DESIGN FOR A BRAIN 8/15 



to avoid it and would blindly evoke the ' lethal ' action. The 

 Homeostat's method for achieving adaptation is thus essentially 

 useless when its environment contains such 4 lethal ' discontinuities. 



The living organism, however, is also apt to fail with just the 

 same type of environment. The pike that collided with the 

 glass plate while chasing minnows failed at first to avoid collision 

 precisely because of the suddenness of the transition from not 

 seeing clear glass to feeling the impact on its nose. This flaw 

 in the living organism's defences has, in fact, long been known 

 and made use of by the hunter. The stalking cat's movements 

 are such as will maintain as long as possible, for the prey, the 

 appearance of a peaceful landscape, to be changed with the 

 utmost possible suddenness into one of mortal threat. In the 

 whole process the suddenness is essential. Consider too the 

 essential features of any successful trap; and the necessity, in 

 poisoning vermin, of ensuring that the first dose is lethal. 



If, then, the ultrastable system usually fails when attempting to 

 adapt to an environment with sudden discontinuities, so too does 

 the living organism. 



8/15. Another weakness shown by the ultrastable system's 

 method is that success is dependent on the system's using a suitable 

 period of delay between each trial. Thus, the system shown in 

 Figure 7/23/1 must persist in Trial IV long enough for the repre- 

 sentative point to get away from the region of the critical states. 

 Both extremes of delay may be fatal: too hurried a change from 

 trial to trial may not allow time for ' success ' to declare itself; 

 and too prolonged a testing of a wrong trial may allow serious 

 damage to occur. The optimal duration of a trial is clearly the 

 time taken by information to travel from the step-mechanisms 

 that initiate the trial, through the environment, to the essential 

 variable that shows the outcome. If the ultrastable system re- 

 quires the duration to be adjusted, so does the living organism; for 

 there can be little doubt that on many occasions living organisms 

 have missed success either by abandoning a trial too quickly, or 

 by persisting too long with a trial that was actually useless. (The 

 topic is referred to again in S. 17/10.) 



The same difficulty, then, confronts both ultrastable system and 

 living organism. 



120 



