REGULATION IN BIOLOGICAL SYSTEMS 10/7 



about the times reaching the essential variables. In the same way, 

 the skilled provider for a family may go through difficult times 

 without his family reahsing that anything unusual has happened. 

 The family of an unskilled provider would have discovered it. 



In general, then, an essential feature of the good regulator is that 

 it blocks the flow of variety from disturbances to essential variables. 



10/7. The blocking may take place in a variety of ways, which 

 prove, however, on closer examination to be fundamentally the 

 same. Two extreme forms will illustrate the range. 



One way of blocking the flow (from the source of disturbance D 

 to the essential variable E) is to interpose something that acts as a 

 simple passive block to the disturbances. Such is the tortoise's 

 shell, which reduces a variety of impacts, blows, bites, etc. to a 

 negligible disturbance of the sensitive tissues within. In the same 

 class are the tree's bark, the seal's coat of blubber, and the human 

 skull. 



At the other extreme from this static defence is the defence by 

 skilled counter-action — the defence that gets information about the 

 disturbance to come, prepares for its arrival, and then meets the 

 disturbance, which may be complex and mobile, with a defence that 

 is equally complex and mobile. This is the defence of the fencer, 

 in some deadly duel, who wears no armour and who trusts to his 

 skill in parrying. This is the defence used mostly by the higher 

 organisms, who have developed a nervous system precisely for the 

 carrying out of this method. 



When considering this second form we should be careful to notice 

 the part played by information and variety in the process. The 

 fencer must watch his opponent closely, and he must gain informa- 

 tion in all ways possible if he is to survive. For this purpose he is 

 born with eyes, and for this purpose he learns how to use them. 

 Nevertheless, the end result of this skill, if successful, is shown by his 

 essential variables, such as his blood-volume, remaining within 

 normal limits, much as if the duel had not occurred. Information 

 flows freely to the non-essential variables, but the variety in the 

 distinction "duel or no-duel" has been prevented from reaching the 

 essential variables. 



Through the remaining chapters we shall be considering this type 

 of active defence, asking such questions as: what principles must 

 govern it? What mechanisms can achieve it? And, what is to be 

 done when the regulation is very difficult? 



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