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 evening primrose, Œnothera 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 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 elementary species. One of these was O. brevistylis, which occurred only as female plants. The other new species was a smooth-leafed form with a more beautiful foliage than O. lamarckiana. This is O. lævifola. 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.
ŒNOTHERA LAMARCKIANA
Elementary Species
These two new forms, as well as the common O. lamarckiana, were collected, and from these plants there have arisen the three groups or 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 O. 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-fertilized plants of the lamarckiana form, so that the new plants appearing in each horizontal line are the descendants in each generation of lamarckiana parents. It will be observed that the species, O. oblongata, appeared again and again in considerable numbers, and the same is true for several of the other forms also. Only the two species, O. gigas and O. scintillans, appeared very rarely.
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 fertilized with pollen from the flowers on the same plant, or of the same kind of plant, they gave rise to forms like themselves, thus showing that they are true elementary species.[[26]] It is also a point of some interest to observe that all these forms differed from each other in a large number of particulars.
[26]. O. lata is always female, and cannot, therefore, be self-fertilized. When crossed with O. lamarckiana there is produced fifteen to twenty per cent of pure lata individuals.
Only one form, O. scintillans, that appeared eight times, is not constant as are the other species. When self-fertilized its seeds produce always three other forms, O. scintillans, O. oblongata, and O. lamarckiana. It differs in this respect from all the other elementary species, which mutate not more than once in ten thousand individuals.
From the seeds of one of the new forms, O. lævifolia, collected in the field, plants were reared, some of which were O. lamarckiana and others O. lævifolia. They were allowed to grow together, and their descendants gave rise to the same forms found in the lamarckiana family, described above, namely, O. lata, elliptica, nannella, rubrinervis, and also two new species, O. spatulata and leptocarpa.