Jerison's now classic volume, Evolution of the Brain and Intelligence (1973), drew inspiration from the words of Karl Lashley
The only neurological character for which a correlation with behavioral capacity in different animals is supported by significant evidence is the total mass of tissue, or rather, the index of cephalization, measured by the ratio of brain to body weight, which seems to represent the amount of brain tissue in excess of that required for transmitting impulses to and from the integrative centers.
Jerison expanded Lashley's views in key respects. First, he postulated that brain size is a natural biological statistic that can be readily used to estimate other anatomical characteristics of the brain including the size of other neural structures, the degree of fissurization, and neuronal density. He recognized, however, one major exception to this rule – the size of the olfactory bulbs appears to have little relationship to that of other brain structures. Second, Jerison articulated the principle of proper mass – ‘the mass of neural tissue controlling a particular function is appropriate to the amount of information processing involved in performing the function.’ (Jerison, 1973, p. 8). Third, Jerison recognized that brain mass varies with body mass, and he developed a model of brain/body size relationships according to which, with increasing body size, brain size expands in a mathematically predictable way in each vertebrate class.