ORIGIN OF THE CEREBRAL HEMISPHERES 451 



involved the elaboration of separate correlation centers in the 

 forebrain for each of these reflex patterns, different in each species 

 of animal according to its mode of life. The diverse patterns 

 of cerebral architecture exhibited in the vertebrate series are, 

 therefore, structural expressions of these functional relationships, 

 namely, the various parts played by the olfactory and the differ- 

 ent non-olfactory sensory systems. 



The form relations of these correlation centers which have 

 been assumed by the different groups of fishes are exceedingly 

 diverse, each structural pattern probably reflecting some partic- 

 ular grouping of the sensorimotor elements of behavior char- 

 acteristic of the species. It would almost seem as if nature had 

 tried many experiments, each of which was successful within a 

 certain environmental range. For life in an aquatic medium 

 the most successful of these appears to have been the teleostean 

 type; but this type, though capable of unlimited modification 

 of detail on the plane of relatively simple forms of reflex behavior, 

 has not proved adequate for differentiation in the direction 

 leading up to the individually modifiable and inteUigent forms 

 of behavior. 



Moreover, none of the more highly specialized kinds of fishes 

 were able to make the structural readjustments required to 

 maintain themselves in inland waters during the period of 

 continental elevation and consequent drouth known to have 

 occurred in late Silurian times. On the other hand, certain very 

 generalized species of ganoids were able to survive these periods 

 of desiccation by developing accessory respiratory organs and 

 so modifying the brain and its membranes as to facilitate its 

 aeration in a reduced supply of oxygen. Of the several modi- 

 fications of brain form which met the requirements of these 

 adverse conditions, one in particular has proved susceptible of 

 unlimited further differentiation. 



Those primitive ganoids of Silurian or early Devonian times 

 which w^ere able to perfect the air-breathing apparatus and so 

 to leave the water as amphibians probably possessed fully 

 evaginated cerebral hemispheres similar to those of Protopterus 

 and Lepidosiren, for all modern amphibians, larval and adult. 



