66 THE ORIGIN OF VERTEBRATES 
available for thorough investigation and comparison. There are no trilobites 
still alive, but in Branchipus and Apus we possess the nearest approach to the 
trilobite organization among living crustaceans. 
So strongly do all these different lines of argument point to the origin of 
vertebrates from arthropods as to make it imperative to reconsider the position 
of that school of anatomists who derived vertebrates from annelids by reversing 
the back and front of the animal. Let us not turn the animal over, but 
re-consider the position, the infundibular tube of the vertebrate still representing 
the esophagus of the invertebrate, the cerebral hemispheres and basal ganglia 
the supra-cesophageal ganglia, the crura cerebri the cesophageal commissures, 
and the infra-infundibular part of the brain the infra-cesophageal ganglia. It 
is immediately apparent that just as the invertebrate esophagus leads into the 
large cephalic stomach, so the infundibular tube leads into the large cavity of 
the brain known as the third ventricle, which, together with the other ventricles, 
forms in the embryo a large anterior dilated part of the neural tube. In the 
arthropod this cephalic stomach leads into the straight narrow intestine; in the 
vertebrate the fourth ventricle leads into the straight narrow canal of the spinal 
cord. In the arthropod the intestine terminates in the anus; in the vertebrate 
embryo the canal of the spinal cord terminates in the anus by way of the 
neurenteric canal. Keep the animal unreversed, and immediately the whole 
mystery of the tubular nature of the central nervous system is revealed, for it 
is seen that the nervous matter, which corresponds bit by bit with that of the 
arthropod, has surrounded to a greater or less extent and amalgamated with 
the tube of the arthropod alimentary canal, and thus formed the so-called 
central nervous system of the vertebrate. 
The manner in which the nervous material has invaded the walls of the tube 
is clearly shown both by the study of the comparative anatomy of the central 
nervous system in the vertebrate and also by its development in the embryo. 
This theory implies that the vertebrate alimentary canal is a new formation 
necessitated by the urgency of the case, and, indeed, there was cause for urgency, 
for the general plan of the evolution of the invertebrate from the ccelenterate 
involved the piercing of the anterior portion of the central nervous system by the 
cesophagus, while, at the same time, upward progress meant brain-development ; 
brain-development meant concentration of nervous matter at the anterior end 
of the animal, with the result that in the highest scorpion and spider-like 
animals the brain-mass has so grown round and compressed the food-tube that 
nothing but fiuid pabulum can pass through into the stomach; the whole group 
have become blood-suckers. These kinds of animals—the sea-scorpions—were 
the dominant race when the vertebrates first appeared: here in the natural com- 
petition among members of the dominant race the difficulty must have become 
acute. Further upward evolution demanded a larger and larger brain with the 
ensuing consequence of a greater and greater difficulty of food-supply. Nature’s 
mistake was rectified and further evolution secured, not by degeneration in the 
brain-region, for that means degradation not upward progress, but by the 
formation of a new food-channel, in consequence of which the brain was free 
to develop to its fullest extent. Thus the great and mighty kingdom of the 
Vertebrata was evolved with its culminating organism—man—whose massive 
brain with all its possibilities could never have been evolved if he had still been 
