Part 5 (1/2)
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 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 ”b.a.l.l.s” 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 ma.s.s of the coelenterates 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.
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.
CHAPTER IV
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 muscle 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 haemal arches. These unite with the ma.s.ses of cartilage surrounding the notochord to form cartilaginous vertebrae, which may be stiffened by an infiltration of carbonate of lime. The vertebral column of sharks has reached this stage. Then the cartilaginous vertebrae 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 conception of the number of experiments which were tried in different groups before the definite mode of forming a bony vertebra was attained. At the same time the skull was developing in a somewhat similar manner. But the skull is far more complex in origin and undergoes far more numerous and important changes than the simpler vertebral column.
Into its history we have no time to enter.
And what shall we say of bone itself as a mere material or tissue, with its admirable lightness, compactness, and flawlessness. And every bone in our body is a triumph of engineering architecture. No engineer could better recognize the direction of strain and stress, and arrange his rods and columns, arches and b.u.t.tresses, to suitably meet them, than these problems are solved in the long bone of our thigh. And they must be lengthened while the child is leaping upon them. An engineer is justly proud if he can rebuild or lengthen a bridge without delaying the pa.s.sage of a single train. But what would he say if you asked him to rebuild a locomotive, while it was running even twenty miles an hour? And yet a similar problem had to be solved in our bodies.
But the vertebral column is not perfected by fish. The vertebrae with few exceptions are hollow in front and behind, biconcave; and between each two vertebrae there is a large cavity still occupied by the notochord. Thus these vertebrae join one another by their edges, like two shallow wine-gla.s.ses placed rim to rim. Only gradually is the notochord crowded out so that the vertebrae join by their whole adjacent surfaces. Even in highest forms, for the sake of mobility, they are united by washer-like disks of cartilage. Biconcave vertebrae persisted through the oldest amphibia, reptiles, and birds. But finally a firm backbone and skull were attained.
2. The appendages. Of these we can say but little. The fish has oar-like fins, attached to the body by a joint, but themselves unjointed. By the amphibia legs, with the same regions as our own and with five toes, have already appeared. The development of the leg out of the fin is one of the most difficult and least understood problems of vertebrate comparative anatomy. The legs are at first weak and scarcely capable of supporting the body. Only gradually do they strengthen into the fore- and hind-legs of mammals, or into the legs and wings of birds and old flying reptiles.
3. Changes in the circulatory and respiratory systems. The fish lives altogether in the water and breathes by gills, but the dipnoi among fishes breathes by lungs as well as gills. As long as respiration takes place by gills alone, the circulation is simple; the blood flows from the heart to the gills, and thence directly all over the body; the oxygenated blood from the gills does not return directly to the heart. But the blood from the lungs does return to the heart; and there at first mixes in the ventricle with the impure blood which has returned from the rest of the body. Gradually a part.i.tion arises in the ventricle, dividing it into a right and left half. Thus the two circulations of the venous blood to the lungs, and of the oxygenated blood over the body, are more and more separated until, in higher reptiles, they become entirely distinct.
As the animal came on land and breathed the air, more completely oxygenated blood was carried to the organs, and their activity was greatly heightened. As more and more heat was produced by the combustion in muscular and nervous tissues, and less was lost by conduction, the temperature of the body rose, and in birds and mammals becomes constant several degrees above the highest summer temperature of the surrounding air.
The changes in the brain affect mainly the large and small brain.
The cerebellum increases with the greater locomotive powers of the animal. But its development is evidently limited. The large brain, or cerebrum, is in fish hardly as heavy as the mid-brain; in amphibia the reverse is true. In higher recent reptiles the cerebrum would somewhat outweigh all the other portions of the brain put together. In mammals it extends upward and backward, has already in lower forms overspread the mid-brain, and is beginning to cover the small brain. But this was not so in the earliest mammals. Here the cerebrum was small, more like that of reptiles. But during the tertiary period the large brain began to increase with marvellous rapidity. It was very late in arriving at the period of rapid development, but it kept on after all the other organs of the body had settled down into comparative rest, perhaps retrogression.
We have given thus a rapid sketch in outline of the changes in the most characteristic systems between fish and mammals. Some of the changes which took place in mammals were along the same lines, but one at least is so new and unexpected that this highest cla.s.s demands more careful and detailed examination.
The mammal is a vertebrate. Hence all its organs are at their best.
But mammals stand, all things considered, at the head of vertebrates. The skeleton is firm and compact. The muscles are beautifully moulded and fitted to the skeleton so as to produce the greatest effect with the least ma.s.s and weight of tissue. The sense-organs are keen, and the eye and ear especially delicate, and fitted for perception at long range. Yet in all these respects they are surpa.s.sed by birds. As a mere anatomical machine the bird always seems to me superior to the mammal. It is not easy to see why it failed, as it has, to reach the goal of possibility of indefinite development and dominance in the animal world. Why he stopped short of the higher brain development I cannot tell. The fact remains that the mammal is pre-eminent in brain power, and that this gave him the supremacy.
But mammals came very late to the throne, and the probability of their ever gaining it must for ages have appeared very doubtful.