Z K. S. LASHLEY 



a real advance in functional complexity. The work of Dubois 

 and of Lapicque ( '23) has done much to clear up this problem 

 by defining the ratio of brain to body weights and Dubois' 

 coefficient of cephalization presents a possible means of ac- 

 curately correlating cerebral development with intelligence. 

 But the lack of any quantitative determination of differences 

 in the complexity of behavior of different species of animals 

 makes it impossible as yet to evaluate this coefficient. The 

 work of Szymanski ('12), Turner ('13), and von Frisch ('14) 

 on learning^ in insects shows them little inferior to lower 

 mammals, and among mammals we cannot say that a rela- 

 tively higher brain weight is a certain indication of superior 

 intelligence, since we have no sure measure of the latter. 



Within the single species the correlation is still less certain. 

 Basset ('14) found that a strain of rats of less than average 

 brain weight was inferior in learning ability to a normal 

 strain, but his data, as Paterson ( '17) has pointed out, are not 

 statistically valid. Crude comparisons of the brain weights 

 of superior, normal, and criminal men show the former 

 groups as having slightly higher weights (Donaldson, '03), 

 but are of doubtful significance because of the uncertainty 

 of the evidence for a real difference in the complexity of the 

 behavior of the groups (Goring, '13; Fernald, Hayes, and 

 Dawley, '20). 



For ganglion cell number, as indicated by surface area of 

 the cerebrum or degree of fissurization, the relationship is 

 even less clear. The primates show a relatively greater sur- 

 face area than lower mammals, and the anthropoids the great- 

 est surface area of the primates, but, as Monakow ('14) has 

 pointed out, many of the ungulates show much greater fissuri- 

 zation of the cortex than carnivora with probably much higher 

 intelligence, so that the exact significance of surface area 



^ The relation between learning ability and intelligence is by no means estab- 

 lished, since we cannot define either function except in terms of specific instances. 

 Our measures of learning in animals require both the solving of problems and 

 retention of the solutions, and I use the term learning here in the broader sense 

 as including at least some phases of intelligent behavior. 



