14 THE BRAIN OF THE TIGER SALAMANDER 



that of the most primitive amphibians and of the generahzed fishes 

 ancestral to them. 



The internal texture of the brains of the generalized amphibians 

 which are described in this work closely resembles that of the most 

 primitive extant fishes; but the brain as a whole is organized on a 

 higher plane, so that it can more readily be compared with those of 

 reptiles, lower mammals, and man. For this reason the salamanders 

 occupy a strategic position in the phylogenetic series. This examina- 

 tion has brought to light incipient stages of many complicated hu- 

 man structures and some guiding principles of both morphogenesis 

 and physiological action that are instructive. 



When the first amphibians emerged from the water, they had all 

 the land to themselves ; there were no living enemies there except one 

 another. During aeons of this internecine warfare they carried pro- 

 tective armor; but in later times, during the Age of Reptiles, these 

 more efficient fighters exterminated the clumsy armored amphibians. 

 The more active frogs and toads survived, and so also did the slug- 

 gish salamanders and their allies, but only by retiring to concealment 

 in sheltered places. 



In Devonian times, probably about three hundred million years 

 ago, various species of fishes made excursions to the land and ac- 

 quired structures adapted for temporary sojourn out of water. Some 

 of the primitive crossopterygian fishes went further and, after a fish- 

 like larval period, experienced a metamorphosis into air-breathing 

 tetrapods. They became amphibians. These were fresh-water species, 

 and the immediate cause of this evolutionary change was extensive 

 continental desiccation during the Devonian period. While their 

 streams and pools were drying up, those fishes which had accessory 

 organs of respiration in addition to the gills of typical fishes, were 

 able to survive and, through further transformations, become air- 

 breathing land animals. An excellent summary of the paleontological 

 evidence upon which the history of the evolution of fishes has been 

 reconstructed has been published by Romer ('46). 



Two prominent features of this revolutionary change involved the 

 organs of respiration and locomotion, with corresponding changes in 

 the nervous apparatus of control. These systems of organs are typical 

 representatives of the two major subdivisions of all vertebrate bodies 

 and their functions — the visceral and the somatic. The visceral func- 

 tions and the visceral nervous system will receive scant consideration 



