The Story of Evolution/Chapter X
In an earlier chapter it was stated that the story of life is a story of gradual and continuous advance, with occasional periods of more rapid progress. Hitherto it has been, in these pages, a slow and even advance from one geological age to another, one level of organisation to another. This, it is true, must not be taken too literally. Many a period of rapid change is probably contained, and blurred out of recognition, in that long chronicle of geological events. When a region sinks slowly below the waves, no matter how insensible the subsidence may be, there will often come a time of sudden and vast inundations, as the higher ridges of the coast just dip below the water-level and the lower interior is flooded. When two invading arms of the sea meet at last in the interior of the sinking continent, or when a land-barrier that has for millions of years separated two seas and their populations is obliterated, we have a similar occurrence of sudden and far-reaching change. The whole story of the earth is punctuated with small cataclysms. But we now come to a change so penetrating, so widespread, and so calamitous that, in spite of its slowness, we may venture to call it a revolution.
Indeed, we may say of the remaining story of the earth that it is characterised by three such revolutions, separated by millions of years, which are very largely responsible for the appearance of higher types of life. The facts are very well illustrated by an analogy drawn from the recent and familiar history of Europe.
The socio-political conditions of Europe in the eighteenth century, which were still tainted with feudalism, were changed into the socio-political conditions of the modern world, partly by a slow and continuous evolution, but much more by three revolutionary movements. First there was the great upheaval at the end of the eighteenth century, the tremors of which were felt in the life of every country in Europe. Then, although, as Freeman says, no part of Europe ever returned entirely to its former condition, there was a profound and almost universal reaction. In the 'thirties and 'forties, differing in different countries, a second revolutionary disturbance shook Europe. The reaction after this upheaval was far less severe, and the conditions were permanently changed to a great extent, but a third revolutionary movement followed in the next generation, and from that time the evolution of socio-political conditions has proceeded more evenly.
The story of life on the earth since the Coal-forest period is similarly quickened by three revolutions. The first, at the close of the Carboniferous period, is the subject of this chapter. It is the most drastic and devastating of the three, but its effect, at least on the animal world, will be materially checked by a profound and protracted reaction. At the end of the Chalk period, some millions of years later, there will be a second revolution, and it will have a far more enduring and conspicuous result, though it seem less drastic at the time. Yet there will be something of a reaction after a time, and at length a third revolution will inaugurate the age of man. If it is clearly understood that instead of a century we are contemplating a period of at least ten million years, and instead of a decade of revolution we have a change spread over a hundred thousand years or more, this analogy will serve to convey a most important truth.
The revolutionary agency that broke into the comparatively even chronicle of life near the close of the Carboniferous period, dethroned its older types of organisms, and ushered new types to the lordship of the earth, was cold. The reader will begin to understand why I dwelt on the aspect of the Coal-forest and its surrounding waters. There was, then, a warm, moist earth from pole to pole, not even temporarily chilled and stiffened by a few months of winter, and life spread luxuriantly in the perpetual semi-tropical summer. Then a spell of cold so severe and protracted grips the earth that glaciers glitter on the flanks of Indian and Australian hills, and fields of ice spread over what are now semitropical regions. In some degree the cold penetrates the whole earth. The rich forests shrink slowly into thin tracts of scrubby, poverty-stricken vegetation. The loss of food and the bleak and exacting conditions of the new earth annihilate thousands of species of the older organisms, and the more progressive types are moulded into fitness for the new environment. It is a colossal application of natural selection, and amongst its results are some of great moment.
In various recent works one reads that earlier geologists, led astray by the nebular theory of the earth's origin, probably erred very materially in regard to the climate of primordial times, and that climate has varied less than used to be supposed. It must not be thought that, in speaking of a "Permian revolution," I am ignoring or defying this view of many distinguished geologists. I am taking careful account of it. There is no dispute, however, about the fact that the Permian age witnessed an immense carnage of Carboniferous organisms, and a very considerable modification of those organisms which survived the catastrophe, and that the great agency in this annihilation and transformation was cold. To prevent misunderstanding, nevertheless, it will be useful to explain the controversy about the climate of the earth in past ages which divides modern geologists.
The root of the difference of opinion and the character of the conflicting parties have already been indicated. It is a protest of the "Planetesimalists" against the older, and still general, view of the origin of the earth. As we saw, that view implies that, as the heavier elements penetrated centreward in the condensing nebula, the gases were left as a surrounding shell of atmosphere. It was a mixed mass of gases, chiefly oxygen, hydrogen, nitrogen, and carbon-dioxide (popularly known as "carbonic acid gas"). When the water-vapour settled as ocean on the crust, the atmosphere remained a very dense mixture of oxygen, nitrogen, and carbon-dioxide—to neglect the minor gases. This heavy proportion of carbon-dioxide would cause the atmosphere to act as a glass-house over the surface of the earth, as it does still to some extent. Experiment has shown that an atmosphere containing much vapour and carbon-dioxide lets the heat-rays pass through when they are accompanied by strong light, but checks them when they are separated from the light. In other words, the primitive atmosphere would allow the heat of the sun to penetrate it, and then, as the ground absorbed the light, would retain a large proportion of the heat. Hence the semi-tropical nature of the primitive earth, the moisture, the dense clouds and constant rains that are usually ascribed to it. This condition lasted until the rocks and the forests of the Carboniferous age absorbed enormous quantities of carbon-dioxide, cleared the atmosphere, and prepared an age of chill and dryness such as we find in the Permian.
But the planetesimal hypothesis has no room for this enormous percentage of carbon-dioxide in the primitive atmosphere. Hinc illoe lachrymoe: in plain English, hence the acute quarrel about primitive climate, and the close scanning of the geological chronicle for indications that the earth was not moist and warm until the end of the Carboniferous period. Once more I do not wish to enfeeble the general soundness of this account of the evolution of life by relying on any controverted theory, and we shall find it possible to avoid taking sides.
I have not referred to the climate of the earth in earlier ages, except to mention that there are traces of a local "ice-age" about the middle of the Archaean and the beginning of the Cambrian. As these are many millions of years removed from each other and from the Carboniferous, it is possible that they represent earlier periods more or less corresponding to the Permian. But the early chronicle is so compressed and so imperfectly studied as yet that it is premature to discuss the point. It is, moreover, unnecessary because we know of no life on land in those remote periods, and it is only in connection with life on land that we are interested in changes of climate here. In other words, as far as the present study is concerned, we need only regard the climate of the Devonian and Carboniferous periods. As to this there is no dispute; nor, in fact, about the climate from the Cambrian to the Permian.
As the new school is most brilliantly represented by Professor Chamberlin,[1] it will be enough to quote him. He says of the Cambrian that, apart from the glacial indications in its early part, "the testimony of the fossils, wherever gathered, implies nearly uniform climatic conditions... throughout all the earth wherever records of the Cambrian period are preserved" (ii, 273). Of the Ordovician he says: "All that is known of the life of this era would seem to indicate that the climate was much more uniform than now throughout the areas where the strata of the period are known" (ii, 342). In the Silurian we have "much to suggest uniformity of climate"—in fact, we have just the same evidence for it—and in the Devonian, when land-plants abound and afford better evidence, we find the same climatic equality of living things in the most different latitudes. Finally, "most of the data at hand indicate that the climate of the Lower Carboniferous was essentially uniform, and on the whole both genial and moist" (ii, 518). The "data," we may recall, are in this case enormously abundant, and indicate the climate of the earth from the Arctic regions to the Antarctic. Another recent and critical geologist, Professor Walther ("Geschichte der Erde und des Lebens," 1908), admits that the coal-vegetation shows a uniformly warm climate from Spitzbergen to Africa. Mr. Drew ("The Romance of Modern Geology," 1909) says that "nearly all over the globe the climate was the same—hot, close, moist, muggy" (p. 219).
The exception which Professor Chamberlin has in mind when he says "most of the data" is that we find deposits of salt and gypsum in the Silurian and Lower Carboniferous, and these seem to point to the evaporation of lakes in a dry climate. He admits that these indicate, at the most, local areas or periods of dryness in an overwhelmingly moist and warm earth. It is thus not disputed that the climate of the earth was, during a period of at least fifteen million years (from the Cambrian to the Carboniferous), singularly uniform, genial, and moist. During that vast period there is no evidence whatever that the earth was divided into climatic zones, or that the year was divided into seasons. To such an earth was the prolific life of the Coal-forest adapted.
It is, further, not questioned that the temperature of the earth fell in the latter part of the Carboniferous age, and that the cold reached its climax in the Permian. As we turn over the pages of the geological chronicle, an extraordinary change comes over the vegetation of the earth. The great Lepidodendra gradually disappear before the close of the Permian period; the Sigillariae dwindle into a meagre and expiring race; the giant Horsetails (Calamites) shrink, and betray the adverse conditions in their thin, impoverished leaves. New, stunted, hardy trees make their appearance: the Walchia, a tree something like the low Araucarian conifers in the texture of its wood, and the Voltzia, the reputed ancestor of the cypresses. Their narrow, stunted leaves suggest to the imagination the struggle of a handful of pines on a bleak hill-side. The rich fern-population is laid waste. The seed-ferns die out, and a new and hardy type of fern, with compact leaves, the Glossopteris, spreads victoriously over the globe; from Australia it travels northward to Russia, which it reaches in the early Permian, and westward, across the southern continent, to South America. A profoundly destructive influence has fallen on the earth, and converted its rich green forests, in which the mighty Club-mosses had reared their crowns above a sea of waving ferns, into severe and poverty-stricken deserts.
No botanist hesitates to say that it is the coming of a cold, dry climate that has thus changed the face of the earth. The geologist finds more direct evidence. In the Werribee Gorge in Victoria I have seen the marks which Australian geologists have discovered of the ice-age which put an end to their Coal-forests. From Tasmania to Queensland they find traces of the rivers and fields of ice which mark the close of the Carboniferous and beginning of the Permian on the southern continent. In South Africa similar indications are found from the Cape to the Transvaal. Stranger still, the geologists of India have discovered extensive areas of glaciation, belonging to this period, running down into the actual tropics. And the strangest feature of all is that the glaciers of India and Australia flowed, not from the temperate zones toward the tropics, but in the opposite direction. Two great zones of ice-covered land lay north and south of the equator. The total area was probably greater than the enormous area covered with ice in Europe and America during the familiar ice-age of the latest geological period.
Thus the central idea of this chapter, the destructive inroad of a colder climate upon the genial Carboniferous world, is an accepted fact. Critical geologists may suggest that the temperature of the Coal-forest has been exaggerated, and the temperature of the Permian put too low. We are not concerned with the dispute. Whatever the exact change of temperature was, in degrees of the thermometer, it was admittedly sufficient to transform the face of the earth, and bring a mantle of ice over millions of square miles of our tropical and subtropical regions. It remains for us to inquire into the causes of this transformation.
It at once occurs to us that these facts seem to confirm the prevalent idea, that the Coal-forests stripped the air of its carbon-dioxide until the earth shivered in an atmosphere thinner than that of to-day. On reflection, however, it will be seen that, if this were all that happened, we might indeed expect to find enormous ice-fields extending from the poles—which we do not find—but not glaciation in the tropics. Others may think of astronomical theories, and imagine a shrinking or clouding of the sun, or a change in the direction of the earth's axis. But these astronomical theories are now little favoured, either by astronomers or geologists. Professor Lowell bluntly calls them "astrocomic" theories. Geologists think them superfluous. There is another set of facts to be considered in connection with the Permian cold.
As we have seen several times, there are periods when, either owing to the shrinking of the earth or the overloading of the sea-bottoms, or a combination of the two, the land regains its lost territory and emerges from the ocean. Mountain chains rise; new continental surfaces are exposed to the sun and rain. One of the greatest of these upheavals of the land occurs in the latter half of the Carboniferous and the Permian. In the middle of the Carboniferous, when Europe is predominantly a flat, low-lying land, largely submerged, a chain of mountains begins to rise across its central part. From Brittany to the east of Saxony the great ridge runs, and by the end of the Carboniferous it becomes a chain of lofty mountains (of which fragments remain in the Vosges, Black Forest, and Hartz mountains), dragging Central Europe high above the water, and throwing the sea back upon Russia to the north and the Mediterranean region to the south. Then the chain of the Ural Mountains begins to rise on the Russian frontier. By the beginning of the Permian Europe was higher above the water than it had ever yet been; there was only a sea in Russia and a southern sea with narrow arms trailing to the northwest. The continent of North America also had meantime emerged. The rise of the Appalachia and Ouachita mountains completes the emergence of the eastern continent, and throws the sea to the west. The Asiatic continent also is greatly enlarged, and in the southern hemisphere there is a further rise, culminating in the Permian, of the continent ("Gondwana Land") which united South America, South Africa, the Antarctic land, Australia and New Zealand, with an arm to India.
In a word, we have here a physical revolution in the face of the earth. The changes were generally gradual, though they seem in some places to have been rapid and abrupt (Chamberlin); but in summary they amounted to a vast revolution in the environment of animals and plants. The low-lying, swampy, half-submerged continents reared themselves upward from the sea-level, shook the marshes and lagoons from their face, and drained the vast areas that had fostered the growth of the Coal-forests. It is calculated (Chamberlin) that the shallow seas which had covered twenty or thirty million square miles of our continental surfaces in the early Carboniferous were reduced to about five million square miles in the Permian. Geologists believe, in fact, that the area of exposed land was probably greater than it is now.
This lifting and draining of so much land would of itself have a profound influence on life-conditions, and then we must take account of its indirect influence. The moisture of the earlier period was probably due in the main to the large proportion of sea-surface and the absence of high land to condense it. In both respects there is profound alteration, and the atmosphere must have become very much drier. As this vapour had been one of the atmosphere's chief elements for retaining heat at the surface of the earth, the change will involve a great lowering of temperature. The slanting of the raised land would aid this, as, in speeding the rivers, it would promote the circulation of water. Another effect would be to increase the circulation of the atmosphere. The higher and colder lands would create currents of air that had not been formed before. Lastly, the ocean currents would be profoundly modified; but the effect of this is obscure, and may be disregarded for the moment.
Here, therefore, we have a massive series of causes and effects, all connected with the great emergence of the land, which throw a broad light on the change in the face of the earth. We must add the lessening of the carbon dioxide in the atmosphere. Quite apart from theories of the early atmosphere, this process must have had a great influence, and it is included by Professor Chamberlin among the causes of the world-wide change. The rocks and forests of the Carboniferous period are calculated to have absorbed two hundred times as much carbon as there is in the whole of our atmosphere to-day. Where the carbon came from we may leave open. The Planetesimalists look for its origin mainly in volcanic eruptions, but, though there was much volcanic activity in the later Carboniferous and the Permian, there is little trace of it before the Coal-forests (after the Cambrian). However that may be, there was a considerable lessening of the carbon-dioxide of the atmosphere, and this in turn had most important effects. First, the removal of so much carbon-dioxide and vapour would be a very effective reason for a general fall in the temperature of the earth. The heat received from the sun could now radiate more freely into space. Secondly, it has been shown by experiment that a richness in carbon-dioxide favours Cryptogamous plants (though it is injurious to higher plants), and a reduction of it would therefore be hurtful to the Cryptogams of the Coal-forest. One may almost put it that, in their greed, they exhausted their store. Thirdly, it meant a great purification of the atmosphere, and thus a most important preparation of the earth for higher land animals and plants.
The reader will begin to think that we have sufficiently "explained" the Permian revolution. Far from it. Some of its problems are as yet insoluble. We have given no explanation at all why the ice-sheets, which we would in a general way be prepared to expect, appear in India and Australia, instead of farther north and south. Professor Chamberlin, in a profound study of the period (appendix to vol. ii, "Geology"), suggests that the new land from New Zealand to Antarctica may have diverted the currents (sea and air) up the Indian Ocean, and caused a low atmospheric pressure, much precipitation of moisture, and perpetual canopies of clouds to shield the ice from the sun. Since the outer polar regions themselves had been semi-tropical up to that time, it is very difficult to see how this will account for a freezing temperature in such latitudes as Australia and India. There does not seem to have been any ice at the Poles up to that time, or for ages afterwards, so that currents from the polar regions would be very different from what they are today. If, on the other hand, we may suppose that the rise of "Gondwana Land" (from Brazil to India) was attended by the formation of high mountains in those latitudes, we have the basis, at least, of a more plausible explanation. Professor Chamberlin rejects this supposition on the ground that the traces of ice-action are at or near the sea-level, since we find with them beds containing marine fossils. But this only shows, at the most, that the terminations of the glaciers reached the sea. We know nothing of the height of the land from which they started.
For our main purpose, however, it is fortunately not necessary to clear up these mysteries. It is enough for us that the Carboniferous land rises high above the surface of the ocean over the earth generally. The shallow seas are drained off its surface; its swamps and lagoons generally disappear; its waters run in falling rivers to the ocean. The dense, moist, warm atmosphere that had so long enveloped it is changed into a thinner mantle of gas, through which, night by night, the sun-soaked ground can discharge its heat into space. Cold winds blow over it from the new mountains; probably vast regions of it are swept by icy blasts from the glaciated lands. As these conditions advance in the Permian period, the forests wither and shrink. Of the extraordinarily mixed vegetation which we found in the Coal-forests some few types are fitted to meet the severe conditions. The seed-bearing trees, the thin, needle-leafed trees, the trees with stronger texture of the wood, are slowly singled out by the deepening cold. The golden age of Cryptogams is over. The age of the Cycad and the Conifers is opening. Survivors of the old order linger in the warmer valleys, as one may see to-day tree-ferns lingering in nooks of southern regions while an Antarctic wind is whistling on the hills above them; but over the broad earth the luscious pasturage of the Coal-forest has changed into what is comparatively a cold desert. We must not, of course, imagine too abrupt a change. The earth had been by no means all swamp in the Carboniferous age. The new types were even then developing in the cooler and drier localities. But their hour has come, and there is great devastation among the lower plant population of the earth.
It follows at once that there would be, on land, an equal devastation and a similar selection in the animal world. The vegetarians suffered an appalling reduction of their food; the carnivores would dwindle in the same proportion. Both types, again, would suffer from the enormous changes in their physical surroundings. Vast stretches of marsh, with teeming populations, were drained, and turned into firm, arid plains or bleak hill-sides. The area of the Amphibia, for instance, was no less reduced than their food. The cold, in turn, would exercise a most formidable selection. Before the Permian period there was not on the whole earth an animal with a warm-blooded (four-chambered) heart or a warm coat of fur or feathers; nor was there a single animal that gave any further care to the eggs it discharged, and left to the natural warmth of the earth to develop. The extermination of species in the egg alone must have been enormous.
It is impossible to convey any just impression of the carnage which this Permian revolution wrought among the population of the earth. We can but estimate how many species of animals and plants were exterminated, and the reader must dimly imagine the myriads of living things that are comprised in each species. An earlier American geologist, Professor Le Conte, said that not a single Carboniferous species crossed the line of the Permian revolution. This has proved to be an exaggeration, but Professor Chamberlin seems to fall into an exaggeration on the other side when he says that 300 out of 10,000 species survived. There are only about 300 species of animals and plants known in the whole of the Permian rocks (Geikie), and most of these are new. For instance, of the enormous plant-population of the Coal-forests, comprising many thousands of species, only fifty species survived unchanged in the Permian. We may say that, as far as our knowledge goes, of every thirty species of animals and plants in the Carboniferous period, twenty-eight were blotted out of the calendar of life for ever; one survived by undergoing such modifications that it became a new species, and one was found fit to endure the new conditions for a time. We must leave it to the imagination to appreciate the total devastation of individuals entailed in this appalling application of what we call natural selection.
But what higher types of life issued from the womb of nature after so long and painful a travail? The annihilation of the unfit is the seamy side, though the most real side, of natural selection. We ignore it, or extenuate it, and turn rather to consider the advances in organisation by which the survivors were enabled to outlive the great chill and impoverishment.
Unfortunately, if the Permian period is an age of death, it is not an age of burials. The fossil population of its cemeteries is very scanty. Not only is the living population enormously reduced, but the areas that were accustomed to entomb and preserve organisms—the lake and shore deposits—are also greatly reduced. The frames of animals and plants now rot on the dry ground on which they live. Even in the seas, where life must have been much reduced by the general disturbance of conditions, the record is poor. Molluscs and Brachiopods and small fishes fill the list, but are of little instructiveness for us, except that they show a general advance of species. Among the Cephalopods, it is true, we find a notable arrival. On the one hand, a single small straight-shelled Cephalopod lingers for a time with the ancestral form; on the other hand, a new and formidable competitor appears among the coiled-shell Cephalopods. It is the first appearance of the famous Ammonite, but we may defer the description of it until we come to the great age of Ammonites.
Of the insects and their fortunes in the great famine we have no direct knowledge; no insect remains have yet been found in Permian rocks. We shall, however, find them much advanced in the next period, and must conclude that the selection acted very effectively among their thousand Carboniferous species.
The most interesting outcome of the new conditions is the rise and spread of the reptiles. No other sign of the times indicates so clearly the dawn of a new era as the appearance of these primitive, clumsy reptiles, which now begin to oust the Amphibia. The long reign of aquatic life is over; the ensign of progress passes to the land animals. The half-terrestrial, half-aquatic Amphibian deserts the water entirely (in one or more of its branches), and a new and fateful dynasty is founded. Although many of the reptiles will return to the water, when the land sinks once more, the type of the terrestrial quadruped is now fully evolved, and from its early reptilian form will emerge the lords of the air and the lords of the land, the birds and the mammals.
To the uninformed it may seem that no very great advance is made when the reptile is evolved from the Amphibian. In reality the change implies a profound modification of the frame and life of the vertebrate. Partly, we may suppose, on account of the purification of the air, partly on account of the decrease in water surface, the gills are now entirely discarded. The young reptile loses them during its embryonic life—as man and all the mammals and birds do to-day—and issues from the egg a purely lung-breathing creature. A richer blood now courses through the arteries, nourishing the brain and nerves as well as the muscles. The superfluous tissue of the gill-structures is used in the improvement of the ear and mouth-parts; a process that had begun in the Amphibian. The body is raised up higher from the ground, on firmer limbs; the ribs and the shoulder and pelvic bones—the saddles by which the weight of the body is adjusted between the limbs and the backbone—are strengthened and improved. Finally, two important organs for the protection and nurture of the embryo (the amnion and the allantois) make their appearance for the first time in the reptile. In grade of organisation the reptile is really nearer to the bird than it is to the salamander.
Yet these Permian reptiles are so generalised in character and so primitive in structure that they point back unmistakably to an Amphibian ancestry. The actual line of descent is obscure. When the reptiles first appear in the rocks, they are already divided into widely different groups, and must have been evolved some time before. Probably they started from some group or groups of the Amphibia in the later Carboniferous, when, as we saw, the land began to rise considerably. We have not yet recovered, and may never recover, the region where the early forms lived, and therefore cannot trace the development in detail. The fossil archives, we cannot repeat too often, are not a continuous, but a fragmentary, record of the story of life. The task of the evolutionist may be compared to the work of tracing the footsteps of a straying animal across the country. Here and there its traces will be amply registered on patches of softer ground, but for the most part they will be entirely lost on the firmer ground. So it is with the fossil record of life. Only in certain special conditions are the passing forms buried and preserved. In this case we can say only that the Permian reptiles fall into two great groups, and that one of these shows affinities to the small salamander-like Amphibia of the Coal-forest (the Microsaurs), while the other has affinities to the Labyrinthodonts.
A closer examination of these early reptiles may be postponed until we come to speak of the "age of reptiles." We shall see that it is probable that an even higher type of animal, the mammal, was born in the throes of the Permian revolution. But enough has been said in vindication of the phrase which stands at the head of this chapter; and to show how the great Primary age of terrestrial life came to a close. With its new inhabitants the earth enters upon a fresh phase, and thousands of its earlier animals and plants are sealed in their primordial tombs, to await the day when man will break the seals and put flesh once more on the petrified bones.
- ↑ An apology is due here in some measure. The work which I quote as of Professor Chamberlin ("Geology," 1903) is really by two authors, Professors Chamberlin and Salisbury. I merely quote Professor Chamberlin for shortness, and because the particular ideas I refer to are expounded by him in separate papers. The work is the finest manual in modern geological literature. I have used it much, in conjunction with the latest editions of Geikie, Le Conte, and Lupparent, and such recent manuals as Walther, De Launay, Suess, etc., and the geological magazines.