THE ROLE OF FACTOR MUTATIONS IN 

 EVOLUTION 



ERNEST B. BABCOCK 

 Professor of Genetics, University of California 



The essential features of the mutation theory of evolu- 

 tion, as proposed by de Vries in 1901, are discontinuity 

 and heritability of those variations which make evolution 

 possible. New forms arise from preexisting forms by 

 saltation; they occur in all directions; they are heritable; 

 some of them are advantageous to the species and these 

 are preserved by natural selection. These features are 

 still recognized as the definitive elements of the mutation 

 theory, but biologists are gradually changing their point 

 of view concerning the real nature of mutations them- 

 selves. De Vries had worked with entire plants as units. 

 He was searching for evidence of species in the making. 

 He believed he had found this evidence when he discov- 

 ered his new evening primroses at Hilversum and found 

 that they transmitted their divergent characters to their 

 progeny. The evidence appeared none the less clear to 

 him, even though the parent species when tested did not 

 always breed true, but continued to produce not only the 

 forms first discovered, but also new ones which did not 

 exist in the original population. 



It is not my purpose to discuss the Oenothera data which 

 have accumulated in such enormous bulk in recent years. 

 Goodspeed and Clausen, in their papers on species hybrids 

 and the reaction-system concept of Mendelian heredity, 

 have provided a strong argument for attributing many 

 of the so-called mutations among the evening primroses 

 to antecedent hybridization between distinct species. 

 These authors have shown that "the occurrence of the 

 'mutants' (in (Enothera) and their subsequent behavior 

 in hybridization admit of logical arrangement and inter- 



