224 UNIVERSITY OF MISSOURI BULLETIN 



of so-called "acquired characters." The value of Lamarck- 

 ianism as an evolutionary doctrine depends of course on its 

 fundamental assumption that parental modifications, acquired 

 during the lifetime of the individual, are transmitted by in- 

 heritance. It must stand or fall with the truth or falsity of 

 this assumption. In the absence of any known or imaginable 

 mechanism for the transmission of such specific modifications 

 to the germ cells and the lack of convincing experimental 

 evidence of the actual inheritance of such changes, the majority 

 of biologists have taken the position that the Lamarckian claims 

 remain unproven. 



Criticisms of the Third Group (Fluctuations and Muta- 

 tions). The third class of criticisms deals with the kind of 

 variations that are effective in the formation of new species of 

 animals and plants and with the manner in which the succes- 

 sive steps are taken in the evolutionary process. 



Ever since the time of Darwin, there have been not a 

 few biologists who have in various ways maintained that the 

 discontinuity observed between species is not the result of 

 the accumulation of fluctuating variations, accompanied by 

 the elimination of intermediate stages in the struggle for exist- 

 ence, but on the contrary is the direct expression of discon- 

 tinuity in variation. The method of species-building is, there- 

 fore, quite different from the process as conceived by Dar- 

 win, for, instead of the slow and gradual operation of natural 

 selection, incipient species arise suddenly and ready-made from 

 the beginning as the direct result of the appearance in indivi- 

 duals of whole, definite modifications that are not connected 

 by intermediate steps with antecedent stages. It was not, 

 however, until the work of the Dutch botanist de Vries that 

 attention was seriously directed to the problem of discontinuity 

 in the origin of species a problem which occupies a conspic- 

 uous position in the present-day evolutionary discussion. 



It has been forcibly urged by de Vries that variations of 

 the continuous or fluctuating type, being merely plus or minus 



