234 



value of r in seven pair of mammalian species, i.e. a sligliHy smaller 

 exponent than wonld correspond lo the proportionality of the brain 

 weight with the surface dimensions of the body ^). The discrepancy 

 appeared to be constant, and the same exponent was found for 

 Birds bj Louis Lapicque and Pikrkk Giiiahd in 1905*), and for 

 Reptiles and Fishes by me in 1913"). The exponent ^9 holds im- 

 doul)tedly for all Vertebrata. Certainly this "sli-ange power" of the 

 body weight cannot be attributed to insufficiency of (he data; it is 

 impossible that we have to do here with a "rough empirical law, 

 as limit of a sum of different functions". The relation found between 

 the weights of the brain and the body must be a simple, rational 

 one. As this exponent indicates the relation of species to species, a 

 relation which must have come about with the origination of the species, 

 I will designate it here as phy logen e tic exponent. 



In the system of coordinates of Fig. 2 tlie body weights in kg. 



200 



150. 



100. 



30 < 



z/ 



10 kc 



20. 



30 



40. 



50. 



Fig. 2 



1) EuG. Dubois, De verhouding van het gewicht der hersenen tot de grootte van 

 het lichaam bij de Zoogdieren. Verhandelingen der Kon. Akademie van Weten- 

 schappen te Amsterdam. Tweede Sectie. Deel V, W. 10. 1897. — Also: Sur Ie 

 rapport du poids de I'encépliale avec la grandeur du corp.s chez les Mammifères, 

 in Bulletins de la Société d'Anlhropologie de Paris 1897, p. 337 — 376. 



') Gompies Rendus des séances de 1' Academie des Sciences. Paris 1905, 1, 

 Tome 140, p. 1057-1059. 



>) These Proceedings, Vol XVI, p. 651—654. 1914. 



