DEVELOPMENT OF NERVE ELEMENTS. 175 



those reactions of the central system which form the 

 basis for the general intelligence of the species require a 

 given complexity in this system, which complexity in 

 turn demands a large number of elements. No matter, 

 therefore, how small the size of the entire animal, its 

 nervous system must possess this minimal number of 

 cells, and in the smallest representatives these are of a 

 relatively large size. For this reason the smaller mem- 

 bers of such a group have proportionately the largest 

 central systems, while the largest members have numeri- 

 cally the most complex central system, and therefore 

 the anatomical basis for a better intelligence. 



In general, then, the variations in the weight of the 

 adult human brain just mentioned are best explained by 

 regarding them as mainly dependent on the size of the 

 constituent nerve elements. The nervous system of 

 animals with a smaller brain-weight than man probably 

 contains both fewer cells, and of these the developed cells 

 are also fewer in number. What the relation is in those 

 with a brain-weight above that of man is not known, but 

 the probabilities are in favour of the numerical superiority 

 of man here also. Before concluding, however, I desire 

 to emphasise two points : first, that the explanations 

 just suggested apply to anatomical differences only, to 

 the mass of encephalon, its physiological capabilities 

 being another matter ; and second, that the expression 

 of these differences in the terms of the constituent cells 

 has the advantage of preventing a too hasty inference 

 from the brute figures indicating weight differences to 

 the subtle physiological possibilities of this complex 

 organ, 



