1 8 THE DARWINIAN THEORY 



can show that in cultivated plants and domesticated 

 animals, and in the several races of men, these 

 changes have uniformly taken place. They can 

 show that the degrees of difference so produced are 

 often, as in dogs, greater than those on which 

 distinctions of species are in other cases founded. 

 They can show that it is a matter of dispute whether 

 some of these modified forms are varieties or modi- 

 fied species. And thus they can show that through- 

 out all organic Nature there is at work a modifying 

 influence of the kind they assign as the cause of 

 these specific differences ; an influence which, though 

 slow in its action, does in time, if the circumstances 

 demand it, produce marked changes ; an influence 

 which, to all appearance, would produce in the 

 millions of years, and under the great varieties of 

 condition which geological records imply, any amount 

 of change." 



It is impossible to depict better than this the 

 condition prior to Darwin. In this essay there is 

 full recognition of the fact of transition, and of its 

 being due to natural influences or causes, acting now 

 and at all times. Yet it remained comparatively 

 unnoticed, because Spencer, like his contemporaries 

 and predecessors, while advocating Evolution, was 

 unable to state explicitly what these causes were. 



We have now traced the main steps in the history 

 of the doctrine of evolution, and have mentioned the 

 names of the chief men with whom this history is 

 most closely associated. This doctrine was rendered, 

 possible by Linnaeus by the introduction of definite 

 and precise nomenclature in the language common 



