SALAMANDERS AND THEIR BRAINS 15 



in this work, for the material at our disposal is not favorable for the 

 study of these tissues. Here we are concerned primarily with the 

 nervous apparatus of overt behavior, that is, of the somatic adjust- 

 ments. 



The most important change in these somatic adjustments during 

 the critical evolutionary period under consideration is the transition 

 from swimming to walking. The fossil record of the transformation of 

 fins into legs is incomplete, but it is adequate to show the salient 

 features of the transformation of crossopterygian fins into am- 

 phibian legs (Romer, '46). In the individual development of every 

 salamander and every frog the internal changes in the organization 

 of the nervous system during the transition from swimming to walk- 

 ing can be clearly seen. And these changes are very significant in our 

 present inquiry because they illustrate some general principles of 

 morphogenesis of the brain more clearly than do any other available 

 data. 



In fishes, swimming is a mass movement requiring the co-ordinated 

 action of most of their muscles in unison, notably the musculature of 

 the trunk and tail. The paired fins are rudders, not organs of propul- 

 sion. The young salamander larva has no paired limbs but swims 

 vigorously. This is a typical total pattern of action as defined by 

 Coghill. The adult salamander after metamorphosis may swim in the 

 water like the larva; and he can also walk on land with radically dif- 

 ferent equipment. Some fishes can crawl out on land, but the modi- 

 fied fins are clumsy and ineflBcient makeshifts compared with the 

 amphibian's mobile legs. 



Quadrupedal locomotion is a very complicated activity compared 

 with the simple mass movement of swimming. The action of the four 

 appendages and of every segment of each of them must be harmoni- 

 ously co-ordinated, with accurate timing of the contraction of many 

 small muscles. These local activities are "partial patterns" of be- 

 havior, in Coghill's sense. From the physiological standpoint there is 

 great advance, in that the primitive total pattern is supplemented, 

 and in higher animals largely replaced, by a complicated system of 

 co-ordinated partial patterns. This is emphasized here because it pro- 

 vides the key to an understanding of many of the differences between 

 the nervous systems of fishes, salamanders, and mammals. Motility, 

 and particularly locomotion, have played a major role in vertebrate 

 evolution, as dramatically told by Gregory ('43). This outline has 

 been filled in by Howell's ('45) interesting comparative survey of the 



