On tlic Mechanical Conception of Nature. 677 



mental processes everything depends upon our 

 being able to conceive this most important factor 

 in a definite theoretical manner, and to comprise 

 under one common point of view its apparently 

 contradictory manifestations of constancy and 

 variability. 



Now every change of considerable extent is 

 certainly considered by Darwin to be the direct or 

 indirect consequence of external actions ; but in- 

 direct action always presupposes a certain small 

 variability (individual variability), without which 

 larger modifications cannot be brought about. 

 Empirically this small amount of variability is 

 doubtless present, but the question 'arises, upon 

 what does it depend ? Can it be conceived as 

 arising mechanically, or is it perhaps just at this 

 point that the metaphysical principle steps in and 

 offers those minute variations which make possible 

 that course of development which, according to 

 this view, is immutably pre-determined ? It is 

 certainly the absence of a theoretical definition of 

 variability which always leaves open a door for 

 smuggling in a teleological power. A mechanical 

 explanation of variability must form the basis of 

 this side of the theory of selection. 



This explanation is not difficult to find. All 

 dissimilarities of organisms must depend upon the 

 individuals having been affected by dissimilar ex- 

 ternal influences during the course of the develop- 

 ment of organic nature. If we ascribe to the 



