558 THE CAUSES OF EVOLUTION 



and more critical investigations were undertaken. As a result of 

 these investigations, the general fact of evolution has been amply 

 confirmed, while the selective process has been assigned to its 

 appropriate place among the causes of evolution. The Mutation 

 Theory of the Dutch botanist, Hugo de Vries, is an example of this 

 post-Darwinian study of evolutionary problems. 



The Work of de Vries. — The author of the Mutation Theory 

 conducted his investigations upon the evening primrose, Enothera 

 lamarckiana, which he obtained from stock found growing wild 

 near Amsterdam, Holland. Although Enothera has since been 

 shown to be less favorable for such studies than some other species, 

 and although the mutations studied by de Vries are now thought 

 to be rare combinations of genes rather than true mutations, 

 the existence of mutations and the correctness of de Vries' 

 views regarding their importance have since been established in 

 other plants and animals, notably the fruit-fly, Drosophila. Since 

 the work of de Vries was thus the clew to these important discov- 

 eries, something may be said regarding the original results that led 

 to the Mutation Theory. 



The distinction between the two types of variations known 

 respectively as fluctuations and mutations has already been 

 explained (p. 486 and cf. Figs. 307 and 308). In studying the eve- 

 ning primrose, de Vries found several aberrant types growing wild 

 with the typical form, E. lamarckiana. When the typical form 

 was self-fertiUzed and the seeds planted in his garden, these new 

 types reappeared in small numbers, year after year and generation 

 after generation. Moreover, the new types bred true, except that 

 they gave occasional aberrant types, as did the parent type, 

 E. lamarckiana. This led de Vries to believe that new types, com- 

 parable in some instances with the varieties or sub-species of plants 

 and animals that are frequently recognizable in nature, were 

 appearing in the evening primrose, and that he was actually observ- 

 ing evolution at work. But the changes observed were the result 

 of mutations, and not, as Darwin had supposed, of fluctuations 

 extending over many generations and producing a cumulative 

 effect in a certain direction. This agrees with the conclusion that, 

 since fluctuations are not inherited, they can have no part in evolu- 

 tionary changes, and that, since mutations are inherited, they must 

 constitute the type of variation that is important in evolution. It 

 remains to be shown that mutations are sufficiently common to 



