CHAPTER 5 



Adaptation as Stability 



5/1. The concept of ' adaptation ' has so far been used without 

 definition ; this vagueness must be corrected. Not only must 

 the definition be precise, but it must be given in terms that 

 conform to the demand of S. 2/8. 



5/2. The suggestion that an animal's behaviour is ' adaptive ' 

 if the animal ' responds correctly to a stimulus ' may be rejected 

 at once. First, it presupposes an action by an experimenter and 

 therefore cannot be applied when the free-living organism and 

 its environment affect each other reciprocally. Secondly, the 

 definition provides no meaning for 4 correctly ' unless it means 

 4 conforming to what the experimenter thinks the animal ought 

 to do '. Such a definition is useless. 



Homeostasis 



5/3. I propose the definition that a form of behaviour is adaptive 

 if it maintains the essential variables (S. 3/14) within physiological 

 limits. The full justification of such a definition would involve 

 its comparison with all the known facts — an impossibly large 

 task. Nevertheless it is fundamental in this subject and I must 

 discuss it sufficiently to show how fundamental it is and how 

 wide is its applicability. 



First I shall outline the facts underlying Cannon's concept of 

 4 homeostasis '. They are not directly relevant to the problem 

 of learning, for the mechanisms are inborn ; but the mechanisms 

 are so clear and well known that they provide an ideal basic 

 illustration. They show that : 



(1) Each mechanism is ' adapted ' to its end. 



(2) Its end is the maintenance of the values of some essential 



variables within physiological limits. 

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