5/7 ADAPTATION AS STABILITY 



difficulty, be shown to be similarly variation-limiting. An attempt 

 to demonstrate them all would be interminable. But to confirm 

 the argument we will examine a motor-car, part by part, in order 

 to show its homeostatic relation to man. 



Travel in a vehicle, as contrasted with travel on foot, keeps 

 several essential variables within narrower limits. The fatigue 

 induced by walking for a long distance implies that some vari- 

 ables, as yet not clearly known, have exceeded limits not trans- 

 gressed when the subject is carried in a vehicle. The reserves 

 of food in the body will be less depleted, the skin on the soles of 

 the feet will be less chafed, the muscles will have endured less 

 strain, in winter the body will have been less chilled, and in 

 summer it will have been less heated, than would have happened 

 had the subject travelled on foot. 



When examined in more detail, many ways are found in which 

 it serves us by maintaining our essential variables within narrower 

 limits. The roof maintains our skin at a constant dryness. The 

 windows protect us from a cold wind, and if open in summer, 

 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 ' stress on 

 bone ' within narrower limits. Good headlights keep the lumin- 

 osity of the road within limits narrower than would occur in their 

 absence. 



The thesis that 4 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. 



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