THEORY OF EVOLUTION 31 



present it has a chemical and physical constitu- 

 tion that gives this response. We find a speci- 

 fic chemical composition and generally a specific 

 physical structure already existing. We have 

 no reason to suppose that such particular reac- 

 tions would take place until a specific chemical 

 configuration had been acquired. Where did 

 this constitution come from? This is the ques- 

 tion that the scientist asks himself. I suppose 

 Bergson would have to reply that it came into 

 existence at the moment that the first specific 

 stimulus was applied. But if this is the answer 

 we have passed at once from the realm of obser- 

 vation to the realm of fancy to a realm that 

 is foreign to our experience ; for such a view as- 

 sumes that chemical and physical reactions are 

 guided by the needs of the organism when the 

 reactions take place inside living beings. 



USE AND DISUSE 

 From Lamarck to Weismann 

 The second of the four great historical ex- 

 planations appeals to a change not immedi- 

 ately connected with the outer world, but to 

 one within the organism itself. 



