294 Evolution and Adaptation 



■ — the time required depending upon the frequency with 

 which the new form appears. This law of Delbceuf, De Vries 

 believes, is correct from the point of view of the mutation 

 theory. It explains, in a very simple way, the existence of 

 numerous species-characters that are entirely useless, such, 

 for instance, as exist between the different elementary species 

 of Draba vcrna. " According to the selection theory only 

 useful characters can survive ; according to the mutation 

 theory, useless characters also may survive, and even those 

 that may be hurtful to a small degree." 



We may now proceed to examine the evidence from which 

 De Vries has been led to the general conclusions given in 

 the preceding pages. De Vries found at Hilversam, near 

 Amsterdam, a locality where a number of plants of the even- 

 ing primrose, Oenothera lamarckiana, grow in large numbers. 

 This plant is an American form that has been imported into 

 Europe. It often escapes from cultivation, as is the case at 

 Hilversam, where for ten years it had been growing wild. 

 Its rapid increase in numbers in the course of a few years 

 may be one of the causes that has led to the appearance of a 

 mutation period. The escaped plants showed fluctuating 

 variations in nearly all of their organs. They also had pro- 

 duced a number of abnormal forms. Some of the plants 

 came to maturity in one year, others in two, or in rare cases 

 in three, years. 



A year after the first finding of these plants De Vries 

 observed two well-characterized forms, which he at once rec- 

 ognized as new elementary species. One of these was O. bre- 

 vistylis, which occurred only as female plants. The other new 

 species was a smooth-leafed form with a more beautiful foli- 

 age than O. lamarckiana. This is O. lavifola. It was found 

 that both of these new forms bred true from self-fertilized 

 seeds. At first only a few specimens were found, each form 

 in a particular part of the field, which looks as though each 

 might have come from the seeds of a single plant. 



