9/2 DESIGN FOR A BRAIN 



conditions. As long as the injurious condition continues, 

 the changes of behaviour continue. The first change of 

 behaviour may not be regulatory [what I call ' adaptive '], 

 nor the second, nor the third, nor the tenth. But if the 

 changes continue, subjecting the organism successively to 

 all possible different conditions, a condition will finally be 

 reached that relieves the organism from the injurious action, 

 provided such a condition exists. Thereupon the changes 

 in behaviour cease and the organism remains in the favourable 

 situation.' 



The resemblance between my statement and his is obvious. 

 Jennings grasped the fundamental fact that aimless change can 

 lead to adaptation provided that some active process rejects the 

 bad and retains the good. He did not, however, give any physical 

 (i.e. non-vital) reason why this selection should occur. He records 

 only that it does occur, and that its occurrence is sufficient to 

 account for adaptation at the primitive level. 



The first example therefore suggests that, provided we are 

 willing to assume that Stentor contains step-functions which (a) 

 affect Stentor* s behaviour, and (b) have critical states that are 

 encountered before the essential variables reach their extreme 

 limits, Stentor may well achieve its final adaptation by using 

 the automatic process of ultrastability. 



9/2. The next example includes more complicating factors but 

 the main features are clear. Mowrer put a rat into a box with 

 a grilled metal floor. The grill could be electrified so as to give 

 shocks to the rat's paws. Inside the box was a pedal which, 

 if depressed, at once stopped the shocks. 



When a rat was put into the box and the electric stimulation 

 started, the rat would produce various undirected activities such 

 as jumping, running, squealing, biting at the grill, and random 

 thrashing about. Sooner or later it would depress the pedal 

 and stop the shocks. After the tenth trial, the application of 

 the shock would usually cause the rat to go straight to the pedal 

 and depress it. These, briefly, are the observed facts. 



Consider the internal linkages in this system. We can suffi- 

 ciently specify what is happening by using six variables, or sets 

 of variables : those shown in the box-diagram below. By con- 

 sidering the known actions of part on part in the real system 

 we can construct the diagram of immediate effects. Thus, the 



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