9/9 DESIGN FOR A BRAIN 



These considerations may clarify the relations between the 

 change of concentration of a sex-hormone in the blood of a 

 mammal and its consequent sexual goal-seeking behaviour. A 

 simple alternation between ' present ' and 4 absent ', or between 

 two levels with a threshold, would be sufficient to account for 

 any degree of complexity in the two behaviours, for the com- 

 plexity is not to be related to the hormone-parameter but to the 

 nervous system that is affected by it. Since the mammalian 

 nervous system is extremely complex, and since it is, at almost 

 every point, sensitive to both physical and chemical influences, 

 there seems to be no reason to suppose that the directiveness of 

 the sex-hormones on the brain's behaviour is essentially different 

 from that of any parameter on the system it controls. (That 

 the sex-hormones evoke specifically sexual behaviour is, of course, 

 explicable by the fact that evolution, through natural selection, 

 has constructed specific mechanisms that react to the hormone 

 in the specific way.) 



Ultrastability and the gene-pattern 



9/9. In S. 1/9 it was pointed out that although the power of 

 adaptation shown by a species ultimately depends on its genetic 

 endowment, yet the number of genes is, in the higher animals, 

 quite insufficient to specify every detail of the final neuronic 

 organisation. It was suggested that in the higher animals, the 

 genes must establish function-rules which will look after the 

 details automatically. 



As the minimal function-rules have now been provided (S. 8/7) 

 it is of interest to examine the specification of the ultrastable 

 system to see how many items will have to be specified geneti- 

 cally if the ovum is to grow into an ultrastable organism. The 

 items are as follows : 



(1) The animal and its environment must form an absolute 



system (S. 3/9) ; 



(2) The system must be actively dynamic ; 



(3) Essential variables must be defined for the species (S. 3/14) ; 



(4) Step-functions are to be provided (S. 8/4) ; 



(5) Their critical states are all to be similar (S. 8/6) ; 



(6) The critical states are to be related in value to the limiting 



values of the essential variables (S. 9/1). 

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