If we do not know that simple pkysico-physical correlation, how can we 

 hope to make meaningful statements about the correlation between gross 

 brain size and cellular structure on the one hand, and about psychical 

 and behavioral attributes on the other? For the physico-physical correlation 

 is basic to the physico-psychical association. We do not have the requisite 

 information at either level. 



Holloway (1968a, p. 145), moving outside the human species, has 

 concluded that: "Gross size of the brain alone does not explain differences 

 of behavior within the primate order." He is at pains to point out that 



such correlations [as between brain-size and specific behavioral traits like memory, 

 insight, forethought, symboli/ation] are not causal analyses, and that a parameter 

 such as brain weight in grams, or volume in ml, or area [of cortex] in sq. mm, 

 cannot explain the differences in behavior which are observed. [Ibid., p. 125; 

 italics mine] 



A more encompassing theory, Holloway states, should entail not merely 

 the changes in brain size that have occurred in evolution but the internal 

 reorganization of the cellular material of the brain. It is precisely at this 

 level that we are most ignorant. 



If we compare different species of mammals, many studies have 

 demonstrated that larger brains are correlated with clearly defined cellular 

 and chemical features. For instance, the bigger the brain, the lower is the 

 density of nerve cells in that brain (Nissl 1898; von Bonin 1948; Tower and 

 Elliott 1952; Shariff 1953; Tower 1954). Further, it has been claimed that 

 neurons are bigger and nerve cell processes longer and more complex in 

 bigger brains. The glia/neuron ratio is likewise higher. The claims and their 

 validity have been well-summarized by Holloway (1968a). As he points out, 

 an increase in dendritic branching means more synapses and more con- 

 nectivity, and with this goes more complex behavior. 



By this kind of analysis of different species increase in brain size is 

 coming to be meaningfully analyzed in terms of its structural units. More 

 complete knowledge of these units, in turn, may provide a more rational basis 

 for understanding increasingly complex behavior. All this has been shown to 

 apply in a series of living forms that are assumed to represent an evolution- 

 ary progression from one form to another up the scale. To a lesser extent, 

 similar changes have been shown to apply to the ontogenetic development of 

 individuals within a species. For the adult level, however, I reiterate the view 

 expressed earlier: we do not have any clear picture of the histological and 

 chemical differences between large and small brains among members of the 



