294 Evolution and Adaptation 
—the time required depending upon the frequency with 
which the new form appears. This law of Delboeuf, 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 verna. “ 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, nothera 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. dre- 
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. levifola. 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. 
