190 PLANT PHYSIOLOGY AND ECOLOGY 



period of theoretical discussion of slight value. This was especially 

 unfortunate, since it almost completely obscured the fact that the 

 value of Darwin's hyjDotheses could be tested by experiment alone. 

 Hence in this period it is necessary for us to consider only the 

 conclusions of Henslow and De Vries. The former obtained much 

 new material from a field neglected by Darwin, viz., the origin 

 of adaptations in nature, while the latter by means of careful 

 experiments placed beyond question the origin of new forms 

 through mutation. Henslow ^ believes that Darwin was wrong in 

 assuming that ''indefinite variability is a much more common result 

 of changed conditions than definite variability." His own opinion 

 is that "in nature variations are always definite and not excep- 

 tionally so : the consequence is that ' all or nearly all the individuals 

 become modified in the same way' (and) the result is that a new 

 variety and thence a new species 'would be produced without 

 the aid of natural selection.'" His final conclusion is that "the 

 origin of species is due to the joint action alone of the two great 

 factors of evolution variability and environment without the 

 aid of natural selection." 



De Vries 2 states that w^hile "the current belief assumes that 

 species are slowly changed into new types, in contradiction to this 

 conception, the theory of mutation assumes that new species and 

 varieties are produced from existing ones by sudden leaps." His 

 conclusions are based chiefly upon the experimental study of an 

 evening primrose, (Enothera lamarckiana. He found that out of 

 a hundred or more species in nature, this was the only one that 

 suddenly produced new forms, i.e., mutations. From it he ob- 

 tained in the field and in garden cultures twelve mutations or 

 " new elementary species." These arose suddenly from the parent 

 stock, without any connection with the habitat, and came true 

 from seed. A careful examination of De Vries' results leaves no 

 doubt that mutation is proved to be one of the methods by which 

 new forms originate. That it is the only method of origin is 

 certainly not true. Moreover, it is perfectly evident that De Vries' 

 experiments upon (Enothera are quite inadequate to prove it the 

 chief method, as he would have us think. It is difficult, more- 



' Henslow, George. The Origin of Plant Structures by Self-adaptation 

 to the Environment, IX, 1895. 



'De Vries, Hugo. Die Mutationstheorie, 1901; Species and Varieties, 

 their Origin by Mutation. 1905. 



