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are not yet differentiated, but every part of the 

 body reacts directly to stimulus. At a rather 

 later stage of evolution we find that nerves have 

 been developed, and that these run in definite 

 directions ; from which it seems natural, on any 

 theory of evolution, to suppose that even before 

 the specialisation of tissue to the performance 

 of nervous functions, the liberation of energy by 

 stimulus had usually followed these paths, and 

 that the repeated occurrence of the changes in- 

 volved in such liberation had left traces in the 

 permanent modification ot structure ; modifications 

 which may be looked on as constituting an ele- 

 mentary form of memory. According to the 

 selective theory of evolution, the factor which 

 determines the perpetuation and progress of any 

 modification of structure is the advantage which 

 that modification gives to the organism. Now the 

 advantage given by nerve specialisation must have 

 been mainly the better guidance of action. It is 

 difficult to suppose that, in the earlier stages of 

 evolution, this guidance went much farther than 

 to ensure the persistence of activity so long as 

 sensation remained pleasant, and the intermission 

 of activity when sensation became unpleasant. The 

 first step towards the development of anything 

 that could be called a mental function would 

 probably be taken in respect to the obtaining of 

 food. When organisms had developed in such a 

 D 



