On the Mechanical Conception of Nature. 675 



garded microscopically or macroscopically, various 

 parts become combined, and these therefore offer 

 numerous possibilities of modification, so that the 

 governing laws are more complex, and appear less 

 restricted and unchangeable. In neither instance 

 do we know the final causes which always lead to 

 a given state of equilibrium ; in the case of a 

 crystal it has not occurred to anybody to ascribe 

 the harmonious disposition of the parts to a teleo- 

 logical power; why then should we assume such 

 a force in the organism, and thus discontinue the 

 attempt, which has already been commenced, to 

 refer to its natural causes that harmony of 

 parts which is here certainly present and equally 

 conformable to law ? 



On these grounds the assertion that the theory 

 of selection is not an attempt at a " mechanical " 

 explanation of organic development appears to 

 me to be incorrect. Variability and heredity, as 

 well as correlation, admit of being conceived as 

 purely mechanical, and must be thus regarded so 

 long as no more cogent reasons can be adduced 

 for believing that some force other than physico- 

 chemical lies concealed therein. 



But we certainly cannot remain at the purely 

 empirical conception as laid down by Darwin in 

 his admirable work on the " Origin of Species." 

 If the theory of selection is to furnish a method 

 of mechanical explanation, it is essential that its 

 factors should be formulated in a precise mechani- 



