634 MUTATION THEORY OF PROFESSOR DE VRIES. 



doctrine of variability, the .second to that of mutation. In the first- 

 mentioned case those individuals survive which tind their life condi- 

 tions most favorable, and they are therefore generally the most 

 vigorous. By this process local races originate, and by it acclimati- 

 zation is made possible. If the new life conditions cease, then the 

 adapted races revert to the original type. 



"Natural selection in the struggle for existence between the newly 

 originated elementary species is quite different. These originate sud- 

 denly, unmodiated, and multiply themselves if nothing stands in the 

 way, because they are for the most part completely, or in a high 

 degree, heritable. If, then, the increase leads to a struggle for exist- 

 ence, the weaker succumb and are rooted out. According as the older 

 or the younger form happens to be the better suited for the life condi- 

 tions will one or the other survive. By this struggle for existence 

 species are no more likely to originate than they are by the struggle 

 between the variants of one and the same type, but evidently from 

 quite a different cause. To be able to come into competition with 

 one another, species must exist. The contest decides which of them 

 shall survive and which shall perish. These 'species selections/ in 

 the course of their evolution, have, without doubt, rooted out immense 

 numbers and retained only a small proportion. Briefly stated, I 

 assert, of course on the ground of the mutation theory, that by the 

 struggle for existence and by natural selection species do not origi- 

 nate, but perish. 



"(5) Herbert Spencer's well-known expression, "the survival of the 

 fittest,' is of course divisible into two propositions: The survival of 

 the fittest individuals within the constant species, or the formation of 

 local races, and the survival of the fittest species as the foundation 

 of the doctrine of descent. The two propositions are independent of 

 each other and belong to different categories. 



"(<>) According to the mutation theory, species have not originated 

 by gradual selection, continued through hundreds or thousands of 

 years, but by sudden steps, even if the changes are very small. Unlike 

 the variations, which are progressive changes in a direct line, those 

 metamorphoses which are designated as mutation branch off in new 

 directions. Furthermore, so far as experience goes, they occur at 

 random — that is, in the most diverse directions. They appear only 

 from time to time, and then probably under the operation of determi- 

 nate causes." 



It should be borne in mind that throughout this work the author 

 always restricts the term "mutation" to the designation of sudden phy- 

 logenetic changes, which he distinguishes sharply from all forms of 

 mere variation, however pronounced they may be, and also that in the 

 following remarks I also use that term in the same restricted sense. 

 Tin 1 horticultural forms of variation are especially discussed in the 

 closing section of Volume I. 



The second section of Volume I is entitled "The origination of 

 elementary species in the genus OEnothera," and constitutes its larger 

 part. It consists of an elaborate statement, with numerous illustra- 

 tions, of the author's experimental studies of the subject of mutation 

 which he instituted in a systematic manner in L886, and which he has 



