OTHER THEORIES OF SPECIES-FORMING. 337 



appear to various people insufficient or sufficient according 



to their respective ideas of what is needed in 



De Vries and the way of fact material for the satisfactory 



the mutations . ,. r -i . .1 i 



theory. founding of a theory, it is the special vir- 



tue of de Vries to have attempted on behalf 

 of heterogenesis. 



De Yries 1 introduces his now classic two-volume pres- 

 entation of his views on evolution and species-forming 

 ("Die Mutationstheorie," 1901-1903) with the following 

 paragraph : 



"Als Mutationstheorie bezeichne ich den Satz, dass die 

 Eigenschaften der Organismen aus scharf von einander 

 unterschiedenen Einheiten aufgebaut sind. Diese Einheiten 

 konnen zu Gruppen verbunden sein, und in verwandten 

 Arten kehren dieselben Einheiten und Gruppen wieder. 

 Ubergange, wie sie tins die ausseren Formen der Pflanzen 

 und Thiere so zahlreich darbieten, giebt es aber zwischen 

 diesen Einheiten ebensowenig, wie zwischen den Molekiilen 

 der Chemie." And again in the first paragraph of the 

 preface to his book "Species and Varieties" (an edited 

 transcription of his American lectures on species-forming, 

 delivered in California in 1904) he says : ". . . but the 

 way in which one species originates from another has not 

 been adequately explained. The current belief assumes that 

 species are slowly changed into new types. In contradic- 

 tion to this conception the theory of mutation assumes that 

 new species and varieties are produced from existing forms 

 by sudden leaps. The parent-type itself remains unchanged 

 throughout this process, and may repeatedly give birth to 

 new forms. These may arise simultaneously and in groups, 

 or separately at more or less widely distributed periods." 



Obviously there is no ambiguity here as to the relation 

 of species-forming by mutation to species-forming by 

 gradual modification through selection or fluctuating varia- 

 tions. In the words of de Vries : "Species have not arisen 



