42 THE ORIGIN OF VERTEBRATES 



The second explanation is hardly worth serious consideration, for 

 it supposes that the nervous system, for no possible reason, was laid 

 down in its most important parts — the brain-region — as an epithelial 

 tube with latent potential nervous functions ; that even up to the 

 highest vertebrate yet evolved these nervous functions are still in 

 abeyance over the whole of the choroid plexuses and the roof of the 

 fourth ventricle. Further, it supposes that this prophetic epithelial 

 tube originally developed into true nervous material only in certain 

 parts, and that these parts, curiously enough, formed a nervous 

 system absolutely comparable to that of the arthropod, while the 

 dormant prophetic epithelial part was formed so as just to mimic, 

 in relation to the nervous part, the alimentary canal of that same 

 arthropod. 



The mere facts of the case are sufficient to show the glaring 

 absurdity of such an explanation. This is not the way Nature works ; 

 it is not consistent with natural selection to suppose that in a low 

 form nervous material can be laid down as non-nervous epithelial 

 material in order to provide in some future ages for the great increase 

 in the nervous system. 



Every method of investigation points to the same conclusion, 

 whether the method is embryological, anatomical, or pathological. 



First, take the embryological evidence. On the ground that the 

 individual development reproduces to a certain extent the phylo- 

 genetic development, the peculiarities of the formation of the central 

 nervous system in the vertebrate embryo ought to receive an appro- 

 priate explanation in any theory of phylogenetic development. 

 Hitherto such explanation has been totally lacking ; any suggestion 

 of the manner in which a tubular nervous system may have been 

 formed takes no account whatever of the differences between different 

 parts of the tube ; its dilated cephalic end with its infundibular 

 projection ventrally, its small straight spinal part, and its termination 

 in the anus. My theory, on the other hand, is in perfect harmony 

 with the embryological history, and explains it point by point. 



From the very first origin of the central nervous system there 

 is evidence of two structures— the one nervous, and the other an 

 epithelial surface-layer which ultimately forms a tube ; this was 

 first described by Scott in Petromyzon, and later by Assheton in the 

 frog. In the latter case the external epithelial layer is pigmented, 

 while the underlying nervous layer contains no pigment ; a marked 



