402 EVOLUTION AND GENETICS 



sion, and was clearly explained by Johannsen's work on pure 

 lines, which was described in Chapter XIX. 



The Mutation Theory of De Vries. Darwin recognized the 

 occurrence of sudden departures from parental characters, or 

 saltations, but made little use of them. They later became the 

 basis for the mutation theory of De Vries, which disposes effectively 

 of the question of specific change by disclosing the possibility 

 of minute mutations serving as materials for natural selection. 

 Such variations accomplish the actual shifting of specific charac- 

 ters which in ordinary fluctuating variations is limited to the 

 isolation of pure lines. 



De Vries, a Dutch botanist, conducted a long series of experi- 

 ments with the evening primrose, Oenothera lamarckiana, as the 

 basis for his mutation theory. He found that this species was 

 highly variable in its natural habitat near Hilversum, in the 

 vicinity of Amsterdam, where it was observed for several years. 

 De Vries speaks of some of its variations as new species. "Some 

 of them were observed directly on the field, either as stems or as 

 rosettes. The latter could be transplanted into my garden for 

 further observation, and the stems yielded seeds to be sown under 

 like control. Others were too weak to live a sufficiently long time 

 in the field. . . . These various methods have led to the discovery 

 of over a dozen new types, never previously observed or described." 

 (Fig. 208.) 



To give an account of the differences between the species or 

 mutants so produced, would be beyond the needs of this work. 

 It is sufficient to note that the degree of difference between mutants 

 and the forms from which they originated was great enough to 

 lead De Vries to call them species; it was as great as differences 

 between other related species of plants, and greater than exists 

 between some divisions commonly called species. In addition a 

 mutant once produced continued to produce offspring like itself — an 

 essential factor which distinguishes true mutants from mere sports. 



A significant feature of De Vries' results was the production of 

 a species sciniillans, which in turn gave rise to mutants. This 

 species was produced, like others, from self-fertilized seeds of 

 Oenothera lamarckiana, and among the mutants that it produced 

 were many lamarckiana. some oblonga, and a few lata, in addition 

 to a moderate percentage of true scintillans. Similar reverse mu- 

 tation has since been secured by Muller in Drosophila. 



