The Story of Evolution/Chapter XIV
In accordance with the view of the later story of the earth which was expressed on an earlier page, we now come to the second of the three great revolutions which have quickened the pulse of life on the earth. Many men of science resent the use of the word revolution, and it is not without some danger. It was once thought that the earth was really shaken at times by vast and sudden cataclysms, which destroyed its entire living population, so that new kingdoms of plants and animals had to be created. But we have interpreted the word revolution in a very different sense. The series of changes and disturbances to which we give the name extended over a period of hundreds of thousands of years, and they were themselves, in some sense, the creators of new types of organisms. Yet they are periods that stand out peculiarly in the comparatively even chronicle of the earth. The Permian period transformed the face of the earth; it lifted the low-lying land into a massive relief, drew mantles of ice over millions of miles of its surface, set volcanoes belching out fire and fumes in many parts, stripped it of its great forests, and slew the overwhelming majority of its animals. On the scale of geological time it may be called a revolution.
It must be confessed that the series of disturbances which close the Secondary and inaugurate the Tertiary Era cannot so conveniently be summed up in a single formula. They begin long before the end of the Mesozoic, and they continue far into the Tertiary, with intervals of ease and tranquillity. There seems to have been no culminating point in the series when the uplifted earth shivered in a mantle of ice and snow. Yet I propose to retain for this period—beginning early in the Cretaceous (Chalk) period and extending into the Tertiary—the name of the Cretaceous Revolution. I drew a fanciful parallel between the three revolutions which have quickened the earth since the sluggish days of the Coal-forest and the three revolutionary movements which have changed the life of modern Europe. It will be remembered that, whereas the first of these European revolutions was a sharp and massive upheaval, the second consisted in a more scattered and irregular series of disturbances, spread over the fourth and fifth decades of the nineteenth century; but they amounted, in effect, to a revolution.
So it is with the Cretaceous Revolution. In effect it corresponds very closely to the Permian Revolution. On the physical side it includes a very considerable rise of the land over the greater part of the globe, and the formation of lofty chains of mountains; on the botanical side it means the reduction of the rich Mesozoic flora to a relatively insignificant population, and the appearance and triumphant spread of the flowering plants, on the zoological side it witnesses the complete extinction of the Ammonites, Deinosaurs, and Pterosaurs, an immense reduction of the reptile world generally, and a victorious expansion of the higher insects, birds, and mammals; on the climatic side it provides the first definite evidence of cold zones of the earth and cold seasons of the year, and seems to represent a long, if irregular, period of comparative cold. Except, to some extent, the last of these points, there is no difference of opinion, and therefore, from the evolutionary point of view, the Cretaceous period merits the title of a revolution. All these things were done before the Tertiary period opened.
Let us first consider the fundamental and physical aspect of this revolution, the upheaval of the land. It began about the close of the Jurassic period. Western and Central Europe emerged considerably from the warm Jurassic sea, which lay on it and had converted it into an archipelago. In North-western America also there was an emergence of large areas of land, and the Sierra and Cascade ranges of mountains were formed about the same time. For reasons which will appear later we must note carefully this rise of land at the very beginning of the Cretaceous period.
However, the sea recovered its lost territory, or compensation for it, and the middle of the Cretaceous period witnessed a very considerable extension of the waters over America, Europe, and southern Asia. The thick familiar beds of chalk, which stretch irregularly from Ireland to the Crimea, and from the south of Sweden to the south of France, plainly tell of an overlying sea. As is well known, the chalk consists mainly of the shells or outer frames of minute one-celled creatures (Thalamophores) which float in the ocean, and form a deep ooze at its bottom with their discarded skeletons. What depth this ocean must have been is disputed, and hardly concerns us. It is clear that it must have taken an enormous period for microscopic shells to form the thick masses of chalk which cover so much of southern and eastern England. On the lowest estimates the Cretaceous period, which includes the deposit of other strata besides chalk, lasted about three million years. And as people like to have some idea of the time since these things happened, I may add that, on the lowest estimate (which most geologists would at least double), it is about three million years since the last stretches of the chalk-ocean disappeared from the surface of Europe.
But while our chalk cliffs conjure up a vision of England lying deep—at least twenty or thirty fathoms deep—below a warm ocean, in which gigantic Ammonites and Belemnites and sharks ply their deadly trade, they also remind us of the last phase of the remarkable life of the earth's Middle Ages. In the latter part of the Cretaceous the land rises. The chalk ocean of Europe is gradually reduced to a series of inland seas, separated by masses and ridges of land, and finally to a series of lakes of brackish water. The masses of the Pyrenees and Alps begin to rise; though it will not be until a much later date that they reach anything like their present elevation. In America the change is even greater. A vast ridge rises along the whole western front of the continent, lifting and draining it, from Alaska to Cape Horn. It is the beginning of the Rocky Mountains and the Andes. Even during the Cretaceous period there had been rich forests of Mesozoic vegetation covering about a hundred thousand square miles in the Rocky Mountains region. Europe and America now begin to show their modern contours.
It is important to notice that this great uprise of the land and the series of disturbances it entails differ from those which we summed up in the phrase Permian Revolution. The differences may help us to understand some of the changes in the living population. The chief difference is that the disturbances are more local, and not nearly simultaneous. There is a considerable emergence of land at the end of the Jurassic, then a fresh expansion of the sea, then a great rise of mountains at the end of the Cretaceous, and so on. We shall find our great mountain-masses (the Pyrenees, Alps, Himalaya, etc.) rising at intervals throughout the whole of the Tertiary Era. However, it suffices for the moment to observe that in the latter part of the Mesozoic and early part of the Tertiary there were considerable upheavals of the land in various regions, and that the Mesozoic Era closed with a very much larger proportion of dry land, and a much higher relief of the land, than there had been during the Jurassic period. The series of disturbances was, says Professor Chamberlin, "greater than any that had occurred since the close of the Palaeozoic."
From the previous effect of the Permian upheaval, and from the fact that the living population is now similarly annihilated or reduced, we should at once expect to find a fresh change in the climate of the earth. Here, however, our procedure is not so easy. In the Permian age we had solid proof in the shape of vast glaciated regions. It is claimed by continental geologists that certain early Tertiary beds in Bavaria actually prove a similar, but smaller, glaciation in Europe, but this is disputed. Other beds may yet be found, but we saw that there was not a general upheaval, as there had been in the Permian, and it is quite possible that there were few or no ice-fields. We do not, in fact, know the causes of the Permian icefields. We are thrown upon the plant and animal remains, and seem to be in some danger of inferring a cold climate from the organic remains, and then explaining the new types of organisms by the cold climate. This, of course, we shall not do. The difficulty is made greater by the extreme disinclination of many recent geologists, and some recent botanists who have too easily followed the geologists, to admit a plain climatic interpretation of the facts. Let us first see what the facts are.
In the latter part of the Jurassic we find three different zones of Ammonites: one in the latitude of the Mediterranean, one in the latitude of Central Europe, and one further north. Most geologists conclude that these differences indicate zones of climate (not hitherto indicated), but it cannot be proved, and we may leave the matter open. At the same time the warm-loving corals disappear from Europe, with occasional advances. It is said that they are driven out by the disturbance of the waters, and, although this would hardly explain why they did not spread again in the tranquil chalk-ocean, we may again leave the point open.
In the early part of the Cretaceous, however, the Angiosperms (flowering plants) suddenly break into the chronicle of the earth, and spread with great rapidity. They appear abruptly in the east of the North American continent, in the region of Virginia and Maryland. They are small in stature and primitive in structure. Some are of generalised forms that are now unknown; some have leaves approaching those of the oak, willow, elm, maple, and walnut; some may be definitely described as fig, sassafras, aralia, myrica, etc. Eastern America, it may be recalled, is much higher than western until the close of the Cretaceous period. The Angiosperms do not spread much westward; they appear next in Greenland, and, before the middle of the Cretaceous, in Portugal. They have travelled over the North Atlantic continent, or what remains of it. The process seems very rapid as we write it, but it must be remembered that the first half of the Cretaceous period means a million or a million and a half years.
The cycads, and even the conifers, shrink before the higher type of tree. The landscape, in Europe and America, begins to wear a modern aspect. Long before the end of the Cretaceous most of the modern genera of Angiosperm trees have developed. To the fig and sassafras are now added the birch, beech, oak, poplar, walnut, willow, ivy, mulberry, holly, laurel, myrtle, maple, oleander, magnolia, plane, bread-fruit, and sweet-gum. Most of the American trees of to-day are known. The sequoias (the giant Californian trees) still represent the conifers in great abundance, with the eucalyptus and other plants that are now found only much further south. The ginkgoes struggle on for a time. The cycads dwindle enormously. Of 700 specimens in one early Cretaceous deposit only 96 are Angiosperms; of 460 species in a later deposit about 400 are Angiosperms. They oust the cycads in Europe and America, as the cycads and conifers had ousted the Cryptogams. The change in the face of the earth would be remarkable. Instead of the groves of palm-like cycads, with their large and flower-like fructifications, above which the pines and firs and cypresses reared their sombre forms, there were now forests of delicate-leaved maples, beeches, and oaks, bearing nutritious fruit for the coming race of animals. Grasses also and palms begin in the Cretaceous; though the grasses would at first be coarse and isolated tufts. Even flowers, of the lily family (apparently), are still detected in the crushed and petrified remains.
We will give some consideration later to the evolution of the Angiosperms. For the moment it is chiefly important to notice a feature of them to which the botanist pays less attention. In his technical view the Angiosperm is distinguished by the structure of its reproductive apparatus, its flowers, and some recent botanists wonder whether the key to this expansion of the flowering plants may not be found in a development of the insect world and of its relation to vegetation. In point of fact, we have no geological indication of any great development of the insects until the Tertiary Era, when we shall find them deploying into a vast army and producing their highest types. In any case, such a view leaves wholly unexplained the feature of the Angiosperms which chiefly concerns us. This is that most of them shed the whole of their leaves periodically, as the winter approaches. No such trees had yet been known on the earth. All trees hitherto had been evergreen, and we need a specific and adequate explanation why the earth is now covered, in the northern region, with forests of trees which show naked boughs and branches during a part of the year.
The majority of palaeontologists conclude at once, and quite confidently, from this rise and spread of the deciduous trees, that a winter season has at length set in on the earth, and that this new type of vegetation appears in response to an appreciable lowering of the climate. The facts, however, are somewhat complex, and we must proceed with caution. It would seem that any general lowering of the temperature of the earth ought to betray itself first in Greenland, but the flora of Greenland remains far "warmer," so to say, than the flora of Central Europe is to-day. Even toward the close of the Cretaceous its plants are much the same as those of America or of Central Europe. Its fossil remains of that time include forty species of ferns, as well as cycads, ginkgoes, figs, bamboos, and magnolias. Sir A. Geikie ventures to say that it must then have enjoyed a climate like that of the Cape or of Australia to-day. Professor Chamberlin finds its flora like that of "warm temperate" regions, and says that plants which then flourished in latitude 72 degrees are not now found above latitude 30 degrees.
There are, however, various reasons to believe that it is unsafe to draw deductions from the climate of Greenland. There is, it is true, some exaggeration in the statement that its climate was equivalent to that of Central Europe. The palms which flourished in Central Europe did not reach Greenland, and there are differences in the northern Molluscs and Echinoderms which—like the absence of corals above the north of England—point to a diversity of temperature. But we have no right to expect that there would be the same difference in temperature between Greenland and Central Europe as we find to-day. If the warm current which is now diverted to Europe across the Atlantic—the Gulf Stream—had then continued up the coast of America, and flowed along the coast of the land that united America and Europe, the climatic conditions would be very different from what they are. There is a more substantial reason. We saw that during the Mesozoic the Arctic continent was very largely submerged, and, while Europe and America rise again at the end of the Cretaceous, we find no rise of the land further north. A difference of elevation would, in such a world, make a great difference in temperature and moisture.
Let us examine the animal record, however, before we come to any conclusion. The chronicle of the later Cretaceous is a story of devastation. The reduction of the cyeads is insignificant beside the reduction or annihilation of the great animals of the Mesozoic world. The skeletons of the Deinosaurs become fewer and fewer as we ascend the upper Cretaceous strata. In the uppermost layer (Laramie) we find traces of a last curious expansion—the group of horned reptiles, of the Triceratops type, which we described as the last of the great reptiles. The Ichthyosaurs and Plesiosaurs vanish from the waters. The "sea-serpents" (Mososaurs) pass away without a survivor. The flying dragons, large and small, become entirely extinct. Only crocodiles, lizards, turtle, and snakes cross the threshold of the Tertiary Era. In one single region of America (Puerco beds) some of the great reptiles seem to be making a last stand against the advancing enemy in the dawn of the Tertiary Era, but the exact date of the beds is disputed, and in any case their fight is soon over. Something has slain the most formidable race that the earth had yet known, in spite of its marvellous adaptation to different environments in its innumerable branches.
We turn to the seas, and find an equal carnage among some of its most advanced inhabitants. The great cuttlefish-like Belemnites and the whole race of the Ammonites, large and small, are banished from the earth. The fall of the Ammonites is particularly interesting, and has inspired much more or less fantastic speculation. The shells begin to assume such strange forms that observers speak occasionally of the "convulsions" or "death-contortions" of the expiring race. Some of the coiled shells take on a spiral form, like that of a snail's shell. Some uncoil the shell, and seem to be returning toward the primitive type. A rich eccentricity of frills and ornamentation is found more or less throughout the whole race. But every device—if we may so regard these changes—is useless, and the devastating agency of the Cretaceous, whatever it was, removes the Ammonites and Belemnites from the scene. The Mollusc world, like the world of plants and of reptiles, approaches its modern aspect.
In the fish world, too, there is an effective selection in the course of the Cretaceous. All the fishes of modern times, except the large family of the sharks, rays, skates, and dog-fishes (Elasmobranchs), the sturgeon and chimaera, the mud-fishes, and a very few other types, are Teleosts, or bony-framed fishes—the others having cartilaginous frames. None of the Teleosts had appeared until the end of the Jurassic. They now, like the flowering plants on land, not only herald the new age, but rapidly oust the other fishes, except the unconquerable shark. They gradually approach the familiar types of Teleosts, so that we may say that before the end of the Cretaceous the waters swarmed with primitive and patriarchal cod, salmon, herring, perch, pike, bream, eels, and other fishes. Some of them grew to an enormous size. The Portheus, an American pike, seems to have been about eight feet long; and the activity of an eight-foot pike may be left to the angler's imagination. All, however, are, as evolution demands, of a generalised and unfamiliar type: the material out of which our fishes will be evolved.
Of the insects we have very little trace in the Cretaceous. We shall find them developing with great richness in the following period, but, imperfect as the record is, we may venture to say that they were checked in the Cretaceous. There were good conditions for preserving them, but few are preserved. And of the other groups of invertebrates we need only say that they show a steady advance toward modern types. The sea-lily fills the rocks no longer; the sea-urchin is very abundant. The Molluscs gain on the more lowly organised Brachiopods.
To complete the picture we must add that higher types probably arose in the later Cretaceous which do not appear in the records. This is particularly true of the birds and mammals. We find them spreading so early in the Tertiary that we must put back the beginning of the expansion to the Cretaceous. As yet, however, the only mammal remains we find are such jaws and teeth of primitive mammals as we have already described. The birds we described (after the Archaeopteryx) also belong to the Cretaceous, and they form another of the doomed races. Probably the modern birds were already developing among the new vegetation on the higher ground.
These are the facts of Cretaceous life, as far as the record has yielded them, and it remains for us to understand them. Clearly there has been a great selective process analogous to, if not equal to, the winnowing process at the end of the Palaeozoic. As there has been a similar, if less considerable, upheaval of the land, we are at once tempted to think that the great selective agency was a lowering of the temperature. When we further find that the most important change in the animal world is the destruction of the cold-blooded reptiles, which have no concern for the young, and the luxuriant spread of the warm-blooded animals, which do care for their young, the idea is greatly confirmed. When we add that the powerful Molluscs which are slain, while the humbler Molluscs survive, are those which—to judge from the nautilus and octopus—love warm seas, the impression is further confirmed. And when we finally reflect that the most distinctive phenomenon of the period is the rapid spread of deciduous trees, it would seem that there is only one possible interpretation of the Cretaceous Revolution.
This interpretation—that cold was the selecting agency—is a familiar idea in geological literature, but, as I said, there are recent writers who profess reserve in regard to it, and it is proper to glance at, or at least look for, the alternatives.
Before doing so let us be quite clear that here we have nothing to do with theories of the origin of the earth. The Permian cold—which, however, is universally admitted—is more or less entangled in that controversy; the Cretaceous cold has no connection with it. Whatever excess of carbon-dioxide there may have been in the early atmosphere was cleared by the Coal-forests. We must set aside all these theories in explaining the present facts.
It is also useful to note that the fact that there have been great changes in the climate of the earth in past time is beyond dispute. There is no denying the fact that the climate of the earth was warm from the Arctic to the Antarctic in the Devonian and Carboniferous periods: that it fell considerably in the Permian: that it again became at least "warm temperate" (Chamberlin) from the Arctic to the Antarctic in the Jurassic, and again in the Eocene: that some millions of square miles of Europe and North America were covered with ice and snow in the Pleistocene, so that the reindeer wandered where palms had previously flourished and the vine flourishes to-day; and that the pronounced zones of climate which we find today have no counterpart in any earlier age. In view of these great and admitted fluctuations of the earth's temperature one does not see any reason for hesitating to admit a fall of temperature in the Cretaceous, if the facts point to it.
On the other hand, the alternative suggestions are not very convincing. We have noticed one of these suggestions in connection with the origin of the Angiosperms. It hints that this may be related to developments of the insect world. Most probably the development of the characteristic flowers of the Angiosperms is connected with an increasing relation to insects, but what we want to understand especially is the deciduous character of their leaves. Many of the Angiosperms are evergreen, so that it cannot be said that the one change entailed the other. In fact, a careful study of the leaves preserved in the rocks seems to show the deciduous Angiosperms gaining on the evergreens at the end of the Cretaceous. The most natural, it not the only, interpretation of this is that the temperature is falling. Deciduous trees shed their leaves so as to check their transpiration when a season comes on in which they cannot absorb the normal amount of moisture. This may occur either at the on-coming of a hot, dry season or of a cold season (in which the roots absorb less). Everything suggests that the deciduous tree evolved to meet an increase of cold, not of heat.
Another suggestion is that animals and plants were not "climatically differentiated" until the Cretaceous period; that is to say, that they were adapted to all climates before that time, and then began to be sensitive to differences of climate, and live in different latitudes. But how and why they should suddenly become differentiated in this way is so mysterious that one prefers to think that, as the animal remains also suggest, there were no appreciable zones of climate until the Cretaceous. The magnolia, for instance, flourished in Greenland in the early Tertiary, and has to live very far south of it to-day. It is much simpler to assume that Greenland changed—as a vast amount of evidence indicates—than that the magnolia changed.
Finally, to explain the disappearance of the Mesozoic reptiles without a fall in temperature, it is suggested that they were exterminated by the advancing mammals. It is assumed that the spreading world of the Angiospermous plants somewhere met the spread of the advancing mammals, and opened out a rich new granary to them. This led to so powerful a development of the mammals that they succeeded in overthrowing the reptiles.
There are several serious difficulties in the way of this theory. The first and most decisive is that the great reptiles have practically disappeared before the mammals come on the scene. Only in one series of beds (Puerco) in America, representing an early period of the Tertiary Era, do we find any association of their remains; and even there it is not clear that they were contemporary. Over the earth generally the geological record shows the great reptiles dying from some invisible scourge long before any mammal capable of doing them any harm appears; even if we suppose that the mammal mainly attacked the eggs and the young. We may very well believe that more powerful mammals than the primitive Mesozoic specimens were already developed in some part of the earth—say, Africa—and that the rise of the land gave them a bridge across the Mediterranean to Europe. Probably this happened; but the important point is that the reptiles were already almost extinct. The difficulty is even greater when we reflect that it is precisely the most powerful reptiles (Deinosaurs) and least accessible reptiles (Pterosaurs, Ichthyosaurs, etc.) which disappear, while the smaller land and water reptiles survive and retreat southward—where the mammals are just as numerous. That assuredly is not the effect of an invasion of carnivores, even if we could overlook the absence of such carnivores from the record until after the extinction of the reptiles in most places.
I have entered somewhat fully into this point, partly because of its great interest, but partly lest it be thought that I am merely reproducing a tradition of geological literature without giving due attention to the criticisms of recent writers. The plain and common interpretation of the Cretaceous revolution—that a fall in temperature was its chief devastating agency—is the only one that brings harmony into all the facts. The one comprehensive enemy of that vast reptile population was cold. It was fatal to the adult because he had a three-chambered heart and no warm coat; it was fatal to the Mesozoic vegetation on which, directly or indirectly, he fed; it was fatal to his eggs and young because the mother did not brood over the one or care for the other. It was fatal to the Pterosaurs, even if they were warm-blooded, because they had no warm coats and did not (presumably) hatch their eggs; and it was equally fatal to the viviparous Ichthyosaurs. It is the one common fate that could slay all classes. When we find that the surviving reptiles retreat southward, only lingering in Europe during the renewed warmth of the Eocene and Miocene periods, this interpretation is sufficiently confirmed. And when we recollect that these things coincide with the extinction of the Ammonites and Belemnites, and the driving of their descendants further south, as well as the rise and triumph of deciduous trees, it is difficult to see any ground for hesitating.
But we need not, and must not, imagine a period of cold as severe, prolonged, and general as that of the Permian period. The warmth of the Jurassic period is generally attributed to the low relief of the land, and the very large proportion of water-surface. The effect of this would be to increase the moisture in the atmosphere. Whether this was assisted by any abnormal proportion of carbon-dioxide, as in the Carboniferous, we cannot confidently say. Professor Chamberlin observes that, since the absorbing rock-surface was greatly reduced in the Jurassic, the carbon-dioxide would tend to accumulate in its atmosphere, and help to explain the high temperature. But the great spread of vegetation and the rise of land in the later Jurassic and the Cretaceous would reduce this density of the atmosphere, and help to lower the temperature.
It is clear that the cold would at first be local. In fact, it must be carefully realised that, when we speak of the Jurassic period as a time of uniform warmth, we mean uniform at the same altitude. Everybody knows the effect of rising from the warm, moist sea-level to the top of even a small inland elevation. There would be such cooler regions throughout the Jurassic, and we saw that there were considerable upheavals of land towards its close. To these elevated lands we may look for the development of the Angiosperms, the birds, and the mammals. When the more massive rise of land came at the end of the Cretaceous, the temperature would fall over larger areas, and connecting ridges would be established between one area and another. The Mesozoic plants and animals would succumb to this advancing cold. What precise degree of cold was necessary to kill the reptiles and Cephalopods, yet allow certain of the more delicate flowering plants to live, is yet to be determined. The vast majority of the new plants, with their winter sleep, would thrive in the cooler air, and, occupying the ground of the retreating cycads and ginkgoes would prepare a rich harvest for the coming birds and mammals.