15/2 ADAPTATION IN ITERATED AND SERIAL SYSTEMS 



what happens at each variable is conditional, at that moment, 

 on the values of all the other variables in the system. What, 

 then, does characterise the ordinary terrestrial environments from 

 this point of view ? 



Common observation shows that the ordinary terrestrial 

 environment usually shows several features, which are closely 

 related : 



(1) Many of the variables, often the majority, are constant over 

 appreciable intervals of time, and thus behave as part-functions. 

 Thus, the mammal stands on ground that is almost always im- 

 mobile; tree-trunks keep their positions; a cup placed on a table 

 will stay there till a force of more than a certain amount arrives. 

 If one looks around one, only in the most chaotic surroundings 

 will all the variables be changing. This constancy, this common- 

 ness of part-functions, must, by S. 12/14, be due to commonness 

 of states of equilibrium in the parts that compose the terrestrial 

 environment. Thus the environment of the living organism tends 

 typically to consist of parts that are rich in states of equilibrium. 



(2) Associated with this constancy (naturally enough by S. 

 12/17) is the fact that most variables of the environment have an 

 immediate effect on only a few of the totality of variables. At 

 the moment, for instance, if I dip my pen in the ink-well, hardly 

 a single other variable in the room is affected. Opening of the 

 door may disturb the positions of a few sheets of paper, but will 

 not affect the chairs, the electric light, the books on the shelves, 

 and a host of others. 



We are, in fact, led again to consider the properties of a system 

 whose connexions are fluctuating and conditional — the type 

 encountered before in S. 11/12, and therefore treatable by the 

 same method. I suggest, therefore, that most of the environ- 

 ments encountered on this earth by living organisms contain many 

 part-functions. Conversely, a system of part-functions adequately 

 represents a very wide class of commonly occurring environ- 

 ments. 



As a confirmatory example, here is Jennings' description of an 

 hour in the life of Paramecium, with the part-functions indicated 

 as they occur. 



(It swims upwards and) '. . . thus reaches the surface film.' 



The effects of the surface, being constant at zero throughout the 



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