CHAPTER IU. 
THE PHYSIOLOGY OF THE CRAYFISH—-THE MECHANISM BY 
WHICH THE LIVING ORGANISM ADJUSTS ITSELF TO 
SURROUNDING CONDITIONS AND REPRODUCES ITSELF. 
Ir the hand is brought near a vigorous crayfish, 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 lowered into 
the vessel, the crayfish will sooner or later approach 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 satisfactory character. 
For example, when we say that the crayfish is ‘‘ aware 
of danger,” or ‘“‘knows that meat is good to eat,” what 
