The Story of Evolution/Chapter V
There is, perhaps, no other chapter in the chronicle of the earth that we approach with so lively an interest as the chapter which should record the first appearance of life. Unfortunately, as far as the authentic memorials of the past go, no other chapter is so impenetrably obscure as this. The reason is simple. It is a familiar saying that life has written its own record, the long-drawn record of its dynasties and its deaths, in the rocks. But there were millions of years during which life had not yet learned to write its record, and further millions of years the record of which has been irremediably destroyed. The first volume of the geological chronicle of the earth is the mass of the Archaean (or "primitive") rocks. What the actual magnitude of that volume, and the span of time it covers, may be, no geologist can say. The Archaean rocks still solidly underlie the lowest depth he has ever reached. It is computed, however, that these rocks, as far as they are known to us, have a total depth of nearly ten miles, and seem therefore to represent at least half the story of the earth from the time when it rounded into a globe, or cooled sufficiently to endure the presence of oceans.
Yet all that we read of the earth's story during those many millions of years could be told in a page or two. That section of geology is still in its infancy, it is true. A day may come when science will decipher a long and instructive narrative in the masses of quartz and gneiss, and the layers of various kinds, which it calls the Archaean rocks. But we may say with confidence that it will not discover in them more than a few stray syllables of the earlier part, and none whatever of the earliest part, of the epic of living nature. A few fossilised remains of somewhat advanced organisms, such as shell-fish and worms, are found in the higher and later rocks of the series, and more of the same comparatively high types will probably appear. In the earlier strata, representing an earlier stage of life, we find only thick seams of black shale, limestone, and ironstone, in which we seem to see the ashes of primitive organisms, cremated in the appalling fires of the volcanic age, or crushed out of recognition by the superimposed masses. Even if some wizardry of science were ever to restore the forms that have been reduced to ashes in this Archaean crematorium, it would be found that they are more or less advanced forms, far above the original level of life. No trace will ever be found in the rocks of the first few million years in the calendar of life.
The word impossible or unknowable is not lightly uttered in science to-day, but there is a very plain reason for admitting it here. The earliest living things were at least as primitive of nature as the lowest animals and plants we know to-day, and these, up to a fair level of organisation, are so soft of texture that, when they die, they leave no remains which may one day be turned into fossils. Some of them, indeed, form tiny shells of flint or lime, or, like the corals, make for themselves a solid bed; but this is a relatively late and higher stage of development. Many thousands of species of animals and plants lie below that level. We are therefore forced to conclude, from the aspect of living nature to-day, that for ages the early organisms had no hard and preservable parts. In thus declaring the impotence of geology, however, we are at the same time introducing another science, biology, which can throw appreciable light on the evolution of life. Let us first see what geology tells us about the infancy of the earth.
The distribution of the early rocks suggests that there was comparatively little dry land showing above the surface of the Archaean ocean. Our knowledge of these rocks is not at all complete, and we must remember that some of this primitive land may be now under the sea or buried in unsuspected regions. It is significant, however, that, up to the present, exploration seems to show that in those remote ages only about one-fifth of our actual land-surface stood above the level of the waters. Apart from a patch of some 20,000 square miles of what is now Australia, and smaller patches in Tasmania, New Zealand, and India, nearly the whole of this land was in the far North. A considerable area of eastern Canada had emerged, with lesser islands standing out to the west and south of North America. Another large area lay round the basin of the Baltic; and as Greenland, the Hebrides, and the extreme tip of Scotland, belong to the same age, it is believed that a continent, of which they are fragments, united America and Europe across the North Atlantic. Of the rest of what is now Europe there were merely large islands—one on the border of England and Wales, others in France, Spain, and Southern Germany. Asia was represented by a large area in China and Siberia, and an island or islands on the site of India. Very little of Africa or South America existed.
It will be seen at a glance that the physical story of the earth from that time is a record of the emergence from the waters of larger continents and the formation of lofty chains of mountains. Now this world-old battle of land and sea has been waged with varying fortune from age to age, and it has been one of the most important factors in the development of life. We are just beginning to realise what a wonderful light it throws on the upward advance of animals and plants. No one in the scientific world to-day questions that, however imperfect the record may be, there has been a continuous development of life from the lowest level to the highest. But why there was advance at all, why the primitive microbe climbs the scale of being, during millions of years, until it reaches the stature of humanity, seems to many a profound mystery. The solution of this mystery begins to break upon us when we contemplate, in the geological record, the prolonged series of changes in the face of the earth itself, and try to realise how these changes must have impelled living things to fresh and higher adaptations to their changing surroundings.
Imagine some early continent with its population of animals and plants. Each bay, estuary, river, and lake, each forest and marsh and solid plain, has its distinctive inhabitants. Imagine this continent slowly sinking into the sea, until the advancing arms of the salt water meet across it, mingling their diverse populations in a common world, making the fresh-water lake brackish or salt, turning the dry land into swamp, and flooding the forest. Or suppose, on the other hand, that the land rises, the marsh is drained, the genial climate succeeded by an icy cold, the luscious vegetation destroyed, the whole animal population compelled to change its habits and its food. But this is no imaginary picture. It is the actual story of the earth during millions of years, and it is chiefly in the light of these vast and exacting changes in the environment that we are going to survey the panorama of the advance of terrestrial life.
For the moment it will be enough to state two leading principles. The first is that there is no such thing as a "law of evolution" in the sense in which many people understand that phrase. It is now sufficiently well known that, when science speaks of a law, it does not mean that there is some rule that things MUST act in such and such a way. The law is a mere general expression of the fact that they DO act in that way. But many imagine that there is some principle within the living organism which impels it onward to a higher level of organisation. That is entirely an error. There is no "law of progress." If an animal is fitted to secure its livelihood and breed posterity in certain surroundings, it may remain unchanged indefinitely if these surroundings do not materially change. So the duckmole of Australia and the tuatara of New Zealand have retained primitive features for millions of years; so the aboriginal Australian and the Fuegian have remained stagnant, in their isolation, for a hundred thousand years or more; so the Chinaman, in his geographical isolation, has remained unchanged for two thousand years. There is no more a "conservative instinct" in Chinese than there is a "progressive instinct" in Europeans. The difference is one of history and geography, as we shall see.
To make this important principle still clearer, let us imagine some primitive philosopher observing the advance of the tide over a level beach. He must discover two things: why the water comes onward at all, and why it advances along those particular channels. We shall see later how men of science explain or interpret the mechanism in a living thing which enables it to advance, when it does advance. For the present it is enough to say that new-born animals and plants are always tending to differ somewhat from their parents, and we now know, by experiment, that when some exceptional influence is brought to bear on the parent, the young may differ considerably from her. But, if the parents were already in harmony with their environment, these variations on the part of the young are of no consequence. Let the environment alter, however, and some of these variations may chance to make the young better fitted than the parent was. The young which happen to have the useful variation will have an advantage over their brothers or sisters, and be more likely to survive and breed the next generation. If the change in the environment (in the food or climate, for instance) is prolonged and increased for hundreds of thousands of years, we shall expect to find a corresponding change in the animals and plants.
We shall find such changes occurring throughout the story of the earth. At one important point in the story we shall find so grave a revolution in the face of nature that twenty-nine out of every thirty species of animals and plants on the earth are annihilated. Less destructive and extreme changes have been taking place during nearly the whole of the period we have to cover, entailing a more gradual alteration of the structure of animals and plants; but we shall repeatedly find them culminating in very great changes of climate, or of the distribution of land and water, which have subjected the living population of the earth to the most searching tests and promoted every variation toward a more effective organisation.[1]
And the second guiding principle I wish to lay down in advance is that these great changes in the face of the earth, which explain the progress of organisms, may very largely be reduced to one simple agency—the battle of the land and the sea. When you gaze at some line of cliffs that is being eaten away by the waves, or reflect on the material carried out to sea by the flooded river, you are—paradoxical as it may seem—beholding a material process that has had a profound influence on the development of life. The Archaean continent that we described was being reduced constantly by the wash of rain, the scouring of rivers, and the fretting of the waves on the coast. It is generally thought that these wearing agencies were more violent in early times, but that is disputed, and we will not build on it. In any case, in the course of time millions of tons of matter were scraped off the Archaean continent and laid on the floor of the sea by its rivers. This meant a very serious alteration of pressure or weight on the surface of the globe, and was bound to entail a reaction or restoration of the balance.
The rise of the land and formation of mountains used to be ascribed mainly to the cooling and shrinking of the globe of the earth. The skin (crust), it was thought, would become too large for the globe as it shrank, and would wrinkle outwards, or pucker up into mountain-chains. The position of our greater mountain-chains sprawling across half the earth (the Pyrenees to the Himalaya, and the Rocky Mountains to the Andes), seems to confirm this, but the question of the interior of the earth is obscure and disputed, and geologists generally conceive the rise of land and formation of mountains in a different way. They are due probably to the alteration of pressure on the crust in combination with the instability of the interior. The floors of the seas would sink still lower under their colossal burdens, and this would cause some draining of the land-surface. At the same time the heavy pressure below the seas and the lessening of pressure over the land would provoke a reaction. Enormous masses of rock would be forced toward and underneath the land-surface, bending, crumpling, and upheaving it as if its crust were but a leather coat. As a result, masses of land would slowly rise above the plain, to be shaped into hills and valleys by the hand of later time, and fresh surfaces would be dragged out of the deep, enlarging the fringes of the primitive continents, to be warped and crumpled in their turn at the next era of pressure.
In point of geological fact, the story of the earth has been one prolonged series of changes in the level of land and water, and in their respective limits. These changes have usually been very gradual, but they have always entailed changes (in climate, etc. ) of the greatest significance in the evolution of life. What was the swampy soil of England in the Carboniferous period is now sometimes thousands of feet beneath us; and what was the floor of a deep ocean over much of Europe and Asia at another time is now to be found on the slopes of lofty Alps, or 20,000 feet above the sea-level in Thibet. Our story of terrestrial life will be, to a great extent, the story of how animals and plants changed their structure in the long series of changes which this endless battle of land and sea brought over the face of the earth.
As we have no recognisable remains of the animals and plants of the earliest age, we will not linger over the Archaean rocks. Starting from deep and obscure masses of volcanic matter, the geologist, as he travels up the series of Archaean rocks, can trace only a dim and most unsatisfactory picture of those remote times. Between outpours of volcanic floods he finds, after a time, traces that an ocean and rivers are wearing away the land. He finds seams of carbon among the rocks of the second division of the Archaean (the Keewatin), and deduces from this that a dense sea-weed population already covered the floor of the ocean. In the next division (the Huronian) he finds the traces of extensive ice-action strangely lying between masses of volcanic rock, and sees that thousands of square miles of eastern North America were then covered with an ice-sheet. Then fresh floods of molten matter are poured out from the depths below; then the sea floods the land for a time; and at last it makes its final emergence as the first definitive part of the North American continent, to enlarge, by successive fringes, to the continent of to-day.[2]
This meagre picture of the battle of land and sea, with interludes of great volcanic activity and even of an ice age, represents nearly all we know of the first half of the world's story from geology. It is especially disappointing in regard to the living population. The very few fossils we find in the upper Archaean rocks are so similar to those we shall discuss in the next chapter that we may disregard them, and the seams of carbon-shales, iron-ore, and limestone, suggest only, at the most, that life was already abundant. We must turn elsewhere for some information on the origin and early development of life.
The question of the origin of life I will dismiss with a brief account of the various speculations of recent students of science. Broadly speaking, their views fall into three classes. Some think that the germs of life may have come to the earth from some other body in the universe; some think that life was evolved out of non-living matter in the early ages of the earth, under exceptional conditions which we do not at present know, or can only dimly conjecture; and some think that life is being evolved from non-life in nature to-day, and always has been so evolving. The majority of scientific men merely assume that the earliest living things were no exception to the general process of evolution, but think that we have too little positive knowledge to speculate profitably on the manner of their origin.
The first view, that the germs of life may have come to this planet on a meteoric visitor from some other world, as a storm-driven bird may take its parasites to some distant island, is not without adherents to-day. It was put forward long ago by Lord Kelvin and others; it has been revived by the distinguished Swede, Professor Svante Arrhenius. The scientific objection to it is that the more intense (ultra-violet) rays of the sun would frill such germs as they pass through space. But a broader objection, and one that may dispense us from dwelling on it, is that we gain nothing by throwing our problems upon another planet. We have no ground for supposing that the earth is less capable of evolving life than other planets.
The second view is that, when the earth had passed through its white-hot stage, great masses of very complex chemicals, produced by the great heat, were found on its surface. There is one complex chemical substance in particular, called cyanogen, which is either an important constituent of living matter, or closely akin to it. Now we need intense heat to produce this substance in the laboratory. May we not suppose that masses of it were produced during the incandescence of the earth, and that, when the waters descended, they passed through a series of changes which culminated in living plasm? Such is the "cyanogen hypothesis" of the origin of life, advocated by able physiologists such as Pfluger, Verworn, and others. It has the merit of suggesting a reason why life may not be evolving from non-life in nature to-day, although it may have so evolved in the Archaean period.
Other students suggest other combinations of carbon-compounds and water in the early days. Some suggest that electric action was probably far more intense in those ages; others think that quantities of radium may have been left at the surface. But the most important of these speculations on the origin of life in early times, and one that has the merit of not assuming any essentially different conditions then than we find now, is contained in a recent pronouncement of one of the greatest organic chemists in Europe, Professor Armstrong. He says that such great progress has been made in his science—the science of the chemical processes in living things—that "their cryptic character seems to have disappeared almost suddenly." On the strength of this new knowledge of living matter, he ventures to say that "a series of lucky accidents" could account for the first formation of living things out of non-living matter in Archaean times. Indeed, he goes further. He names certain inorganic substances, and says that the blowing of these into pools by the wind on the primitive planet would set afoot chemical combinations which would issue in the production of living matter.[3]
It is evident that the popular notion that scientific men have declared that life cannot be evolved from non-life is very far astray. This blunder is usually due to a misunderstanding of the dogmatic statement which one often reads in scientific works that "every living thing comes from a living thing." This principle has no reference to remote ages, when the conditions may have been different. It means that to-day, within our experience, the living thing is always born of a living parent. However, even this is questioned by some scientific men of eminence, and we come to the third view.
Professor Nageli, a distinguished botanist, and Professor Haeckel, maintain that our experience, as well as the range of our microscopes, is too limited to justify the current axiom. They believe that life may be evolving constantly from inorganic matter. Professor J. A. Thomson also warns us that our experience is very limited, and, for all we know, protoplasm may be forming naturally in our own time. Mr. Butler Burke has, under the action of radium, caused the birth of certain minute specks which strangely imitate the behaviour of bacteria. Dr. Bastian has maintained for years that he has produced living things from non-living matter. In his latest experiments, described in the book quoted, purely inorganic matter is used, and it is previously subjected, in hermetically sealed tubes, to a heat greater than what has been found necessary to kill any germs whatever.
Evidently the problem of the origin of life is not hopeless, but our knowledge of the nature of living matter is still so imperfect that we may leave detailed speculation on its origin to a future generation. Organic chemistry is making such strides that the day may not be far distant when living matter will be made by the chemist, and the secret of its origin revealed. For the present we must be content to choose the more plausible of the best-informed speculations on the subject.
But while the origin of life is obscure, the early stages of its evolution come fairly within the range of our knowledge. To the inexpert it must seem strange that, whereas we must rely on pure speculation in attempting to trace the origin of life, we can speak with more confidence of those early developments of plants and animals which are equally buried in the mists of the Archaean period. Have we not said that nothing remains of the procession of organisms during half the earth's story but a shapeless seam of carbon or limestone?
A simple illustration will serve to justify the procedure we are about to adopt. Suppose that the whole of our literary and pictorial references to earlier stages in the development of the bicycle, the locomotive, or the loom, were destroyed. We should still be able to retrace the phases of their evolution, because we should discover specimens belonging to those early phases lingering in our museums, in backward regions, and elsewhere. They might yet be useful in certain environments into which the higher machines have not penetrated. In the same way, if all the remains of prehistoric man and early civilisation were lost, we could still fairly retrace the steps of the human race, by gathering the lower tribes and races, and arranging them in the order of their advancement. They are so many surviving illustrations of the stages through which mankind as a whole has passed.
Just in the same way we may marshal the countless species of animals and plants to-day in such order that they will, in a general way, exhibit to us the age-long procession of life. From the very start of living evolution certain forms dropped out of the onward march, and have remained, to our great instruction, what their ancestors were millions of years ago. People create a difficulty for themselves by imagining that, if evolution is true, all animals must evolve. A glance at our own fellows will show the error of this. Of one family of human beings, as a French writer has said, one only becomes a Napoleon; the others remain Lucien, Jerome, or Joseph. Of one family of animals or trees, some advance in one or other direction; some remain at the original level. There is no "law of progress." The accidents of the world and hereditary endowment impel some onward, and do not impel others. Hence at nearly every great stage in the upward procession through the ages some regiment of plants or animals has dropped out, and it represents to-day the stage of life at which it ceased to progress. In other words, when we survey the line of the hundreds of thousands of species which we find in nature to-day, we can trace, amid their countless variations and branches, the line of organic evolution in the past; just as we could, from actual instances, study the evolution of a British house, from the prehistoric remains in Devonshire to a mansion in Park Lane or a provincial castle.
Another method of retracing the lost early chapters in the development of life is furnished by embryology. The value of this method is not recognised by all embryologists, but there are now few authorities who question the substantial correctness of it, and we shall, as we proceed, see some remarkable applications of it. In brief, it is generally admitted that an animal or plant is apt to reproduce, during its embryonic development, some of the stages of its ancestry in past time. This does not mean that a higher animal, whose ancestors were at one time worms, at another time fishes, and at a later time reptiles, will successively take the form of a little worm, a little fish, and a little reptile. The embryonic life itself has been subject to evolution, and this reproduction of ancestral forms has been proportionately disturbed. Still, we shall find that animals will tend, in their embryonic development, to reproduce various structural features which can only be understood as reminiscences of ancestral organs. In the lower animals the reproduction is much less disturbed than in the higher, but even in the case of man this law is most strikingly verified. We shall find it useful sometimes at least in confirming our conclusions as to the ancestry of a particular group.
We have, therefore, two important clues to the missing chapters in the story of evolution. Just as the scheme of the evolution of worlds is written broadly across the face of the heavens to-day, so the scheme of the evolution of life is written on the face of living nature; and it is written again, in blurred and broken characters, in the embryonic development of each individual. With these aids we set out to restore the lost beginning of the epic of organic evolution.
- ↑ This is a very simple expression of "Darwinism," and will be enlarged later. The reader should ignore the occasional statement of non-scientific writers that Darwinism is "dead" or superseded. The questions which are actually in dispute relate to the causes of the variation of the young from their parents, the magnitude of these variations' and the transmission of changes acquired by an animal during its own life. We shall see this more fully at a later stage. The importance of the environment as I have described it, is admitted by all schools.
- ↑ I am quoting Professor Coleman's summary of Archaean research in North America (Address to the Geological Section of the British Association, 1909). Europe, as a continent, has had more "ups and downs" than America in the course of geological time.
- ↑ See his address in Nature, vol. 76, p. 651. For other speculations see Verworn's "General Physiology," Butler Burke's "Origin of Life" (1906), and Dr. Bastian's "Origin of Life" (1911).