90 FOURTH GENERAL MEETING. 



Further the animals to be compared should as much as possible 

 be fair averages as regards size and state of nutrition of their body. 

 If possible we should always use averages of a number of individual 

 weights, but comparing with one another animals of very different 

 size and choosing fair samples of their species, the individual 

 weights may yield reliable results. In man also a mean ratio is to 

 be arrived at by the exclusion of extreme cases and the selection of 

 subjects of medium stoutness. 



It is the great merit of Prof. Max Weber of Amsterdam to have 

 given a list of observations of body-weight and brain-weight in a 

 large number of mammals of different species, accompanied with 

 data, from which in almost every case we are able to draw an 

 inference as to the full-grown or younger state of the animal. 



Putting to the test of our actual knowledge of the general 

 structure of the central nervous system some data furnished by 

 Weber, it is possible to determine with approximate accurac}- the 

 factor of the influence of the size of the body. 



From the point of view of our present knowledge of the central 

 nervous system the brain must be considered as entirely consisting 

 of connecting pieces between sensory and motor nerves, bringing 

 about — in a more or less complicated manner, according to the 

 higher or lower organisation of the animal, — the encephalic com- 

 munication between the organs of receptivity and activity of the 

 body. In nearly allied animals of every natural group, occupying 

 very much the same degree in the scale of organisation, it would 

 appear that the number of these encephalic connecting pieces is 

 proportionate to the number of the sensory and motor nerve-fibres, 

 and (as the size of the nerve-cells changes but little with the bulk 

 of the animal) also approximately their total weight. The different 

 factors influencing the increase of the size of the brain may be 

 taken together. For the moment neglecting the somewhat varying 

 size and the differences in the number of the branches of the nerve- 

 cells, we may arrive at an estimation of the influence of the size of 

 the body upon that of the brain by the following method, first 

 suggested by Dr Snell. Calling e the weight of the encephalon, 

 s the weight of the body of the small animal, and E the weight of 

 the encephalon, and 5' the weight of the body of a nearly allied 

 but very much larger animal, r an exponent, which we want to 

 know, we have 



S"- : s"- = E : e, 

 S 



r{\ogS — \ogs)=\ogE — \oge, 

 log E — log e 

 log ^ - log s 

 By the application of this formula to 7 pairs of mammals of very 



