5/6 ADAPTATION AS STABILITY 



hard objects. Animals, however, are provided with reflexes that 

 tend to minimise the chance of collision and of mechanical injury. 

 A mechanical stress causes injury — laceration, dislocation, or 

 fracture — only if the stress exceeds some definite value, depending 

 on the stressed tissue — skin, ligament, or bone. So these reflexes 

 act to keep the mechanical stresses within physiological limits. 



Many more examples could be given, but all can be included 

 within the same formula. Some external disturbance tends to 

 drive an essential variable outside its normal limits; but the 

 commencing change itself activates a mechanism that opposes the 

 external disturbance. By this mechanism the essential variable 

 is maintained within limits much narrower than would occur if 

 the external disturbance were unopposed. The narrowing is the 

 objective manifestation of the mechanism's adaptation. 



5/5. The mechanisms of the previous section act mostly within 

 the body, but it should be noted that some of them have acted 

 partly through the environment. Thus, if the body-temperature 

 is raised, the nervous system lessens the generation of heat within 

 the body and the body-temperature falls, but only because the 

 body is continuously losing heat to its surroundings. Flushing 

 of the skin cools the body only if the surrounding air is cool; 

 and sweating lowers the body-temperature only if the surround- 

 ing air is unsaturated. Increasing respiration lowers the carbon 

 dioxide content of the blood, but only if the atmosphere contains 

 less than 5 per cent. In each case the chain of cause and effect 

 passes partly through the environment. The mechanisms that 

 work wholly within the body and those that make extensive use 

 of the environment are thus only the extremes of a continuous 

 series. Thus, a thirsty animal seeks water: if it is a fish it does 

 no more than swallow, while if it is an antelope in the veldt it 

 has to go through an elaborate process of search, of travel, and 

 of finding a suitable way down to the river or pond. The homeo- 

 static mechanisms thus extend from those that work wholly 

 within the animal to those that involve its widest-ranging acti- 

 vities ; the principles are uniform throughout. 



Generalised homeostasis 

 5/6. Just the same criterion for ' adaptation ' may be used in 

 judging the behaviour of the free-living animal in its learned 



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