On the MecJianical Conception of Nature. 707 



upon the interference of a metaphysical principle 

 of development. Those to whom the arguments 

 already advanced against such a principle appear 

 insufficient may once more be asked, how and 

 where should this principle properly interfere ? I 

 am of opinion that one effect can have but 

 one sufficient cause ; if this suffices to produce it, 

 no second cause is required. The hand of a 

 watch necessarily turns once round in a circle in a 

 given time as soon as the spring which sets the 

 mechanism in movement is wound up ; in an un- 

 wound watch a skilful finger can perhaps give the 

 same movement to the hand, but it is impossible 

 that the latter can receive both from the operator 

 and from the spring at the same time, the same 

 motion as that which it would receive through 

 either of these two powers alone. In the same 

 manner it appears to me that the variations which 

 lead to transformation cannot be at the same 

 time determined by physical and by metaphysical 

 causes, but must depend upon either one or the 

 other. 



On no side will it be disputed that at least one 

 portion of the processes of organic life depends 

 upon the mechanical co-operation of physical 

 forces. How is it conceivable that sudden pauses 

 should occur in the course of these causal forces, 

 and that a directive power should be substituted 

 therefor, the latter subsequently making way 

 again for the physical forces ? To me this is as 



