10 EVOLUTION AND GENETICS 



ent it has a chemical and physical constitution that 

 gives this response. We find a specific chemical com- 

 position and generally a specific physical structure 

 already existing. We have no reason to suppose that 

 such particular reactions v^ould take place until a 

 specific chemical configuration had been acquired. 

 Where did this constitution come from? This is the 

 question that the scientist asks himself. I suj^pose 

 Bergson would have to reply that it came into exist- 

 ence 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 observation to the realm of 

 fancy — to a realm that is foreign to our experience ; 

 for such a view assumes 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 explana- 

 tions appeals to a change not immediately connected 

 with the outer world, but to one within the organism 

 itself. 



Practice makes perfect is a familiar adage. Not 

 only in human affairs do we find that a part through 

 use becomes a better tool for performing its task, 

 and through disuse degenerates; but in the field of 

 animal behavior we find that manv of the most essen- 

 tial types of behavior have been learned througli 



