igO FROM FISH TO PHILOSOPHER 



alon.' Dominance by the cephalic portion of the nervous 

 system is common among the invertebrates (flatworms, 

 annehd worms, mollusks, insects, and others), but in no 

 instance has such a complex brain been developed as in 

 the vertebrates. The evolution of the vertebrate brain has 

 its roots in the fact that the protovertebrate was a 

 spindle-shaped creature that, when it essayed to enter 

 the fast-moving waters of the Cambrian continents, 

 swam vigorously with one end foremost and needed to 

 concentrate not only the distance receptors, but also the 

 motor control of the mouth parts, the respiratory mech- 

 anism, and the muscles generally, in the front end of the 

 body. 



This brain stem served the ostracoderms and fishes, and, 

 with but slight elaboration, the Amphibia, reptiles, and 

 birds, through geologic ages, only to undergo dramatic 

 evolution in the mammals. In all animals below the 

 mammals, the most anterior part of the brain consists 

 of two nearly separated lobes (cerebral hemispheres) 

 which primarily subserve the function of smell (the old 

 nose brain'). In the reptiles, the cerebral hemispheres 

 have begun to enlarge and to develop, for the first time, 

 an outer layer or cortex composed of relatively large 

 pyramid-shaped cells having many interconnections 

 with each other, as well as with the brain stem. As be- 

 tween the lowest mammals, such as the duckbill and 

 echidna, and man, the cerebral cortex undergoes enor- 

 mous increase in relative size, partly by increase in the 

 number of cell layers and partly by wrinkling or infold- 

 ing of the brain substance so as to increase the stirface 

 area. This increase in number of cells in the cortex is 

 paralleled by a corresponding increase in the number of 

 nervous interconnections enjoyed by each cell, and by 

 the development of large association tracts that connect 

 all parts of the enlarging brain together so that all re- 

 gions, however remote from each other, are intercon- 

 nected. So great is the relative development of the cere- 



