i8o Journal of Comparative Neurology and Psychology. 



representative of one of the primary columns there is presump- 

 tive evidence that in the primitive vertebrate the correspond- 

 ing peripheral structures were present in the same segment. 



Third, in cases of extreme modification of other systems 

 of organs and of the peripheral nervous system the brain re- 

 tains structural features which serve to point the way to the in- 

 terpretation of new structures. The changes of structure and 

 function, so far as yet observed, are less fundamental than the 

 primitive functional divisions of the nervous system. "New 

 structures" arise as modifications within one division, not by 

 structures belonging to one functional division taking on func- 

 tions belonging to another division. It does not happen, for 

 example, that cutaneous structures (sense organs, components, 

 centers, or fiber tracts) are so modified as to serve visceral func- 

 tions, or vice versa. This conservatism of the brain aids in 

 the interpretation of structures which otherwise would be very 

 difficult to understand. By this method of reasoning the evi- 

 dence that the ear is a part of the lateral line system has been 

 completed, and it has been shown that both ear and lateral line 

 system have arisen by modification within the primitive general 

 cutaneous division of the nervous system. The principle will 

 be applied in this paper for the interpretation of the eye and 

 other organs. It is perhaps needless to say that this principle 

 has been only slightly tested and it is not yet known whether 

 it is universally true. While it promises to be of great value, 

 it must be used with extreme caution. 



c. Functional divisions of the nervous system. 



Such considerations as the above led the writer some time 

 since to offer a scheme of the primitive functional divisions of 

 the nervous system. This theory is not only in accord with the 

 theory of nerve components but is an extension of that theory 

 so as to give due recognition to the central organ. It is un- 

 necessary here to enter into explanations or arguments in sup- 

 port of this theory of functional divisions beyond the matter 

 contained in previous papers (69, 70). Here a concise outline 

 of the functional divisions is given, with their subdivisions and 



