Every trait of vertebrates is a promise of a great future. Its internal skeleton gives it the possibility of large size. This gave it in time the victory in the struggle with its competitors, as to whether it should eat or be eaten. It is vigorous and powerful, for all its organs are at the best. It gives the possibility of later, on land, becoming warm-blooded, i.e., of maintaining a constant high temperature. It is thus resistant to climate and hardship. In time its descendants will face the arctic winter as well as the heat of the tropics.
But it has started on the road which leads to mind. The greater size is correlated with longer life. The lessons of experience come to it over and over again, and it can and must learn them. It is the intelligent, remembering, thinking type. The insect had begun to peer into the world of invisible and intangible relations, the vertebrate will some day see them. This much is prophecied in his very structure. He must be heir to an indefinite future.
You have probably noticed that the vertebrate differs greatly from all his predecessors. The gulf between him and them is indeed wide and deep. His origin and ancestry are yet far from certain. But an attempt to decipher his past history, though it may lead to no sure conclusions, will yet be of use to us. Practically all aquatic vertebrates lead a swimming life, neither sessile nor creeping. The embryonic development of our appendages leads to the same conclusion. We must never forget that the embryonic development of the individual recapitulates briefly the history of the development of the race. Now the legs and arms, or fore- and hind-legs, of higher vertebrates and the corresponding paired fins of fish develop in the embryo as portions of a long ridge extending from front to rear of the side of the body.
This justifies the inference that the primitive vertebrate ancestor had a pair of long fins running along the sides of the body, but bending slightly downward toward the rear so as to meet one another and continue as a single caudal fin behind the anal opening. Such fins, like the feathers of an arrow, could be useful only to keep the animal "on an even keel" as it was forced through the water by the lateral sweeps of the tail. They would have been useless for creeping.
But there is another piece of evidence that he was a free swimming form. All vertebrates breathe by gills or lungs, and these are modified portions of the digestive system, of the walls of the œsophagus, from which even the lung is an embryonic outgrowth. Now practically all invertebrates breathe through modified portions of the integument or outer surface of the body, and their gills are merely expansions of this. In the annelid they are projections of the parapodia, in the mollusk expansions of the skin, where the foot or creeping sole joins the body. Why did the vertebrate take a new and strange, and, at first sight, disadvantageous mode of breathing? There must have been some good reason for this. The most natural explanation would seem to be that he had no projections on his outer surface which could develop into gills, and farther, that he could not afford to have any. Now projections on the lower portion of the sides of the body would be an advantage in creeping, but a hindrance in any such mode of swimming as we have described, or indeed in any mode of writhing through the water.
Furthermore, if he lived, not a creeping life on the bottom, but swimming in the water above, he would have to live almost entirely on microscopic animals and embryos; and these would be most easily captured by a current of water brought in at the mouth. The whole branchial apparatus in its simplest forms would seem to be an apparatus for sifting out the microscopic particles of food and only later a purely respiratory apparatus. Moreover, we have seen that the parapodia of annelids naturally point to the development of an external skeleton, for their muscles are already a part of the external body-wall and attached to the already existing horny cuticle. The logical goal of their development was the insect.
Now I do not wish to conceal from you that many good zoölogists believe that the vertebrate is descended from annelids; but for this and other reasons such a descent appears to me very improbable. It would seem far more natural to derive the vertebrate from some free swimming form like the schematic worm, whose largest nerve-cord lay on the dorsal surface because its branches ran to heavy muscles much used in swimming. Later the other nerve-cords degenerated, for such a degeneration of nerve-cords is not at all impossible or improbable. "No thoroughfare" is often written across paths previously followed by blood or nervous impulses, when other paths have been found more economical or effective.
But where did the notochord come from? I do not know. It always forms in the embryo out of the entoderm or layer which becomes the lining of the intestine. Now this is a very peculiar origin for cartilage, and the notochord is a very strange cartilage even if we have not made a mistake in calling it cartilage at all. My best guess would be that it is simply a thickened portion of the upper median surface of the intestine to keep the "balls" of digesting nutriment or other hard particles in the intestine from "grinding" against the nerve-cord as they are crowded along in the process of digestion. Once started its elasticity would be a great aid in swimming.
Professor Brooks has called attention to the fact that the higher a group stands in development, the longer its ancestors have maintained a swimming life. Thus we have noticed that the sponges were the first to settle; then a little later the mass of the cœlenterates followed their example. But the etenophora, the nearest relatives of bilateral animals, have remained free swimming. Then the flat worms and mollusks took to a creeping mode of life, while the annelids and vertebrates still swam. Then the annelids settled to the bottom and crept, and all their descendants remained creeping forms. The vertebrates alone remained swimming, and probably neither they nor their descendants ever crept until they emerged on the land, or as amphibia were preparing for land life. If this be true, it is a fact worthy of our most careful consideration. The swimming life would appear to be neither as easy nor as economical as the creeping. It is certainly hard to believe that food would not have been obtained with less effort and in greater abundance at the bottom than in the water above. The swimming life gave rise to higher and stronger forms; but did its maintenance give immediate advantage in the struggle for existence? This is an exceedingly interesting and important question, and demands most careful consideration. But we shall be better prepared to answer it in a future lecture.