ADAPTATION AS STABILITY 5/6 



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 activi- 

 ties ; the principles are uniform throughout. 



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

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

 reactions. Take the type-problem of the kitten and the fire. 

 When the kitten first approaches an open fire, it may paw at the 

 fire as if at a mouse, or it may crouch down and start to ' stalk ' 

 the fire, or it may attempt to sniff at the fire, or it may walk un- 

 concernedly on to it. Every one of these actions is liable to lead 

 to the animal's being burned. Equally the kitten, if it is cold, 

 may sit far from the fire and thus stay cold. The kitten's 

 behaviour cannot be called adapted, for the temperature of its 

 skin is not kept within normal limits. The animal, in other 

 words, is not acting homeostatically for skin temperature. 

 Contrast this behaviour with that of the experienced cat : on 

 a cold day it approaches the fire to a distance adjusted so that 

 the skin temperature is neither too hot nor too cold. If the fire 

 burns fiercer, the cat will move away until the skin is again warmed 

 to a moderate degree. If the fire burns low the cat will move 

 nearer. If a red-hot coal drops from the fire the cat takes such 

 action as will keep the skin temperature within normal limits. 

 Without making any enquiry at this stage into what has hap- 

 pened to the kitten's brain, we can at least say that whereas 



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