6fil 



of their nervous system and their sha]H% but (hner in si/e, and also 

 in the two sexes of one and tlie same species, the quantity of 

 brain increases : 



A. as tiie quotient of the superticial dimension and the cube-root 

 of the longitudinal dimension. 



B. as the product of the longitudinal dimension and the square 

 of its cube-root. 



II. In individuals of one and the same species and of the same 

 sex, differing in size, the quantity of brain increases as the S(iuare 

 of the cube-root of the longitudinal dimension of the body. 



Consequently we tind between the exponents 0.22.. and 0.55.. a 

 relation of a simple nature. 



2 



Moreover the factor S^-'^^ or L^ in ^ is the square of the deno- 

 minator in A. 



The fact that, in different species, a factor determining the 

 quantity of brain is to be found in the superficial dimension of the 

 body, which is the measui-e of the sensitive surfaces as well as of 

 the muscular force, was discussed at lar^e in my memoir of 1897. 

 It is neither incomprehensible, that individuals of different size in 

 one and the same species distinguish themselves from, for the rest 

 closely resembling species differing in size, because only in the latter 

 case an increase of the quantity of brain proportional to the longitudinal 

 dimension takes place, as a consequence of segmental growth, in- 

 crease of sensu-motorical unities in segmentically constituted species 

 of animals. 



From the investigations of I. Hardesty ^) it appears that in the 

 Elephant, which is 180000 times heavier than the Mouse, and in 

 Man, who is 3628 times heavier than the Mouse, the masses of 

 certain nerve-cells of the spinal-cord are proportional as the imagi- 

 nary longitudinal dimensions of the mentioned species. 



If we admit that to every nerve-fibre a definite central cell-mass 

 answers, then these masses must increase with the number of nerve- 

 fibres, in segmentically constituted animals indeed as the longi- 

 tudinal dimension. 



1 



But what is then the meaning of L^? 



The answer to this question was likewise pre|)ared in my memoir 

 of 1897. It is to be found in the very special relation between the 

 size of the eye and the body in animals of different sizes. The 

 longitudinal dimensions of the body and the eye of these animals 



1) Journal of Comparative Neurology. Vol. 12 (1902), p. 125—182. 



