dandelion a considerable number of varieties may 

 be distinguished. Some have narrow leaves, some broad 

 leaves; on some the leaves are deeply notched, on others 

 almost entire. If we save the seeds of any of these peculiar 

 individuals and plant them we find that the characteristics of 

 the parent plant are inherited. They breed true like distinct 

 species, indeed they may be regarded as little species within 

 the dandelion species. De Vries calls them " elementary " 



