THE MUTATION THEORY 485 



i 

 where a number of plants of the evening primrose, Oenothera lamarck- 



iana, 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 ffilversum, 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 produced 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 recognized as new elemen- 

 tary species. One of these was 0. brevistylis, which occurred only as 

 female plants. The other new species was a smooth-leafed form with 

 a more beautiful foUage than 0. lamarckiana. This is 0. laevijolia. 

 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. 



These two new forms, as well as the common 0. lamarckiana, were 

 collected, and from these plants there have arisen the three groups of 

 families of elementary species that De Vries has studied. In his 

 garden other new forms also arose from those that had been brought 

 under cultivation. The largest group and the most important one 

 is that from the original 0. lamarckiana form. The accompanying 

 table shows the mutations that arose between 1887 and 1899 from 

 these plants. The seeds were selected in each case from self-fertihzed 

 plants of the lamarckiana form, so that the new plants appearing 

 in each horizontal Hne are the descendants in each generation of 

 lamarckiana parents. It will be observed that the species, 0. oblonga, 

 appeared again and again in considerable numbers, and the same is 

 true for several of the other forms also. Only the two species, 0. gigas 

 and 0. scintillans, appeared very rarely (Fig. 93). 



Thus De Vries had, in his seven generations, about fifty thousand 

 plants, and about eight hundred of these were mutations. When the 

 flowers of the new forms were artificially fertiUzed with pollen from 

 the flowers of the same plant, or of the same kind of plant, they gave 

 rise to forms like themselves, thus showing that they are true elemen- 

 tary species. It is also a point of some interest to observe that all 

 these forms differed from each other in a large number of particulars. 



