Duration of the Process of Selection. 87 



nothing more than the likelihood of the chance origin of 

 a useful change, to Wallace on the other hand it meant 

 the operation of time by the gradual cumulation of fluc- 

 tuating variations in the same direction. Other authors 

 sometimes mean one of these things, sometimes the other. 



The ideas involved in the two cases are of course 

 fundamentally different. Darwin's view, although he 

 never definitely formulated it, was that it was these 

 occasional single variations which brought about the con- 

 tinual differentiation of living forms. In short, the es- 

 sential process in the production of new forms is the 

 gradual accumulation by natural selection of these small 

 changes, provided they are useful.^ 



Wallace's view is that the material for species- 

 forming selection is furnished by fluctuating variability; 

 and that these infinitesimal differences are gradually 

 heaped up in the same direction until ultimately they 

 attain the dimensions of specific differences. 



Even when proof can be brought forward, as it can 

 in the case of many cultivated species, that plants are 

 different from what they were one or two thousand years 

 ago, it can scarcely ever be determined historically whether 

 they favor the former or the latter of these two views. 

 Those cases in which the sudden origin of a new form 

 was observed and described by contemporaries, are the 

 exceptions : and they tell in favor of Darwin's view, 

 and never in favor of Wallace's. 



And if it is impossible to discriminate between the two 

 historically, how much less is there any hope of a verdict 

 by the method of analogy, when there are no definite facts 

 to go upon. 



This being the case, we pass on to the consideration 



^ Life and Letters, II, p. 125. 



