Summary and Preface* 



The book is not a treatise on all cerebral mechanisms but an 

 attempt to solve a specific problem : the origin of the nervous 

 system's unique ability to produce adaptive behaviour. The 

 work has as basis the fact that the nervous system behaves 

 adaptively and the hypothesis that it is essentially mechanistic ; 

 it proceeds on the assumption that these two data are not irre- 

 concilable. It attempts to deduce from the observed facts what 

 sort of a mechanism it must be that behaves so differently from 

 any machine made so far. Many other workers have proposed 

 theories on the subject, but they have usually left open the 

 question whether some different theory might not fit the facts 

 equally well. I have attempted to deduce what is necessary, 

 what properties the nervous system must have if it is to behave 

 at once mechanistically and adaptively. 



Proceeding in this way I have deduced that any system which 

 shows adaptation must (1) contain many variables that behave 

 as step-functions, (2) contain many that behave as part-functions, 

 and (3) be assembled largely at random, so that its details are 

 determined not individually but statistically. The last require- 

 ment may seem surprising : man-made machines are usually 

 built to an exact specification, so we might expect a machine 

 assembled at random to be wholly chaotic. But it appears that 

 this is not so. Such a system has a fundamental tendency, shown 

 most clearly when its variables are numerous, to so arrange its 

 internal pattern of action that, in relation to its environment, it 

 becomes stable. If the system were inert this would mean little ; 

 but in a system as active and complex as the brain, it implies that 

 the system will be self-preserving through active and complex 

 behaviour. 



The work may also be regarded as amplifying the view that 

 the nervous system is not only sensitive but ' delicate ' : that its 

 encounters with the environment mark it readily, extensively, 

 and permanently, with traces distributed according to the 

 4 accidents ' of the encounter. Such a distribution might be 

 expected to produce a merely chaotic alteration in the nervous 

 system's behaviour, but this is not so : as the encounters multiply 

 there is a fundamental tendency for the system's adaptation to 

 improve, for the traces tend to such a distribution as will make 

 its behaviour adaptive in the subsequent encounters. 



* The summary is too brief to be accurate ; the full text should be con- 

 sulted for the necessary qualifications. 



V 



