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 oesophagus of the invertebrate, the cerebral hemispheres and basal ganglia 

 the supra-oesophageal ganglia, the crura cerebri the oesophageal commissures, 

 and the infra-infundibular part of the brain the infra-oesophageal ganglia. It 

 is immediately apparent that just as the invertebrate oesophagus 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 

 oesophagus, 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 fluid 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 



