8/10 THE HOMEOSTAT 



any inanimate system which behaved in this way we would say, 

 simply, that the line of behaviour from the state at which 1 and 2 

 started moving was unstable. So, to say in psychological terms 

 that the ' trainer ' has ' punished ' the ' animal ' is equivalent to 

 saying in our terms that the system has a set of parameter-values 

 that make it unstable. 



In general, then, we may identify the behaviour of the animal 

 in ' training ' with that of the ultrastable system adapting to 

 another system of fixed characteristics. 



8/10. How will the ultrastable system behave if it has to adapt 

 to two environments, which alternate ? Such a situation is not 

 uncommon: the diving bird has to adapt to situations both on 

 land and in the water; British birds have to adapt both to full 

 foliage in the summer and to bare branches in the winter; and 

 the kitten has to adapt both to the mouse that tries to escape into 

 a hole and to the bird that tries to escape by flying upwards. 



Such cases are equivalent (by Ss. 6/3 and 7/20) to the case in 

 which there is one environment affected by a parameter with two 

 values. Each value, provided it is sustained long enough for the 

 characteristic behaviours of adaptation to be displayed, gives one 

 form to the environment; and the two forms may, if we please, 

 be thought of as two environments. The question can therefore 

 be investigated by allowing the Homeostat to adapt in the pres- 

 ence of an alternating parameter, each value of which must be 

 sustained long enough so that the change does not interrupt the 

 process of trial and error. 



Let the Homeostat be arranged so that it is partly under uni- 

 selector-, and partly under hand-, control. Let it be started so 

 that it works as an ultrastable system. Select a commutator 

 switch, and from time to time reverse its polarity. This reversal 

 provides the system with the equivalent of two environments 

 which alternate. We can now predict that it will be selective for 

 fields that give adaptation to both environments. For consider 

 what field can be terminal : a field that is terminal for only one of 

 the parameter-values will be lost when the parameter next changes ; 

 but the first field terminal for both will be retained. Figure 

 8/10/1 illustrates the process. At R v R 2 , R 3 , and R± the hand- 

 controlled commutator H was reversed. At first the change of 

 value caused a change of field, shown at A. But the second 



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