Significance of the Available Evidence. 601 



the appearance of transitional forms and only reveal 

 their true nature when tested by breeding- experiments. 



Several of the critics who have expressed themselves 

 more or less favorably on my theory, have pointed out 

 that the greatest danger for it lies in this very point. In 

 a very clear and concise summary of the doctrine of muta- 

 tion Mac Dougal has expressed himself as follows : "The 

 greatest misunderstanding which may likely arise in the 

 consideration of these results will be that founded on the 

 error of confusing fluctuating variability and mutabil- 

 ity/'i 



The distinction between species-forming and fluctu- 

 ating variability was first derived by Darwin from his 

 theory of pangenesis, and this may perhaps explain the 

 antipathy which so many investigators bear towards it.- 



The great majority of writers assume that fluctuating 

 as well as discontinuous variability play a part in the 

 formation of species.^ This view of Darwin, which 

 under Wallace's influence gradually shifted into the 

 background, has in latter years come again prominently 

 to the front ; and the various investigators concede here 

 a less or there a greater share to discontinuous variations 

 or mutations, according to their preconceptions and their 

 experience in investigation. This long series of shades 

 of opinion would seem to indicate that we are not con- 

 cerned here with an independent principle, but with a 

 gradual change of opinion from the prevaih'ng theory 



^ D. T. MacDougal, The Origin of Species by Mutation. Torreya. 

 1902, Vol. II, p. 99. 



'^ See IntraccUular Pangenesis, e.g., p. 214 (English cmI.I. ami 

 Ber. d. d. Ges., 1900, XVIIi; p. 83. 



'Von Wettstein has published a useful summary of his vi-w-^ 

 in the form of a lecture dehvered to tlio Scicntilk and Medical .\ 

 ciation at Karlsbad, and entitled Der Nco-Lamarckismus und scittc 

 Bcsiehungen zum Danvinismus, 1903. 



