ADAPTATION AS STABILITY 5/11 



path mechanically determined. . . . Now contrast with this 

 an instance of behaviour. Take a timid animal such as a 

 guinea-pig from its hole or nest, and put it upon the grass 

 plot. Instead of remaining at rest, it runs back to its hole ; 

 push it in any other direction, and, as soon as you withdraw 

 your hand, it turns back towards its hole ; place any obstacle 

 in its way, and it seeks to circumvent or surmount it, rest- 

 lessly persisting until it achieves its end or until its energy 

 is exhausted.' 



He could hardly have chosen an example showing more clearly 

 the features of stability. 



Survival 



5/9. The forces of the environment, and even the drift of time, 

 tend to displace the essential variables by amounts to which we 

 can assign no limit. For survival, the' essential variables must 

 be kept within their physiological limits. In other words, the 

 values of the essential variables must stay within some definite 

 region in the system's phase-space. It follows therefore that 

 unless the environment is wholly inactive, stability is necessary 

 for survival. 



5/10. If an animal's behaviour always maintains its essential 

 variables within their physiological limits, then the animal can 

 die only of old age. Disease might disturb the essential variables, 

 but the processes of repair and immunity would tend to restore 

 them. But it is equally clear that the environment sometimes 

 causes disturbances for which the body's stabilising powers are 

 inadequate ; infections may prove too virulent, cold too extreme, 

 a famine too severe, or the attack of an enemy too swift. 



The possession of a mechanism which stabilises the essential 

 variables is therefore of advantage : against moderate disturb- 

 ances it may be life-saving even if it eventually fails at some 

 severe disturbance. It promotes, but does not guarantee, survival. 



5/11. Are there aspects of ' adaptation ' not included within the 

 definition of ' stability ' ? Is ' survival ' to be the sole criterion 

 of adaptation ? Is it to be maintained that the Roman soldier 

 who killed Archimedes in Syracuse was better ' adapted ' in his 

 behaviour than Archimedes ? 



65 



