CHAPTER III. 



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 ci'aj-fish, free 

 to move in a large vessel of 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 hnvered into 

 the vessel, the crayfish will sooner or later aj^proach and 

 devour it. 



If we ask why 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 clearly 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 crayfish is " aware 

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



