Il8 HEREDITY AND EVOLUTION IN PLANTS 



If it does it is a mutant; otherwise it is not. It is clear, 

 therefore, that the only way the problem can be followed 

 out is by experiment — hence the term experimental evolu- 

 tion. The next step for de Vries to take, after discovering 

 the two forms that he supposed to be mutants, was to breed 

 them in carefully guarded, pedigreed cultures in his 

 garden, and also to breed the parent form, (Enothera La- 

 marckiana, and see if he could observe the two forms above 

 mentioned, or other mutants, arise from seed produced 

 without crossing with any other species. 



The entire story of this classical series of experiments 

 is too long to be told here. Suffice it to say that de Vries 

 did observe numerous other aberrant forms arise, and also 

 found that they bred true (except for additional muta- 

 tions) when propagated by seed for over 25 years — that 

 is, they were true mutations. 



93. Relation of Mutation Theory to Darwinism.- — The 

 mutation theory is not intended by de Vries to supplant 

 the theory of natural selection, but to demonstrate that 

 the materials upon which selection acts in the formation 

 of new species are mutations, and mutations only — never 

 fluctuating or individual variations. Here lies the essen- 

 tial difference between Darwin and de Vries, for Darwin, 

 though recognizing, and with increasing clearness, that 

 mutation furnishes part of the material to be "selected" 

 by nature, assigned a larger and more important r61e to 

 fluctuating or individual variations. "Species have been 

 modified," he said, "chiefly through the natural selection 

 of numerous successive, slight, favorable variations; aided 

 [however] in an important manner by ... . variations 

 which seem to us in our ignorance to arise spontaneously. 

 It appears that I formerly underrated the frequency and 



