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 
