The method involves many assumptions. It does not adequately take 

 into account regional variations in the density of neurons, in the ratio be- 

 tween neuroglia and neurons, in tlie size of nerve-cell bodies, and in the 

 length and complexity of the dendritic processes of the neurons. Nonethe- 

 less, it provides a novel, approximate gauge of cortical development, which 

 seems to go beyond the earlier intricate attempts of Eugene Dubois (1918a, 

 1918b, 1919, 1920, 1924, 1934, etc.) and others. Above all, the method has 

 been developed for comparisons among species. 



Table 17 conveys the results for a series of hominoids. The table gives 

 a selection of measured endocranial capacities of individual specimens, or 

 means of groups, estimates of body size in gm. or c.c, and then, derived 

 from Jerison's equations, estimates of the total number of neurons in the 

 whole brain and in that part of the brain related to body size, as well as the 

 number of surplus or excess neurons. 



A number of the figures in Table 17 are taken from Jerison's ( 1 963) 

 analysis. Others are based on my own computations or recomputations of 

 capacities and body weights. Thus, for A. afrit anus, he based his estimate 

 on a "brain size" of 500 gm. (which is close to the mean capacity of 494 c.c.) 

 and a body size of 20,000 gm. I have recalculated these values for capaci- 

 ties of 500 c.c. (near the mean of published values), 435 c.c. (lowest published 

 value in the sample) and 540 c.c. (highest value in the sample). The body 

 weight of 20,000 gm. seems to be somewhat too small an estimate, and I 

 have accordingly made the calculations for body weights of 25,000 gm. and 

 35,000 gm., associating the smaller brain sizes with the smaller body weight 

 and the largest A. africanus capacity (540 c.c.) with the larger body weight. 

 The estimates of "extra neurons" obtained by applying Jerison's formulae 

 to these figures are 3.9, 4.3, and 4.5 billion, respectively; Jerison's figure for 

 A. africanus fell near the upper end of this range of values. 



Again, Jerison used a guess of fioo gm. for the brain size of A. boisei 

 (formerly Zinjarithropns); I have used the value of 530 c.c, while adhering 

 to his estimate of body size, namely 50,000 gm. This gave a value of 4.2 

 in place of his estimate of 4.7 billion. 



Despite the altered values, and within the limitations of the method 

 and the assumptions on which it is based, my results have corroborated 

 one of Jerison's conclusions, namely that the australopithecines "were 

 clearly, if only slightly, in advance of the level of brain evolution achieved 

 by the anthropoid apes of our time" (Jerison 1963, p. 288). The pongid 

 values range from 3.4 to 3.6 billion "excess neurons." H. habilis and H. erec- 



% no 



