The lowest fish-like animal at present living is Amphioxus, and on this ground it is argued that the original vertebrate must have approached in organization to that of Amphioxus; it is upon the comparison between the structure of Amphioxus and that of Balanoglossus, that the theory of the origin of vertebrates from forms like the latter animal is based. For my own part, I think that in the first instance, at all events, Amphioxus should be put on one side, although of course its structure must always be kept in mind, for the following reasons:-- {34}Amphioxus, like the tunicates, does not possess the characteristics of other vertebrates. In all vertebrates above these forms the great characteristic is a well-defined brain-region from which arise nerves to organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for the pigmented spot at the anterior extremity of the brain-region is no eye but only a mass of pigment, and the so-called olfactory pit is a very rudimentary and inferior organ of smell. In connection with the nearly complete absence of these two most important sense-organs, the most important part of the central nervous system, the region corresponding to the cerebral hemispheres, is also nearly completely absent. Now, the history of the evolution of the central nervous system in the animal race points directly to its formation as a concentrated mass of nervous material at the anterior extremity of the body, in consequence of the formation of special olfactory and visual organs at that extremity. As already stated, the concentration of nervous material around the mouth as an oral ring was its beginning. In connection with this there arose special sense-organs for the guidance of the animal to its food which took the form of olfactory and optic organs. With the shifting from the radial to the elongated form these sense-organs remained at the anterior or mouth-end of the animal, and owing to their immense importance in the struggle for existence, that part of the central nervous system with which they were connected developed more than any other part, became the leader to which the rest of the nervous system was subservient, and from that time onwards the development of the brain-region was inevitably associated with the upward progress of animal life.