CHAPTER in. 



THE PHYSIOLOGY OF THE CRAYFISH — THE MECHANISM BY 

 WHICH THE LIVING ORGANISM ADJUSTS ITSELF TO 

 SURROUNDING CONDITIONS AND REPRODUCES ITSELF. 



If the hand is brought near a vigorous cra3'fish, free 

 to move in a large vessel o*f water, it will generally give 

 a vigorous flap with its tail, and dart backwards out of 

 reach ; but if a piece of meat is gently lowered into 

 the vessel, the crayfish will sooner or later approach and 

 devour it. 



If we ask wh}^ the crayfish behaves in this fashion, 

 every one has an answer ready. In the first case, it is 

 said that the animal is aware of danger, and therefore 

 hastens away ; in the second, that it knows that meat is 

 good to eat, and therefore walks towards it and makes a 

 meal. And nothing can seem to be simpler or more 

 satisfactory than these replies, until we attempt to con- 

 ceive clearty what they mean ; and, then, the explanation, 

 however simple it may be admitted to be, hardly retains 

 its satisfactor}^ character. 



For example, when we say that the cra3'fish is " aware 

 of danger," or "knows that meat is good to eat," what 



