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.
To those who believe in Evolution and the Darwinian theory of the survival of the fittest, it is simply inconceivable that a soft-bodied animal living in the mud, blind, with a rudimentary brain and rudimentary olfactory organs, such as is postulated when we think of Balanoglossus and Amphioxus, should hold its own and come victorious out of the struggle for existence at a time when the sea was peopled with powerful predaceous scorpion- and crab-like armour-plated animals possessing a well-developed brain, good eyes and olfactory organs, and powerful means of locomotion. Wherever in the scale of animal development Amphioxus may ultimately be placed, it cannot be looked upon as the type of the earliest formed fishes such as appeared in Silurian times.
{ 35}The next lowest group of living fishes is the Marsipobranchii which include the lampreys and hag-fishes. To these naturally we must turn for a clue as to the organization of the earliest fish, for here we find all the characteristics of the vertebrates represented: a well-formed brain-region, well-developed eyes and nose, cranial nerves directly comparable with those of other vertebrates, and even the commencement of vertebræ.