72 HEREDITARY CHARACTERS 



morphic (E. biennis, which had previously been introduced 

 into Europe." l 



Sir W. T. Thiselton-Dyer, contrasting the variability of 

 plants under cultivation with their variability when under 

 natural conditions, says : " In all these cases I think we may 

 safely infer, from the persistent specific stability at the com- 

 mencement of cultivation, that the changes which subse- 

 quently occur would not have occurred in nature. . . . The 

 evidence, on the other hand, that such changes follow cultural 

 conditions as a result is simply overwhelming." 2 



Darwin, writing to Wallace on February 2, 1869, says: " I 

 always thought individual differences more important ; but I 

 was blind and thought that single variations might be pre- 

 served much oftener than I now see is possible or probable. 

 ... I believe I was mainly deceived by single variations 

 offering such simple illustrations, as when man selects." 

 " Single variations " here are what de Vries now calls 

 " mutations," " individual differences " what he calls " fluc- 

 tuating variations." 



The demonstration of de Vries' mutation theory, in so 

 far as experimental evidence goes, must then be regarded as 

 highly unsatisfactory. It rests entirely upon domesticated 

 plants, and mainly upon (Enothera lamarckiana, which is 

 possibly, even probably, an artificially produced hybrid. 

 The theory is, however, accepted by a number of biologists 

 of high standing, and must be considered here on general 

 as well as upon experimental evidence. De Vries claims 

 that new species arise by mutations, and that the small 

 fluctuating variations have nothing to do with evolution ; 

 as though transmissible from parents to offspring, they are 

 not stable. He says : " The current belief assumes that 

 species are slowly changed into new types. In contradiction 

 to this conception the theory of mutation assumes that new 



1 " On the Variations of the Evening Primrose," Journal of Botany, October 

 1907. 



2 "Specific Stability and Mutation," Nature, November 28, 1907, pp. 

 77-79. 



