« IndietroContinua »
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 zoologists 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 im
possible 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, thẹ 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 ctenophora, 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 Iwould 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 lect
The period of development of mollusks, articulates, and vertebrates, is really one. They developed to a certain extent contemporaneously. The development of vertebrates was slow, and they were the last to appear on the stage of geological history.
You must all have noticed that development, during this period, takes on a much more hopeful form than during that described in the last chapter. Then digestion and reproduction were dominant. Now muscle is of the greatest importance. If this fails of development, as in mollusks, the group is doomed to degeneration or at best stagnation. But we have seen. the dawn of a still higher function. In insects and vertebrates the brain is becoming of importance, and absorbing more and more material. This is the promise of something vastly higher and better. Better sense-organs are appearing, fitted to aid in a wider perception of more distant objects. The vertebrate has discovered the right path; though a long journey still lies before it. The night is far spent, the day is at hand.
VERTEBRATES: BACKBONE AND BRAIN
IN tracing man's ancestry from fish upward we ought properly to describe three or four fish, an amphibian, a reptile, and then take up the series of mammalian ancestors. But we have not sufficient time for so extended a study, and a simpler method may answer our purpose fairly well. Let us fix our attention on the few organs which still show the capacity of marked development, and follow each one of these rapidly in its upward course.
We must remember that there are changes in the vegetative organs. The digestive and excretory systems improve. But this improvement is not for the sake of these vegetative functions. Brain and musele demand vastly more fuel, and produce vastly more waste which must be removed. At almost the close of the series the reproductive system undergoes a modification which is almost revolutionary in its results. But we shall find that this modification is necessitated by the smaller amount of material which can be spared for this function; not by its increasing importance, still less its dominance for its own worth. The vertebrate is like an old Roman; everything is subordinated to mental and physical power. He is the world conqueror.
The important changes from fish upward affect the following organs: 1. The skeleton. A light, solid framework must be developed for the body. 2. The appendages start as fins, and end as the legs and arms of man. 3. The circulatory and respiratory systems developed so as to carry with the utmost rapidity and certainty fuel and oxygen to the muscular and nervous high-pressure engines. Or, to change the figure, they are the roads along which supplies and munitions can be carried to the army suddenly mobilized at any point on the frontier. 4. Above all, the brain, especially the cerebrum, the crown and goal of vertebrate structure. The improvement is now practically altogether in the animal organs of locomotion and thought. Still, among these animal organs, the lower systems will lead in point of time. The brain must to a certain extent wait for the skeleton.
1. The skeleton. The axial skeleton consists, in the lowest fish, of the notochord, a cylindrical unsegmented rod of cartilage running nearly the length of the body. This is surrounded by a sheath of connective tissue, at first merely membranous, later becoming cartilaginous or gristly. Pieces of cartilage extend upward over the spinal marrow, and downward around the great aortic artery, forming the neural and hæmal arches. These unite with the masses of cartilage surrounding the notochord to form cartilaginous vertebræ, which may be stiffened by an infiltration of carbonate of lime. The vertebral column of sharks has reached this stage. Then the cartilaginous vertebræ ossify and form a true backbone. I have described the process as if it were very simple. But only the student of comparative osteology can have any con