ADAPTATION AS STABILITY 5/7 



help to cool us. The carpet on the floor acts similarly in winter, 

 helping to prevent the temperature of the feet from falling below 

 its optimal value. The jolts of the road cause, on the skin and 

 bone of the human frame, stresses which are much lessened by 

 the presence of springs. Similar in action are the shock-absorbers 

 and tyres. A collision would cause an extreme deceleration 

 which leads to very high values for the stress on the skin and 

 bone of the passengers. By the brakes these very high values 

 may be avoided, and in this way the brakes keep the variables 

 4 stress on bone ' within narrower limits. Good headlights keep 

 the luminosity of the road within limits narrower than would 

 occur in their absence. 



The thesis that ' adaptation ' means the maintenance of essential 

 variables within physiological limits is thus seen to hold not 

 only over the simpler activities of primitive animals but over 

 the more complex activities of the ' higher ' organisms. 



5/7. Before proceeding further, it must be noted that the word 

 ' adaptation ' is commonly used in two senses which refer to 

 different processes. 



The distinction may best be illustrated by the inborn homeo- 

 static mechanisms : the reaction to cold by shivering, for instance. 

 Such a mechanism may undergo two types of ' adaptation '. 

 The first occurred long ago and was the change from a species 

 too primitive to show such a reaction to a species which, by 

 natural selection, had developed the reaction as a characteristic 

 inborn feature. The second type of ' adaptation ' occurs when 

 a member of the species, born with the mechanism, is subjected 

 to cold and changes from not-shivering to shivering. The first 

 change involved the development of the mechanism itself ; the 

 second change occurs when the mechanism is stimulated into 

 showing its properties. 



In the learning process, the first stage occurs when the animal 

 1 learns ' : when it changes from an animal not having an adapted 

 mechanism to one which has such a mechanism. The second 

 stage occurs when the developed mechanism changes from in- 

 activity to activity. In this chapter we are concerned with the 

 characteristics of the developed mechanism. The processes 

 which led to its development are discussed in Chapter 8. 



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