THE BRAIN OF APES AND OF MAN 259 



Abundance of convolutions ^nd their increase at this 

 or that part of the brain must, it is obvious, increase the 

 active brain substance. But there is some evidence of a 

 special kind as to the significance of increased bulk of 

 the entire brain, apart from the folding of its surface. 

 This is afforded by the brain cavities of the skulls observed 

 in the series of vertebrate animals. The older groups — 

 those " lower," that is farthest removed from man and 

 the animals most like him — have in proportion to the 

 bulk of their bodies much smaller brains than the later- 

 developed groups. Thus fishes have smaller brains than 

 reptiles, and these have much smaller brains than 

 mammals. A cod-fish has in proportion to its bulk of 

 living material a smaller brain than a crocodile or a 

 turtle, and these have a much smaller brain than a pig. 

 Not only so, but earlier kinds of mammals than the pig 

 have a smaller brain proportionately than that animal 

 has, and pigs have a smaller brain in proportion to their 

 bulk than monkeys, and monkeys (as we have seen) a 

 smaller brain than man. This increase of size is, in 

 general, proportionate to an increase in the variety and 

 complexity of the control of the movements of the body 

 and their relation to the activities of the great organs of 

 sense, such as the eyes, and the organs of smell and 

 hearing. 



But there is something more involved in the increase 

 of the brain than this. We now know that the brain of 

 very many kinds of animals has been increasing in size 

 in the later geological periods. Huge reptiles as big as 

 elephants existed on the land surface of the globe before 

 the hairy, warm-blooded mammals which now dominate 

 the situation had developed in number or in size — 

 namely, in the period of and before the chalk which 

 geologists call the Mesozoic or secondary period, to 



