Chapter 13 



REGULATING THE VERY LARGE 



SYSTEM 



13/1. Regulation and control in the very large system is of peculiar 

 interest to the worker in any of the biological sciences, for most of 

 the systems he deals with are complex and composed of almost 

 uncountably many parts. The ecologist may want to regulate the 

 incidence of an infection in a biological system of great size and 

 complexity, with climate, soil, host's reactions, predators, competi- 

 tors, and many other factors playing a part. The economist may want 

 to regulate against a tendency to slump in a system in which prices, 

 availability of labour, consumer's demands, costs of raw materials, 

 are only a few of the factors that play some part. The sociologist 

 faces a similar situation. And the psychotherapist attempts to 

 regulate the working of a sick brain that is of the same order of 

 size as his own, and of fearful complexity. These regulations are 

 obviously very different from those considered in the simple mech- 

 anisms of the previous chapter. At first sight they look so different 

 that one may well wonder whether what has been said so far is not 

 essentially inapplicable. 



13/2. This, however, is not so. To repeat what was said in S.4/ 18, 

 many of the propositions established earher are stated in a form 

 that leaves the size of the system irrelevant. (Sometimes the number 

 of states or the number of variables may be involved, but in such 

 a way that the proposition remains true whatever the actual 

 number.) 



Regulation in biological systems certainly raises difficult problems 

 — that can be admitted freely. But let us be careful, in admitting 

 this, not to attribute the difficulty to the wrong source. Largeness 

 in itself is not the source; it tends to be so regarded partly because 

 its obviousness makes it catch the eye and partly because variations 

 in size tend to be correlated with variations in the source of the real 

 difficulty. What is usually the main cause of difficulty is the variety 

 in the disturbances that must be regulated against. 



The size of the dynamic system that embodies T tends to be 



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