LIMITATIONS 91 



physical causation. To convert a given nature into a 

 higher nature, where the difference is one of kind and 

 not of mere degree, or linear variation as it is called, is, 

 from the point of view of the lower nature, a super- 

 natural operation, requiring a supernatural cause. An 

 ape cannot by its own power evolve into a human being, 

 nor can this limitation be overcome by lengthening the 

 process through a succession of generations. To think 

 so is as illogical as to suppose that a man can lift him- 

 self by his bootstraps if he will only do it gradually. 

 The reply may be made that the evolutionary hypothe- 

 sis does not teach that a species elevates itself in isola- 

 tion, or independently of the forces and laws at work 

 in nature at large. Such an answer merely shifts the 

 battle-ground without changing the issue, for the prin- 

 ciple I have been stating applies to a universe as well 

 as to a species. It is impossible for nature as a whole, 

 originally containing no mind, no personalities, and no 

 purposive elements, to produce these things and oper- 

 ate teleologically, without a causation being involved 

 which is distinctly supernatural to the original cosmos: 

 — as impossible as it is for a non-rational species 

 acting by itself to become rational. Nothing can be 

 evolved which has not previously been involved. Mere 

 nature cannot produce except "after its kind." Me- 

 chanical variation may develop things after their kind, 

 but this constitutes the necessary limit of a purely 

 natural evolution.^ 



1 Such variation is called "linear." Thus the brain may become 

 more complicated in its convolutions, and may increase in size; but 



