1/9 DESIGN FOR A BRAIN 



The genetic control of cerebral function 



1/9. The various species of the animal kingdom differ widely 

 in their powers of learning : Man's intelligence, for instance, is 

 clearly a species-characteristic, for the higher apes, however well 

 trained, never show an intelligence equal to that of the average 

 human being. Clearly the power of learning is determined to 

 some extent by the inherited gene-pattern. In what way does 

 the gene-pattern exert its effect on the learning process ? In 

 particular, what part does it play in the adjustments of part to 

 part which the previous section showed to be fundamental ? 

 Does the gene-pattern determine these adjustments in detail ? 



In Man, the genes number about 50,000 and the neurons number 

 about 10,000,000,000. The genes are therefore far too few to 

 specify every neuronic interconnection. (The possibility that a 

 gene may control several phenotypic features is to some extent 

 balanced by the fact that a single phenotypic feature may require 

 several genes for its determination.) 



But the strongest evidence against the suggestion that the 

 genes exert, in the higher animals, a detailed control over the 

 adjustments of part to part is provided by the evidence of S. 1/4. 

 A dog, for instance, can be made to respond to the sound of a 

 bell either with or without salivation, regardless of its particular 

 gene-pattern. It is impossible, therefore, to relate the control of 

 salivation to the particular genes possessed by the dog. This 

 example, and all the other facts of which it is typical, show that 

 the effect of the gene-pattern on the details of the learning process 

 cannot be direct. 



The effect, then, must be indirect : the genes fix permanently 

 certain function-rules, but do not interfere with the function-rules 

 in their detailed application to particular situations. Three 

 examples of this type of control will be given in order to illustrate 

 its nature. 



In the game of chess, the laws (the function rules) are few and 

 have been fixed for a century ; but their effects are as numerous 

 as the number of positions to which they can be applied. The 

 result is that games of chess can differ from one another though 

 controlled by constant laws. 



A second example is given by the process of evolution through 

 natural selection. Here again the function-rule (the principle of 



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