16 THE BRAIN OF THE TIGER SALAMANDER 



mechanisms of locomotion, and I have elsewhere discussed ('48) Cog- 

 hill's contributions to this theme. 



In the history of vertebrate evolution there were four critical pe- 

 riods: (1) the emergence of the vertebrate pattern of the nervous 

 system from invertebrate ancestry; (2) the transition from aquatic 

 to terrestrial life; (3) the differentiation within the cerebral hemi- 

 spheres of primitive cerebral cortex; (4) the culmination of cortical 

 development in mankind, with elaboration of the apparatus requisite 

 for language and other symbolic (semantic) instrumentation of the 

 mental life. 



1 . The extinct ancestors of the vertebrates in early Silurian times 

 were probably soft and squashy creatures, not preserved as fossils. 

 Some of their aberrant descendants may be recognized among the 

 Enteropneusta, Tunicata, and Amphioxi ; but the first craniate verte- 

 brates preserved as fossils were highly specialized, heavily armored 

 ostracoderms, now all extinct. 



2. The salient features of the second critical period have been men- 

 tioned, and here the surviving amphibians recapitulate in ontogeny 

 many instructive features of the ancestral history. 



3. Amphibians have no cerebral cortex, that is, superficial lami- 

 nated gray matter, in the cerebral hemispheres. This first takes 

 definitive form in the reptiles, though prodromal stages of this dif- 

 ferentiation can be seen in fishes and amphibians, a theme to which 

 we shall return in chapter vii. 



4. The fourth critical period, like the first, does not lie within the 

 scope of this work, though study of the second and third periods 

 brings to light some principles of morphogenesis which may help us 

 to understand the more recondite problems involved in human corti- 

 cal functions. 



It is probable that none of the existing Amphibia are primitive in 

 the sense of survival of the original transitional forms and that the 

 urodeles are not only aberrant but in some cases retrograde (Noble, 

 '31; Evans, '44); yet the organization of their nervous systems is 

 generalized along very primitive lines, and these brains seem to me 

 to be more instructive as types ancestral to mammals than any others 

 that might be chosen. They lack the highly divergent specializations 

 seen in most of the fishes; and, in both external form and internal 

 architecture, comparison with the mammalian pattern can be made 

 with more ease and security. So far as structural differentiation has 



