On the Mechanical Conception of Nature. 659 



colour of leaf-frequenting forms, be always re- 

 ferred to a " regular, designed, fixed tendency to 

 variation acting from within," and causing a 

 " large number of individuals " to vary in a similar 

 manner ? 



There is, however, a grain of truth in the fore- 

 going; variations which occur singly have but 

 little chance of becoming predominant characters, 

 and this is obviously what Darwin concedes. But 

 this is by no means equivalent to the assumption 

 that only those variations which from the first 

 occur in numerous individuals have a chance of 

 being perpetuated. Let us keep to the facts. 

 We have not the slightest reason either for re- 

 garding the white colour of polar animals as the 

 direct action of cold, or for considering that the 

 green colour of foliage-living caterpillars de- 

 pends upon direct action arising from the habit of 

 resting upon the leaves ; 7 both these characters 

 are explicable only by natural selection, and there 

 is nothing to favour the assumption (which Von 

 Hartmann postulates as necessary for success) 

 that many individuals varied into white at the 

 same time. We know no single extra-polar 

 species of a dark colour which frequently, t. e. in 

 many individuals of every generation, varies into 

 white, but we know many species which from time 

 to time produce single white individuals. Now 



1 [See note 2, p. 310. R. M.] 



