The Whence and the Whither of Man Part 2
From egg to sponge is, comparatively speaking, but a step; it is a long march from the egg to the earthworm; and the vertebrate embryo makes a vast journey. But embryonic life is and must remain short.
Hence in higher forms the ancestral stages will often be slurred over and very incompletely represented. And the embryo may, and often does, shorten the path by "short-cuts" impossible to its original ancestor. Still it will in general hold true, and may be recognized as a law of vast importance, that any individual during his embryonic life repeats very briefly the different stages through which his ancestors have pa.s.sed in their development since the beginning of life. Or, briefly stated, ontogenesis, or the embryonic development of the individual, is a brief recapitulation of phylogenesis, or the ancestral development of the phylum or group.
The ill.u.s.tration and proof of this law is the work of the embryologist. We have time to draw only one or two ill.u.s.trations from the embryonic development of birds. We have already seen that the embryonic bird has the long tail of his reptilian ancestor. In early embryonic life it has gill-slits leading from the pharynx to the outside of the neck like those through which the water pa.s.ses in the respiration of fish. The Eustachian tube and the ca.n.a.l of the external ear of man, separated only by the "drum," are nothing but such an old persistent gill-slit. No gills ever develop in these, but the great arteries run to them, and indeed to all parts of the embryo, on almost precisely the same general plan as in the adult fish. Only later is the definite avian circulation gradually acquired.
This law is even more strikingly ill.u.s.trated in the embryonic development of the vertebral column and skull, if we had time to trace their development. And the development of the excretory system points to an ancestor far more primitive than even the fish. Our embryonic development is one of the very strongest evidences of our lowly origin.
Thus we have three sources of information for the study of animal genealogy. First, the comparative anatomy of all the different groups of animals; second, their comparative embryology; and third, their palaeontological history. Each source has its difficulties or defects. But taken all together they give us a genealogical tree which is in the main points correct, though here and there very defective and doubtful in detail. The points in which we are left most in doubt in regard to each ancestor are its modes of life and locomotion, and body form. But these may temporarily vary considerably without affecting to any great extent the general plan of structure and the line of development of the most important deep-seated organs.
I have chosen a line composed of forms taken from the comparative anatomical series. All such present existing forms have probably been modified during the lapse of ages. But I shall try to tell you when they have diverged noticeably from the structure of the primitive ancestor of the corresponding stage. It is much safer for us to study concrete, actual forms than imaginary ones, however real may have been the former existence of the latter. And, after all, their lateral divergence is of small account compared with the great upward and onward march of life, to the right and left of which they have remained stationary or retrograded somewhat, like the tribes which remained on the other side of Jordan and never entered the Promised Land.
To recapitulate: Our question is the Whence and the Whither of man.
To this question the Bible gives a clear and definite answer. Can Science also give an answer, and is this in the main in accord with the answer of Scripture? Science can answer the question only by the historical method of tracing the history of life in the past and observing the goal toward which it tends. If the evolution theory be true, the record of human achievement and progress forms only one short chapter in the history of the ages. If from the records of man's little span of life on the globe we can deduce laws of history on whose truth we can rely, with how much greater confidence and certainty may we rely on laws which have governed all life since its earliest appearance?--always provided that such can be found.
Our first effort must therefore be to trace the great line of development through a few of its most characteristic stages from the simplest living beings up to man. This will be our work in the three succeeding lectures. And to these I must ask you to bring a large store of patience. Anatomical details are at best dry and uninteresting. But these dry facts of anatomy form the foundation on which all our arguments and hopes must rest.
But if you will think long and carefully even of anatomical facts, you will see in and behind them something more and grander than they. You will catch glimpses of the divinity of Nature. Most of us travel threescore years and ten stone-blind in a world of marvellous beauty. Why does the artist see so much more in every fence-corner and on every hill-side than we, set face to face with the grandest landscapes? Primarily, I believe, because he is sympathetic, and looks on Nature as a comrade as near and dear as any human sister and companion. As Professor Huxley has said, "they get on rarely together." She speaks to the artist; to us she is dumb, and ought to be, for we are boorishly careless of her and her teachings.
Nature, to be known, must be loved. And though you have all the knowledge of a von Humboldt, and do not love her, you will never understand her or her teachings. You will go through life with her, and yet parted from her as by an adamantine wall.
I do not suppose that the author of the book of Job had ever studied geology, or mineralogy, or biology, but read him, and see whether this old prince of scientific heroes had loved, and understood, and caught the spirit of Nature. And what a grand, free spirit it was, and what a giant it made of him. I do not believe that Paul ever had a special course of anatomy or botany. But if he had not pondered long and lovingly on the structure of his body, and the germination of the seed, he never could have written the twelfth and fifteenth chapters of the first letter to the Corinthians. And time fails to speak of David and all the writers of the Psalms, and of those heroic souls misnamed the "Minor" Prophets.
Study the teachings of our Lord. How he must have considered the lilies of the field, and that such a tiny seed as that of the mustard could have produced so great an herb, and noticed and thought on the thorns and the tares and the wheat, and watched the sparrows, and pondered and wondered how the birds were fed. All his teaching was drawn from Nature. And all the study in the world could never have taught him what he knew, if it had not been a loving and appreciative study.
There is one strange and interesting pa.s.sage in John's Gospel, xv.
1: "I am the true vine." My father used to tell us that the Greek word [Greek: alethine], rendered true, is usually employed of the genuine in distinction from the counterfeit, the reality in distinction from the shadow and image. Is not this perhaps the clew to our Lord's use of natural imagery? Nature was always the presentation to his senses of the divine thought and purpose. He studied the words of the ancient Scripture, he found the same words and teachings clearly and concretely embodied in the processes of Nature. The interpretation of the Parable of the Sower was no mere play of fancy to him; it was the genuine and fundamental truth, deeper and more real than the existence of the sower, the soil, and the seed. The spiritual truth was the substance; the tangible soil and seed really only the shadow. And thus all Nature was to him divine.
We all of us need to offer the prayer of the blind man, "Lord, that our eyes may be opened." Let us learn, too, from the old heathen giant, Antaeus, who, after every defeat and fall, rose strengthened and vivified from contact with his mother Earth. You will experience in life many a desperate struggle, many a hard fall. There is at such times nothing in the world so strengthening, healing, and life-giving as the thoughts and encouragements which Nature pours into the hearts and minds of her loving disciples. She will set you on your feet again, infused with new life, filled with an unconquerable spirit, with unfaltering courage, and an iron will to fight once more and win. In every battle her inspiring words will ring in your ears, and she will never fail you. We may not see her deepest realities, her rarest treasures of thought and wisdom; but if we will listen lovingly for her voice, we may be a.s.sured that she will speak to us many a word of cheer and encouragement, of warning and exhortation. For, to paraphrase the language of the nineteenth Psalm, "She has no speech nor language, her voice is not heard. But her rule is gone out throughout all the earth, and her words to the end of the world."
CHAPTER II
PROTOZOA TO WORMS: CELLS, TISSUES, AND ORGANS
The first and lowest form in our ancestral series is the amoeba, a little fresh-water animal from 1/500 to 1/1000 of an inch in diameter. Under the microscope it looks like a little drop of mucilage. This semifluid, mucilaginous substance is the Protoplasm.
Its outer portion is clear and transparent, its inner more granular.
In the inner portion is a little spheroidal body, the nucleus. This is certainly of great importance in the life of the animal; but just what it does, or what is its relation to the surrounding protoplasm we do not yet know. There is also a little cavity around which the protoplasm has drawn back, and on which it will soon close in again, so that it pulsates like a heart. It is continually taking in water from the body, or the outside, and driving it out again, and thus aids in respiration and excretion. The animal has no organs in the proper sense of the word, and yet it has the rudiments of all the functions which we possess.
A little projection of the outer, clearer layer of protoplasm, a pseudopodium, appears; into this the whole animal may flow and thus advance a step, or the projection may be withdrawn. And this power of change of form is a lower grade of the contractility of our muscular cells. p.r.i.c.k it with a needle and it contracts. It recognizes its food even at a microscopic distance; it appears therefore to feel and perceive. Perhaps we might say that it has a mind and will of its own. It is safer to say that it is irritable, that is, it reacts to stimuli too feeble to be regarded as the cause of its reaction. It engulfs microscopic plants, and digests them in the internal protoplasm by the aid of an acid secretion. It breathes oxygen, and excretes carbonic acid and urea, through its whole body surface. Its mode of gaining the energy which it manifests is therefore apparently like our own, by combustion of food material.
[Ill.u.s.tration: 1. AMOEBA PROTEUS. HERTWIG, FROM LEIDY.
_ek_, ectosarc; _en_, endosarc; _N_, food particles; _n_, nucleus; _cv_, contractile vesicle.]
It grows and reaches a certain size, then constricts itself in the middle and divides into two. The old amoeba has divided into two young ones, and there is no parent left to die, and death, except by violence, does not occur. But this absence of death in other rather distant relatives of the amoeba, and probably in the amoeba itself, holds true only provided that, after a series of self-divisions, reproduction takes place after another mode. Two rather small and weak individuals fuse together in one animal of renewed vigor, which soon divides into two larger and stronger descendants. We have here evidently a process corresponding to the fertilization of the egg in higher animals; yet there is no egg, spermatozoon, or s.e.x.
It is a little ma.s.s of protoplasm containing a nucleus, and corresponds, therefore, to one of the cells, most closely to the egg-cell or spermatozoon of higher animals. If every living being is descended from a single cell, the fertilized egg, it is not hard to believe that all higher animals are descended from an ancestor having the general structure or lack of structure of the amoeba.
But is the amoeba really structureless? Probably it has an exceedingly complex structure, but our microscopes and technique are still too imperfect to show more than traces of it. Says Hertwig: "Protoplasm is not a single chemical substance, however complicated, but a mixture of many substances, which we must picture to ourselves as finest particles united in a wonderfully complicated structure."
Truly protoplasm is, to borrow Mephistopheles' expression concerning blood, a "quite peculiar juice." And the complexity of the nucleus is far more evident than that of the protoplasm. Is protoplasm itself the result of a long development? If so, out of what and how did it develop? We cannot even guess. But the beginning of life may, apparently must, have been indefinitely farther back than the simplest now existing form. The study of the amoeba cannot fail to raise a host of questions in the mind of any thoughtful man.
As we have here the animal reduced, so to speak, to lowest terms, it may be well to examine a little more closely into its physiology and compare it briefly with our own.
The amoeba eats food as we do, but the food is digested directly in the internal protoplasm instead of in a stomach; and once digested it diffuses to all parts of the cell; here it is built up into compounds of a more complex structure, and forms an integral part of the animal body. The dead food particle has been transformed into living protoplasm, the continually repeated miracle of life.
But it does not remain long in this condition. In contact with the oxygen from the air it is soon oxidized, burned up to furnish the energy necessary for the motion and irritability of the body. We are all of us low-temperature engines. The digestive function exists in all animals merely to bring the food into a soluble, diffusible form, so that it can pa.s.s to all parts of the body and be used for fuel or growth. In our body a circulatory system is necessary to carry food and oxygen to the cells and to remove their waste. For most of our cells lie at a distance from the stomach, lungs, and kidney. But in a small animal the circulatory system is often unnecessary and fails. Breathing and excretion take place through the whole surface of the body. The body of the frog is devoid of scales, so that the blood is separated from the surrounding water only by a thin membrane, and it breathes and excretes to a certain extent in the same way.
But another factor has to be considered. If we double each dimension of our amoeba, we shall increase its surface four times, its ma.s.s eight-fold. Now the power of absorbing oxygen and excreting waste is evidently proportional to the excretory and respiratory surface, and much the same is true of digestion. But the amount of oxygen required, and of waste to be removed is proportional to the ma.s.s; for every particle of protoplasm requires food and oxygen, and produces waste. The particles of protoplasm in our new, larger amoeba can therefore receive only half as much oxygen as before, and rid themselves of their waste only half as fast. There is danger of what in our bodies would be called suffocation and blood-poisoning. The amoeba having attained a certain size meets this emergency by dividing into two small individuals, the division is a physical adaptation. But the many-celled animal cannot do this; it must keep its cells together. It gains the additional surface by folding and plaiting. And the complicated internal structure of higher animals is in its last a.n.a.lysis such a folding and plaiting in order to maintain the proper ratio between the exposed surface of the cells and their ma.s.s. And each cell in our bodies lives in one sense its own individual life, only bathed in the lymph and receiving from it its food and oxygen instead of taking it from the water.
But in another sense the cells of our body live an entirely different life, for they form a community. Division of labor has taken place between them, they are interdependent, correlated with one another, subject therefore to the laws of the whole community or organism. There are many respects in which it is impossible to compare Robinson Crusoe with a workman in a huge watch factory; yet they are both men.
Both the amoeba and we live in the closest relation to our environment, and conformity to it is evidently necessary: life has been defined as the adjustment of internal relations to external conditions. We continually take food, use it for energy and growth, and return the simpler waste compounds. We are all of us, as Professor Huxley has said, "whirlpools on the surface of Nature;"
when the whirl of exchange of particles ceases we die. We have seen that the fusion of two amoebae results in a new rejuvenated individual. Why is a mixture of two protoplasms better than one? We can frame hypotheses; we know nothing about it. What of the mind of the amoeba? A host of questions throng upon us and we can answer no one of them. All the great questions concerning life confront us here in the lowest term of the animal series, and appear as insoluble as in the highest.
Our second ancestral form is also a fresh-water animal, the hydra.
This is a little, vase-shaped animal, which usually lives attached to gra.s.s-stems or sticks, but has the power to free itself and hang on the surface of the water or to slowly creep on the bottom. The mouth is at the top of the vase, and the simple, undivided cavity within the vase is the digestive cavity. Around the mouth is a ring of from four to ten hollow tentacles, whose cavities communicate freely underneath with the digestive cavity. Not only is food taken in at the mouth, but indigestible material is thrown out here. The animal may thus be compared to a nearly cylindrical sack with a circle of tubes attached to it above. The body consists of two layers of cells, the ectoderm on the outside and the entoderm lining the digestive cavity. Between these two is a structureless, elastic membrane, which tends to keep the body moderately expanded.
The food is captured by the tentacles; but digestion takes place only partially in the digestive cavity, for each surrounding cell engulfs small particles of food and digests them within itself. The entodermal cells behave in this respect much like a colony of amoebae. The cells of both layers have at their bases long muscular fibrils, those of the ectodermal cells running longitudinally, those of the entoderm transversely. The animal can thus contract its body in both directions, or, if the body contain water and the transverse muscles are contracted, the pressure of the water lengthens the body and tends to extend the tentacles.
On the outside of the elastic membrane, just beneath the ectoderm, is a plexus or cobweb of nervous cells and fibrils. As in every nervous system, three elements are here to be found. 1. An afferent or sensory nerve-fibril, which under adequate stimulus is set in vibration by some cell of the epidermis or ectoderm, which is therefore called a sensory cell. 2. A central or ganglion cell, which receives the sensory impulse, translates it into consciousness, and is the seat of whatever powers of perception, thought, or will the animal possesses. This also gives rise to the efferent or motor impulses, which are conveyed by (3) a motor fibril to the corresponding muscle, exciting its contraction. But there are also nerve-fibrils connecting the different ganglion cells, so that they may act in unison. In the higher animals we shall find these central or ganglion cells condensed in one or a few ma.s.ses or ganglia. But here they are scattered over the whole surface of the elastic supporting membrane.
The reproductive organs for the production of eggs and spermatozoa form little protuberances on the outside of the body below the tentacles. But hydra reproduces mostly by budding; new individuals growing out of the side of the old one, like branches from the trunk of a tree, but afterward breaking free and leading an independent life. There are special forms of cells besides those described; nettle cells for capturing food, interst.i.tial cells, etc., but these do not concern us.
The distance from the single-celled amoeba to hydra is vast, probably really greater than that between any other successive terms of our series. It may therefore be useful to consider one or two intermediate forms and the parallel embryonic stages of higher animals, and to see how the higher many-celled animal originates from the unicellular stage.
The amoeba is an ill.u.s.tration of a great kingdom of similar, practically unicellular forms, which have played no unimportant part in the geological history of the globe. These are the protozoa. They include, first of all, the foraminifera, which usually have sh.e.l.ls composed of carbonate of lime. These sh.e.l.ls, settling to the bottom of the ocean, have acc.u.mulated in vast beds, and when compacted and raised above the surface, form chalk, limestone, or marble, according to the degree and mode of their hardening.
The protozoa include also the flagellata, a great, very poorly defined ma.s.s of forms occupying the boundary between the plant and animal kingdoms. They are usually unicellular, and their protoplasm is surrounded by a thin, structureless membrane. This prevents their putting out pseudopodia as organs of motion. Instead of these they have at one end of the ovoid or pear-shaped body a long, whiplash-like process or thread, a flagellum, and by swinging this they propel themselves through the water. These flagellata seem to have a rather marked tendency to form colonies. The first individual gives rise to others by division. But the division is not complete; the new individuals remain connected by the undivided rear end of the body. And such a colony may come to contain a large number of individuals.
[Ill.u.s.tration: 2. MAGOSPHaeRA PLANULA. LANG, FROM HAECKEL.]
Such a colony is represented by magosphaera. This is a microscopic globular form, discovered by Professor Haeckel on the coast of Norway. It consists of a large number of conical or pear-shaped individual cells, whose apices are turned toward the centre of the sphere. The cells are cemented together by a mucilaginous substance.
Around their exposed larger ends, which form the surface of the sphere, are rows of flagella, by whose united action the colony rolls through the water. After a time each individual absorbs its flagella, the colony is broken up, the different individuals settle to the bottom, and each gives rise by division to a new colony. This group of cells may be considered as a colony or as an individual.
Each term is defensible.
Volvox is also a spheroidal organism, composed often of a very large number of flagellated cells. But it differs from magosphaera in certain important respects. In the first place its cells have chlorophyl, the green coloring matter of plants. It lives therefore on unorganized fluid nourishment, carbon dioxide, nitrates, etc. It is a plant. But certain characteristics render it probable that it once lived on solid food and was therefore an animal. For where almost the sole difference between plants and animals is in the fluid or solid character of their food, a change from the one form into the other is not as difficult or improbable as one might naturally think. And plants and animals are here so near together, and travelling by roads so nearly parallel, that, even if volvox never was an animal, it might still serve very well to ill.u.s.trate a stage through which animals must have pa.s.sed.
The cells of volvox do not form a solid ma.s.s, but have arranged themselves in a single layer on the outer surface of the sphere. For a time, under favorable circ.u.mstances, volvox reproduces very much like magosphaera, and each cell can give rise to a new, many-celled individual. But after a time, especially under unfavorable circ.u.mstances, a new mode of reproduction appears. Certain cells withdraw from the outer layer into the interior of the colony. Here they are nourished by the other cells and develop into true reproductive elements, eggs and spermatozoa. Fertilization, that is, the union of egg and spermatozoon, or mainly of their nuclei, takes place; and the fertilized egg develops into a new organism. But the other cells, which have been all the time nourishing these, seem now to lack nutriment, strength, or vitality to give rise to a new colony. They die.
We find thus in volvox division of labor and corresponding difference of structure or differentiation; certain cells retain the power of fusing with other corresponding cells, and thus of rejuvenescence and of giving rise to a new organism. And these cells, forming a series through all generations, are evidently immortal like the protozoa. Natural death cannot touch them. These are the reproductive cells. The other cells nourish and transport them and carry on the work of excretion and respiration. These latter correspond practically to our whole body. We call them somatic cells. In volvox they are entirely subservient to, and exist for, the reproductive cells, and die when they have completed their service of these. The body is here only a vehicle for ova.
Furthermore, in volvox there has arisen such an interdependence of cells that we can no longer speak of it as a colony. The colony has become an individual by division of labor and the resulting differentiation in structure.
But hydra gives us but a poor idea of the coelenterata, to which kingdom it belongs. The higher coelenterata have nearly or quite all the tissues of higher animals--muscular, connective, glandular, etc. And by tissues we mean groups of cells modified in form and structure for the performance of a special work or function. The protozoa developed the cell for all time to come, the coelenterata developed the tissues which still compose our bodies. But they had them mainly in a diffuse form. A sort of digestive and reproductive system they did possess. But the work of arranging these tissues and condensing them into compact organs was to be done by the next higher group, the worms.
Let us now take a glance at certain stages of embryonic development which correspond to these earliest ancestral forms. We should expect some such correspondence from the fact already stated that the embryonic development of the individual is a brief recapitulation of the ancestral development of the species or larger group. The egg of the lowest vertebrate, amphioxus, shows these changes in a simple and apparently primitive form.
[Ill.u.s.tration: 3. IMMATURE EGG-Sh.e.l.l FROM OVARY OF ECHINODERM.
HATSCHEK, FROM HERTWIG.]