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 coelenterate 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 competition 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 {67}compelled to pass the whole of his food through the narrow oesophageal tube, still existent in him as the infundibular tube. This, then, is the working hypothesis upon which this book is written. If this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it follows that we have various fixed points of comparison in the central nervous systems of the two groups of animals from which to search for further clues. It further follows that from such starting-point every organ of importance in the body of the arthropod ought to be visible in the corresponding position in the vertebrate, either as a functional or rudimentary organ. The subsequent chapters will deal with this detailed comparison of organs in the arthropod and vertebrate respectively. {68}CHAPTER II _THE EVIDENCE OF THE ORGANS OF VISION_ Different kinds of eye.--Simple and compound retinas.--Upright and inverted retinas.--Median eyes.--Median or pineal eyes of Ammocoetes and their optic ganglia.--Comparison with other median eyes.--Lateral eyes of vertebrates compared with lateral eyes of crustaceans.--Peculiarities of the lateral eye of the lamprey.--Meaning of the optic diverticula.--Evolution of vertebrate eyes.--Summary.