10/4 THE RECURRENT SITUATION 



theoretically (because of its complete generality), is of little 

 importance in practice, for its occurrence in the terrestrial world 

 is rare (though it may occur more commonly in models or in pro- 

 cesses of adaptation set up in large computers). Were it common, 

 a brain would be useless (/. to C, S. 13/5). In fact, brains have 

 been developed because the terrestrial environment usually con- 

 fronts the organism with a Grand Disturbance that has a major 

 degree of constraint within its component parts, of which the 

 organism can take advantage. Thus the organism commonly 

 faces a world that repeats itself, that is consistent to some degree 

 in obeying laws, that is not wholly chaotic. The greater the 

 degree of constraint, the more can the adapting organism specialise 

 against the particular forms of environment thsft do occur. As 

 it specialises so will its efficiency against the particular form of 

 environment increase. If the reader feels the ultrastable system, 

 as described so far, to be extremely low in efficiency, this is because 

 it is as yet quite unspecialised ; and the reader is evidently uncon- 

 sciously pitting it against a set of environments that he has 

 restricted in some way not yet stated explicitly in this book. 



10/3. The chapters that follow will consider several constraints 

 of outstanding commonness and will show how the appropriate 

 specialisations exemplify the above propositions in several ways. 

 They will consider certain ways in which the ordinary terrestrial 

 environments fail to show the full range ; and we will see how these 

 restrictions indicate ways in which the living organism can 

 specialise so as to take advantage of them. 



The recurrent situation 



10/4. In this chapter we will consider the case, of great importance 

 in real life, in which the occasional disturbances (class" 2 of S. 9/14) 

 are sometimes repetitive, and in which a response, if adaptive on 

 the disturbance's first appearance, is also adaptive when the same 

 disturbance appears for the second, third, and later times. 



We must not take for granted that one response will be adaptive 

 to all occurrences of the disturbance, for there are cases in which 

 what is appropriate to a disturbance depends on how many times 

 it has appeared before. An outstanding example is given by the 

 rat facing that environment (a natural one by S. 3/1) in which food 



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