THE UNIQUE MAMMAL 7 



tion of all students of evolution. It has generally been regarded 

 as the most important feature, in a practical point of view, 

 differentiating man from the other mammals. This is, of course, 

 because of the association of greater mental capacities and abili- 

 ties with the larger and more highly organized brain. The dis- 

 tinguished Dutch anatomist and anthropologist Eugene Dubois, 

 who was the discoverer and principal student of that extraor- 

 dinary "missing link" between man and his humbler forebears 

 called Pithecanthropus erectuSy has for many years laid stress 

 upon the value of brain size in proportion to body size as a 

 quantitative index of evolutionary progress. In 1897 he put 

 forward a formula for what he called the "coefficient of cephali- 

 zation." This formula stated that for any species of mammal 

 the coefficient of cephalization K was equal to the brain weight 

 in grams divided by the body weight in grams raised to the 

 5/9^^ power. Numerically evaluating this coefficient on the basis 

 of actual weighings, he first found that the cephalizations of 

 the gorilla, the chimpanzee, the orang, and the gibbons are all 

 approximately equivalent and equal to about one-fourth of the 

 value of the cephalization coefficient for man. The cephalization 

 coefficients for the tiger, the cat, the buffalo, and the goat proved 

 again, according to Dubois, to be approximately the same in 

 each of these forms, and equal to about one-eighth of the value 

 for man. For rodents like the rat and the mouse the cephaliza- 

 tion coefficient was much smaller, amounting to only about one- 

 quarter of that for the carnivores and herbivores just mentioned. 

 The well-known French student of the brain-weight problem, 

 Louis Lapicque, and his collaborators came forward in 1905 

 with evidence from birds confirmatory of the same rule, and in 

 1913 Dubois claimed to have established it for reptiles and 

 fishes. So that now he regards it as a law of evolution for 

 vertebrates in general. 



Dubois made an interesting generalization from his numerical 

 results, to the effect that as one goes up the evolutionary series 

 it appears that the coefficient of cephalization tends approxi- 

 mately to increase in a geometrical progression of the term 2 j 



