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.



CHAPTER XV. THE TERTIARY ERA

We have already traversed nearly nine-tenths of the story of terrestrial life, without counting the long and obscure Archaean period, and still find ourselves in a strange and unfamiliar earth. With the close of the Chalk period, however, we take a long stride in the direction of the modern world. The Tertiary Era will, in the main, prove a fresh period of genial warmth and fertile low-lying regions. During its course our deciduous trees and grasses will mingle with the palms and pines over the land, our flowers will begin to brighten the landscape, and the forms of our familiar birds and mammals, even the form of man, will be discernible in the crowds of animals. At its close another mighty period of selection will clear the stage for its modern actors.

A curious reflection is prompted in connection with this division of the earth's story into periods of relative prosperity and quiescence, separated by periods of disturbance. There was—on the most modest estimate—a stretch of some fifteen million years between the Cambrian and the Permian upheavals. On the same chronological scale the interval between the Permian and Cretaceous revolutions was only about seven million years, and the Tertiary Era will comprise only about three million years. One wonders if the Fourth (Quaternary) Era in which we live will be similarly shortened. Further, whereas the earth returned after each of the earlier upheavals to what seems to have been its primitive condition of equable and warm climate, it has now entirely departed from that condition, and exhibits very different zones of climate and a succession of seasons in the year. One wonders what the climate of the earth will become long before the expiration of those ten million years which are usually assigned as the minimum period during which the globe will remain habitable.

It is premature to glance at the future, when we are still some millions of years from the present, but it will be useful to look more closely at the facts which inspire this reflection. From what we have seen, and shall further see, it is clear that, in spite of all the recent controversy about climate among our geologists, there has undeniably been a progressive refrigeration of the globe. Every geologist, indeed, admits "oscillations of climate," as Professor Chamberlin puts it. But amidst all these oscillations we trace a steady lowering of the temperature. Unless we put a strained and somewhat arbitrary interpretation on the facts of the geological record, earlier ages knew nothing of our division of the year into pronounced seasons and of the globe into very different climatic zones. It might plausibly be suggested that we are still living in the last days of the Ice-Age, and that the earth may be slowly returning to a warmer condition. Shackleton, it might be observed, found that there has been a considerable shrinkage of the south polar ice within the period of exploration. But we shall find that a difference of climate, as compared with earlier ages, was already evident in the middle of the Tertiary Era, and it is far more noticeable to-day.

We do not know the causes of this climatic evolution—the point will be considered more closely in connection with the last Ice-Age—but we see that it throws a flood of light on the evolution of organisms. It is one of the chief incarnations of natural selection. Changes in the distribution of land and water and in the nature of the land-surface, the coming of powerful carnivores, and other agencies which we have seen, have had their share in the onward impulsion of life, but the most drastic agency seems to have been the supervention of cold. The higher types of both animals and plants appear plainly in response to a lowering of temperature. This is the chief advantage of studying the story of evolution in strict connection with the geological record. We shall find that the record will continue to throw light on our path to the end, but, as we are now about to approach the most important era of evolution, and as we have now seen so much of the concrete story of evolution, it will be interesting to examine briefly some other ways of conceiving that story.

We need not return to the consideration of the leading schools of evolution, as described in a former chapter. Nothing that we have seen will enable us to choose between the Lamarckian and the Weismannist hypothesis; and I doubt if anything we are yet to see will prove more decisive. The dispute is somewhat academic, and not vital to a conception of evolution. We shall, for instance, presently follow the evolution of the horse, and see four of its toes shrink and disappear, while the fifth toe is enormously strengthened. In the facts themselves there is nothing whatever to decide whether this evolution took place on the lines suggested by Weismann, or on the lines suggested by Lamarck and accepted by Darwin. It will be enough for us merely to establish the fact that the one-toed horse is an evolved descendant of a primitive five-toed mammal, through the adaptation of its foot to running on firm ground, its teeth and neck to feeding on grasses, and so on.

On the other hand, the facts we have already seen seem to justify the attitude of compromise I adopted in regard to the Mutationist theory. It would be an advantage in many ways if we could believe that new species arose by sudden and large variations (mutations) of the young from the parental type. In the case of many organs and habits it is extremely difficult to see how a gradual development, by a slow accentuation of small variations, is possible. When we further find that experimenters on living species can bring about such mutations, and when we reflect that there must have been acute disturbances in the surroundings of animals and plants sometimes, we are disposed to think that many a new species may have arisen in this way. On the other hand, while the palaeontological record can never prove that a species arose by mutations, it does sometimes show that species arise by very gradual modification. The Chalk period, which we have just traversed, affords a very clear instance. One of our chief investigators of the English Chalk, Dr. Rowe, paid particular attention to the sea-urchins it contains, as they serve well to identify different levels of chalk. He discovered, not merely that they vary from level to level, but that in at least one genus (Micraster) he could trace the organism very gradually passing from one species to another, without any leap or abruptness. It is certainly significant that we find such cases as this precisely where the conditions of preservation are exceptionally good. We must conclude that species arise, probably, both by mutations and small variations, and that it is impossible to say which class of species has been the more numerous.

There remain one or two conceptions of evolution which we have not hitherto noticed, as it was advisable to see the facts first. One of these is the view—chiefly represented in this country by Professor Henslow—that natural selection has had no part in the creation of species; that the only two factors are the environment and the organism which responds to its changes. This is true enough in the sense that, as we saw, natural selection is not an action of nature on the "fit," but on the unfit or less fit. But this does not in the least lessen the importance of natural selection. If there were not in nature this body of destructive agencies, to which we apply the name natural selection, there would be little—we cannot say no—evolution. But the rising carnivores, the falls of temperature, etc., that we have studied, have had so real, if indirect, an influence on the development of life that we need not dwell on this.

Another school, or several schools, while admitting the action of natural selection, maintain that earlier evolutionists have made nature much too red in tooth and claw. Dr. Russel Wallace from one motive, and Prince Krapotkin from another, have insisted that the triumphs of war have been exaggerated, and the triumphs of peace, or of social co-operation, far too little appreciated. It will be found that such writers usually base their theory on life as we find it in nature to-day, where the social principle is highly developed in many groups of animals. This is most misleading, since social co-operation among animals, as an instrument of progress, is (geologically speaking) quite a recent phenomenon. Nearly every group of animals in which it is found belongs, to put it moderately, to the last tenth of the story of life, and in some of the chief instances the animals have only gradually developed social life. [*] The first nine-tenths of the chronicle of evolution contain no indication of social life, except—curiously enough—in such groups as the Sponges, Corals, and Bryozoa, which are amongst the least progressive in nature. We have seen plainly that during the overwhelmingly greater part of the story of life the predominant agencies of evolution were struggle against adverse conditions and devouring carnivores; and we shall find them the predominant agencies throughout the Tertiary Era.

* Thus the social nature of man is sometimes quoted as one of the chief causes of his development. It is true that it has much to do with his later development, but we shall see that the statement that man was from the start a social being is not at all warranted by the facts. On the other hand, it may be pointed out that the ants and termites had appeared in the Mesozoic. We shall see some evidence that the remarkable division of labour which now characterises their life did not begin until a much later period, so that we have no evidence of social life in the early stages.

Yet we must protest against the exaggerated estimate of the conscious pain which so many read into these millions of years of struggle. Probably there was no consciousness at all during the greater part of the time. The wriggling of the worm on which you have accidentally trodden is no proof whatever that you have caused conscious pain. The nervous system of an animal has been so evolved as to respond with great disturbance of its tissue to any dangerous or injurious assault. It is the selection of a certain means of self-preservation. But at what level of life the animal becomes conscious of this disturbance, and "feels pain," it is very difficult to determine. The subject is too vast to be opened here. In a special investigation of it. [*] I concluded that there is no proof of the presence of any degree of consciousness in the invertebrate world even in the higher insects; that there is probably only a dull, blurred, imperfect consciousness below the level of the higher mammals and birds; and that even the consciousness of an ape is something very different from what educated Europeans, on the ground of their own experience, call consciousness. It is too often forgotten that pain is in proportion to consciousness. We must beware of such fallacies as transferring our experience of pain to a Mesozoic reptile, with an ounce or two of cerebrum to twenty tons of muscle and bone.

* "The Evolution of Mind" (Black), 1911.

One other view of evolution, which we find in some recent and reputable works (such as Professor Geddes and Thomson's "Evolution," 1911), calls for consideration. In the ordinary Darwinian view the variations of the young from their parents are indefinite, and spread in all directions. They may continue to occur for ages without any of them proving an advantage to their possessors. Then the environment may change, and a certain variation may prove an advantage, and be continuously and increasingly selected. Thus these indefinite variations may be so controlled by the environment during millions of years that the fish at last becomes an elephant or a man. The alternative view, urged by a few writers, is that the variations were "definitely directed." The phrase seems merely to complicate the story of evolution with a fresh and superfluous mystery. The nature and precise action of this "definite direction" within the organism are quite unintelligible, and the facts seem explainable just as well—or not less imperfectly—without as with this mystic agency. Radiolaria, Sponges, Corals, Sharks, Mudfishes, Duckbills, etc., do not change (except within the limits of their family) during millions of years, because they keep to an environment to which they are fitted. On the other hand, certain fishes, reptiles, etc., remain in a changing environment, and they must change with it. The process has its obscurities, but we make them darker, it seems to me, with these semi-metaphysical phrases.

It has seemed advisable to take this further glance at the general principles and current theories of evolution before we extend our own procedure into the Tertiary Era. The highest types of animals and plants are now about to appear on the stage of the earth; the theatre itself is about to take on a modern complexion. The Middle Ages are over; the new age is breaking upon the planet. We will, as before, first survey the Tertiary Era as a whole, with the momentous changes it introduces, and then examine, in separate chapters, the more important phases of its life.

It opens, like the preceding and the following era, with "the area of land large and its relief pronounced." This is the outcome of the Cretaceous revolution. Southern Europe and Southern Asia have risen, and shaken the last masses of the Chalk ocean from their faces; the whole western fringe of America has similarly emerged from the sea that had flooded it. In many parts, as in England (at that time a part of the Continent), there is so great a gap between the latest Cretaceous and the earliest Tertiary strata that these newly elevated lands must evidently have stood out of the waters for a prolonged period. On their cooler plains the tragedy of the extinction of the great reptiles comes to an end. The cyeads and ginkgoes have shrunk into thin survivors of the luxuriant Mesozoic groves. The oak and beech and other deciduous trees spread slowly over the successive lands, amid the glare and thunder of the numerous volcanoes which the disturbance of the crust has brought into play. New forms of birds fly from tree to tree, or linger by the waters; and strange patriarchal types of mammals begin to move among the bones of the stricken reptiles.

But the seas and the rains and rivers are acting with renewed vigour on the elevated lands, and the Eocene period closes in a fresh age of levelling. Let us put the work of a million years or so in a sentence. The southern sea, which has been confined almost to the limits of our Mediterranean by the Cretaceous upheaval, gradually enlarges once more. It floods the north-west of Africa almost as far as the equator; it covers most of Italy, Turkey, Austria, and Southern Russia; it spreads over Asia Minor, Persia, and Southern Asia, until it joins the Pacific; and it sends a long arm across the Franco-British region, and up the great valley which is now the German Ocean.

From earlier chapters we now expect to find a warmer climate, and the record gives abundant proof of it. To this period belongs the "London Clay," in whose thick and—to the unskilled eye—insignificant bed the geologist reads the remarkable story of what London was two or three million years ago. It tells us that a sea, some 500 or 600 feet deep, then lay over that part of England, and fragments of the life of the period are preserved in its deposit. The sea lay at the mouth of a sub-tropical river on whose banks grew palms, figs, ginkgoes, eucalyptuses, almonds, and magnolias, with the more familiar oaks and pines and laurels. Sword-fishes and monstrous sharks lived in the sea. Large turtles and crocodiles and enormous "sea-serpents" lingered in this last spell of warmth that Central Europe would experience. A primitive whale appeared in the seas, and strange large tapir-like mammals—remote ancestors of our horses and more familiar beasts—wandered heavily on the land. Gigantic primitive birds, sometimes ten feet high, waded by the shore. Deposits of the period at Bournemouth and in the Isle of Wight tell the same story of a land that bore figs, vines, palms, araucarias, and aralias, and waters that sheltered turtles and crocodiles. The Parisian region presented the same features.

In fact, one of the most characteristic traces of the southern sea which then stretched from England to Africa in the south and India in the east indicates a warm climate. It will be remembered that the Cretaceous ocean over Southern Europe had swarmed with the animalcules whose dead skeletons largely compose our chalk-beds. In the new southern ocean another branch of these Thalamophores, the Nummulites, spreads with such portentous abundance that its shells—sometimes alone, generally with other material—make beds of solid limestone several thousand feet in thickness. The pyramids are built of this nummulitic limestone. The one-celled animal in its shell is, however, no longer a microscopic grain. It sometimes forms wonderful shells, an inch or more in diameter, in which as many as a thousand chambers succeed each other, in spiral order, from the centre. The beds containing it are found from the Pyrenees to Japan.

That this vast warm ocean, stretching southward over a large part of what is now the Sahara, should give a semitropical aspect even to Central Europe and Asia is not surprising. But this genial climate was still very general over the earth. Evergreens which now need the warmth of Italy or the Riviera then flourished in Lapland and Spitzbergen. The flora of Greenland—a flora that includes magnolias, figs, and bamboos—shows us that its temperature in the Eocene period must have been about 30 degrees higher than it is to-day. [*] The temperature of the cool Tyrol of modern Europe is calculated to have then been between 74 and 81 degrees F. Palms, cactuses, aloes, gum-trees, cinnamon trees, etc., flourished in the latitude of Northern France. The forests that covered parts of Switzerland which are now buried in snow during a great part of the year were like the forests one finds in parts of India and Australia to-day. The climate of North America, and of the land which still connected it with Europe, was correspondingly genial.

* The great authority on Arctic geology, Heer, who makes this calculation, puts this flora in the Miocene. It is now usually considered that these warmer plants belong to the earlier part of the Tertiary era.

This indulgent period (the Oligocene, or later part of the Eocene), scattering a rich and nutritious vegetation with great profusion over the land, led to a notable expansion of animal life. Insects, birds, and mammals spread into vast and varied groups in every land. Had any of the great Mesozoic reptiles survived, the warmer age might have enabled them to dispute the sovereignty of the advancing mammals. But nothing more formidable than the turtle, the snake, and the crocodile (confined to the waters) had crossed the threshold of the Tertiary Era, and the mammals and birds had the full advantage of the new golden age. The fruits of the new trees, the grasses which now covered the plains, and the insects which multiplied with the flowers afforded a magnificent diet. The herbivorous mammals became a populous world, branching into numerous different types according to their different environments. The horse, the elephant, the camel, the pig, the deer, the rhinoceros gradually emerge out of the chaos of evolving forms. Behind them, hastening the course of their evolution, improving their speed, arms, and armour, is the inevitable carnivore. He, too, in the abundance of food, grows into a vast population, and branches out toward familiar types. We will devote a chapter presently to this remarkable phase of the story of evolution.

But the golden age closes, as all golden ages had done before it, and for the same reason. The land begins to rise, and cast the warm shallow seas from its face. The expansion of life has been more rapid and remarkable than it had ever been before, in corresponding periods of abundant food and easy conditions; the contraction comes more quickly than it had ever done before. Mountain masses begin to rise in nearly all parts of the world. The advance is slow and not continuous, but as time goes on the Atlas, Alps, Pyrenees, Apennines, Caucasus, Himalaya, Rocky Mountains, and Andes rise higher and higher. When the geologist looks to-day for the floor of the Eocene ocean, which he recognises by the shells of the Nummulites, he finds it 10,000 feet above the sea-level in the Alps, 16,000 feet above the sea-level in the Himalaya, and 20,000 feet above the sea-level in Thibet. One need not ask why the regions of London and Paris fostered palms and magnolias and turtles in Tertiary times, and shudder in their dreary winter to-day.

The Tertiary Era is divided by geologists into four periods: the Eocene, Oligocene, Miocene, and Pliocene. "Cene" is our barbaric way of expressing the Greek word for "new," and the classification is meant to mark the increase of new (or modern and actual) types of life in the course of the Tertiary Era. Many geologists, however, distrust the classification, and are disposed to divide the Tertiary into two periods. From our point of view, at least, it is advisable to do this. The first and longer half of the Tertiary is the period in which the temperature rises until Central Europe enjoys the climate of South Africa; the second half is the period in which the land gradually rises, and the temperature falls, until glaciers and sheets of ice cover regions where the palm and fig had flourished.

The rise of the land had begun in the first half of the Tertiary, but had been suspended. The Pyrenees and Apennines had begun to rise at the end of the Eocene, straining the crust until it spluttered with volcanoes, casting the nummulitic sea off large areas of Southern Europe. The Nummulites become smaller and less abundant. There is also some upheaval in North America, and a bridge of land begins to connect the north and south, and permit an effective mingling of their populations. But the advance is, as I said, suspended, and the Oligocene period maintains the golden age. With the Miocene period the land resumes its rise. A chill is felt along the American coast, showing a fall in the temperature of the Atlantic. In Europe there is a similar chill, and a more obvious reason for it. There is an ascending movement of the whole series of mountains from Morocco and the Pyrenees, through the Alps, the Caucasus, and the Carpathians, to India and China. Large lakes still lie over Western Europe, but nearly the whole of it emerges from the ocean. The Mediterranean still sends an arm up France, and with another arm encircles the Alpine mass; but the upheaval continues, and the great nummulitic sea is reduced to a series of extensive lakes, cut off both from the Atlantic and Pacific. The climate of Southern Europe is probably still as genial as that of the Canaries to-day. Palms still linger in the landscape in reduced numbers.

The last part of the Tertiary, the Pliocene, opens with a slight return of the sea. The upheaval is once more suspended, and the waters are eating into the land. There is some foundering of land at the south-western tip of Europe; the "Straits of Gibraltar" begin to connect the Mediterranean with the Atlantic, and the Balearic Islands, Corsica, and Sardinia remain as the mountain summits of a submerged land. Then the upheaval is resumed, in nearly every part of the earth.

Nearly every great mountain chain that the geologist has studied shared in this remarkable movement at the end of the Tertiary Era. The Pyrenees, Alps, Himalaya, etc., made their last ascent, and attained their present elevation. And as the land rose, the aspect of Europe and America slowly altered. The palms, figs, bamboos, and magnolias disappeared; the turtles, crocodiles, flamingoes, and hippopotamuses retreated toward the equator. The snow began to gather thick on the rising heights; then the glaciers began to glitter on their flanks. As the cold increased, the rivers of ice which flowed down the hills of Switzerland, Spain, Scotland, or Scandinavia advanced farther and farther over the plains. The regions of green vegetation shrank before the oncoming ice, the animals retreated south, or developed Arctic features. Europe and America were ushering in the great Ice-Age, which was to bury five or six million square miles of their territory under a thick mantle of ice.

Such is the general outline of the story of the Tertiary Era. We approach the study of its types of life and their remarkable development more intelligently when we have first given careful attention to this extraordinary series of physical changes. Short as the Era is, compared with its predecessors, it is even more eventful and stimulating than they, and closes with what Professor Chamberlin calls "the greatest deformative movements in post-Cambrian history." In the main it has, from the evolutionary point of view, the same significant character as the two preceding eras. Its middle portion is an age of expansion, indulgence, exuberance, in which myriads of varied forms are thrown upon the scene, its later part is an age of contraction, of annihilation, of drastic test, in which the more effectively organised will be chosen from the myriads of types. Once more nature has engendered a vast brood, and is about to select some of her offspring to people the modern world. Among the types selected will be Man.



CHAPTER XVI. THE FLOWER AND THE INSECT

AS we approach the last part of the geological record we must neglect the lower types of life, which have hitherto occupied so much of our attention, so that we may inquire more fully into the origin and fortunes of the higher forms which now fill the stage. It may be noted, in general terms, that they shared the opulence of the mid-Tertiary period, produced some gigantic specimens of their respective families, and evolved into the genera, and often the species, which we find living to-day. A few illustrations will suffice to give some idea of the later development of the lower invertebrates and vertebrates.

Monstrous oysters bear witness to the prosperity of that ancient and interesting family of the Molluscs. In some species the shells were commonly ten inches long; the double shell of one of these Tertiary bivalves has been found which measured thirteen inches in length, eight in width, and six in thickness. In the higher branch of the Mollusc world the naked Cephalopods (cuttle-fish, etc.) predominate over the nautiloids—the shrunken survivors of the great coiled-shell race. Among the sharks, the modern Squalodonts entirely displace the older types, and grow to an enormous size. Some of the teeth we find in Tertiary deposits are more than six inches long and six inches broad at the base. This is three times the size of the teeth of the largest living shark, and it is therefore believed that the extinct possessor of these formidable teeth (Carcharodon megalodon) must have been much more than fifty, and was possibly a hundred, feet in length. He flourished in the waters of both Europe and America during the halcyon days of the Tertiary Era. Among the bony fishes, all our modern and familiar types appear.

The amphibia and reptiles also pass into their modern types, after a period of generous expansion. Primitive frogs and toads make their first appearance in the Tertiary, and the remains are found in European beds of four-foot-long salamanders. More than fifty species of Tertiary turtles are known, and many of them were of enormous size. One carapace that has been found in a Tertiary bed measures twelve feet in length, eight feet in width, and seven feet in height to the top of the back. The living turtle must have been nearly twenty feet long. Marine reptiles, of a snake-like structure, ran to fifteen feet in length. Crocodiles and alligators swarmed in the rivers of Europe until the chilly Pliocene bade them depart to Africa.

In a word, it was the seven years of plenty for the whole living world, and the expansive development gave birth to the modern types, which were to be selected from the crowd in the subsequent seven years of famine. We must be content to follow the evolution of the higher types of organisms. I will therefore first describe the advance of the Tertiary vegetation, the luxuriance of which was the first condition of the great expansion of animal life; then we will glance at the grand army of the insects which followed the development of the flowers, and at the accompanying expansion and ramification of the birds. The long and interesting story of the mammals must be told in a separate chapter, and a further chapter must be devoted to the appearance of the human species.

We saw that the Angiosperms, or flowering plants, appeared at the beginning of the Cretaceous period, and were richly developed before the Tertiary Era opened. We saw also that their precise origin is unknown. They suddenly invade a part of North America where there were conditions for preserving some traces of them, but we have as yet no remains of their early forms or clue to their place of development. We may conjecture that their ancestors had been living in some elevated inland region during the warmth of the Jurassic period.

As it is now known that many of the cycad-like Mesozoic plants bore flowers—as the modern botanist scarcely hesitates to call them—the gap between the Gymnosperms and Angiosperms is very much lessened. There are, however, structural differences which forbid us to regard any of these flowering cycads, which we have yet found, as the ancestors of the Angiosperms. The most reasonable view seems to be that a small and local branch of these primitive flowering plants was evolved, like the rest, in the stress of the Permian-Triassic cold; that, instead of descending to the warm moist levels with the rest at the end of the Triassic, and developing the definite characters of the cycad, it remained on the higher and cooler land; and that the rise of land at the end of the Jurassic period stimulated the development of its Angiosperm features, enlarged the area in which it was especially fitted to thrive, and so permitted it to spread and suddenly break into the geological record as a fully developed Angiosperm.

As the cycads shrank in the Cretaceous period, the Angiosperms deployed with great rapidity, and, spreading at various levels and in different kinds of soils and climates, branched into hundreds of different types. We saw that the oak, beech, elm, maple, palm, grass, etc., were well developed before the end of the Cretaceous period. The botanist divides the Angiosperms into two leading groups, the Monocotyledons (palms, grasses, lilies, orchises, irises, etc.) and Dicotyledons (the vast majority), and it is now generally believed that the former were developed from an early and primitive branch of the latter. But it is impossible to retrace the lines of development of the innumerable types of Angiosperms. The geologist has mainly to rely on a few stray leaves that were swept into the lakes and preserved in the mud, and the evidence they afford is far too slender for the construction of genealogical trees. The student of living plants can go a little further in discovering relationships, and, when we find him tracing such apparently remote plants as the apple and the strawberry to a common ancestor with the rose, we foresee interesting possibilities on the botanical side. But the evolution of the Angiosperms is a recent and immature study, and we will be content with a few reflections on the struggle of the various types of trees in the changing conditions of the Tertiary, the development of the grasses, and the evolution of the flower. In other words, we will be content to ask how the modern landscape obtained its general vegetal features.

Broadly speaking, the vegetation of the first part of the Tertiary Era was a mixture of sub-tropical and temperate forms, a confused mass of Ferns, Conifers, Ginkgoales, Monocotyledons, and Dicotyledons. Here is a casual list of plants that then grew in the latitude of London and Paris: the palm, magnolia, myrtle, Banksia, vine, fig, aralea, sequoia, eucalyptus, cinnamon tree, cactus, agave, tulip tree, apple, plum, bamboo, almond, plane, maple, willow, oak, evergreen oak, laurel, beech, cedar, etc. The landscape must have been extraordinarily varied and beautiful and rich. To one botanist it suggests Malaysia, to another India, to another Australia.

It is really the last gathering of the plants, before the great dispersion. Then the cold creeps slowly down from the Arctic regions, and begins to reduce the variety. We can clearly trace its gradual advance. In the Carboniferous and Jurassic the vegetation of the Arctic regions had been the same as that of England; in the Eocene palms can flourish in England, but not further north; in the Pliocene the palms and bamboos and semi-tropical species are driven out of Europe; in the Pleistocene the ice-sheet advances to the valleys of the Thames and the Danube (and proportionately in the United States), every warmth-loving species is annihilated, and our grasses, oaks, beeches, elms, apples, plums, etc., linger on the green southern fringe of the Continent, and in a few uncovered regions, ready to spread north once more as the ice creeps back towards the Alps or the Arctic circle. Thus, in few words, did Europe and North America come to have the vegetation we find in them to-day.

The next broad characteristic of our landscape is the spreading carpet of grass. The interest of the evolution of the grasses will be seen later, when we shall find the evolution of the horse, for instance, following very closely upon it. So striking, indeed, is the connection between the advance of the grasses and the advance of the mammals that Dr. Russel Wallace has recently claimed ("The World of Life," 1910) that there is a clear purposive arrangement in the whole chain of developments which leads to the appearance of the grasses. He says that "the very puzzling facts" of the immense reptilian development in the Mesozoic can only be understood on the supposition that they were evolved "to keep down the coarser vegetation, to supply animal food for the larger Carnivora, and thus give time for higher forms to obtain a secure foothold and a sufficient amount of varied form and structure" (p. 284).

Every insistence on the close connection of the different strands in the web of life is welcome, but Dr. Wallace does not seem to have learned the facts accurately. There is nothing "puzzling" about the Mesozoic reptilian development; the depression of the land, the moist warmth, and the luscious vegetation of the later Triassic and the Jurassic amply explain it. Again, the only carnivores to whom they seem to have supplied food were reptiles of their own race. Nor can the feeding of the herbivorous reptiles be connected with the rise of the Angiosperms. We do not find the flowering plants developing anywhere in those vast regions where the great reptiles abounded; they invade them from some single unknown region, and mingle with the pines and ginkgoes, while the cyeads alone are destroyed.

The grasses, in particular, do not appear until the Cretaceous, and do not show much development until the mid-Tertiary; and their development seems to be chiefly connected with physical conditions. The meandering rivers and broad lakes of the mid-Tertiary would have their fringes of grass and sedge, and, as the lakes dried up in the vicissitudes of climate, large areas of grass would be left on their sites. To these primitive prairies the mammal (not reptile) herbivores would be attracted, with important results. The consequences to the animals we will consider presently. The effect on the grasses may be well understood on the lines so usefully indicated in Dr. Wallace's book. The incessant cropping, age after age, would check the growth of the larger and coarser grasses give opportunity to the smaller and finer, and lead in time to the development of the grassy plains of the modern world. Thus one more familiar feature was added to the landscape in the Tertiary Era.

As this fresh green carpet spread over the formerly naked plains, it began to be enriched with our coloured flowers. There were large flowers, we saw, on some of the Mesozoic cycads, but their sober yellows and greens—to judge from their descendants—would do little to brighten the landscape. It is in the course of the Tertiary Era that the mantle of green begins to be embroidered with the brilliant hues of our flowers.

Grant Allen put forward in 1882 ("The Colours of Flowers") an interesting theory of the appearance of the colours of flowers, and it is regarded as probable. He observed that most of the simplest flowers are yellow; the more advanced flowers of simple families, and the simpler flowers of slightly advanced families, are generally white or pink; the most advanced flowers of all families, and almost all the flowers of the more advanced families, are red, purple, or blue; and the most advanced flowers of the most advanced families are always either blue or variegated. Professor Henslow adds a number of equally significant facts with the same tendency, so that we have strong reason to conceive the floral world as passing through successive phases of colour in the Tertiary Era. At first it would be a world of yellows and greens, like that of the Mesozoic vegetation, but brighter. In time splashes of red and white would lie on the face of the landscape; and later would come the purples, the rich blues, and the variegated colours of the more advanced flowers.

Why the colours came at all is a question closely connected with the general story of the evolution of the flower, at which we must glance. The essential characteristic of the flower, in the botanist's judgment, is the central green organ which you find—say, in a lily—standing out in the middle of the floral structure, with a number of yellow-coated rods round it. The yellow rods bear the male germinal elements (pollen); the central pistil encloses the ovules, or female elements. "Angiosperm" means "covered-seed plant," and its characteristic is this protection of the ovules within a special chamber, to which the pollen alone may penetrate. Round these essential organs are the coloured petals of the corolla (the chief part of the flower to the unscientific mind) and the sepals, often also coloured, of the calyx.

There is no doubt that all these parts arose from modifications of the leaves or stems of the primitive plant; though whether the bright leaves of the corolla are directly derived from ordinary leaves, or are enlarged and flattened stamens, has been disputed. And to the question why these bright petals, whose colour and variety of form lend such charm to the world of flowers, have been developed at all, most botanists will give a prompt and very interesting reply. As both male and female elements are usually in one flower, it may fertilise itself, the pollen falling directly on the pistil. But fertilisation is more sure and effective if the pollen comes from a different individual—if there is "cross fertilisation." This may be accomplished by the simple agency of the wind blowing the pollen broadcast, but it is done much better by insects, which brush against the stamens, and carry grains of the pollen to the next flower they visit.

We have here a very fertile line of development among the primitive flowers. The insects begin to visit them, for their pollen or juices, and cross-fertilise them. If this is an advantage, attractiveness to insects will become so important a feature that natural selection will develop it more and more. In plain English, what is meant is that those flowers which are more attractive to insects will be the most surely fertilised and breed most, and the prolonged application of this principle during hundreds of thousands of years will issue in the immense variety of our flowers. They will be enriched with little stores of honey and nectar; not so mysterious an advantage, when we reflect on the concentration of the juices in the neighbourhood of the seed. Then they must "advertise" their stores, and the strong perfumes and bright colours begin to develop, and ensure posterity to their possessors. The shape of the corolla will be altered in hundreds of ways, to accommodate and attract the useful visitor and shut out the mere robber. These utilities, together with the various modifying agencies of different environments, are generally believed to have led to the bewildering variety and great beauty of our floral world.

It is proper to add that this view has been sharply challenged by a number of recent writers. It is questioned if colours and scents do attract insects; though several recent series of experiments seem to show that bees are certainly attracted by colours. It is questioned if cross-fertilisation has really the importance ascribed to it since the days of Darwin. Some of these writers believe that the colours and the peculiar shape which the petals take in some flowers (orchises, for instance) have been evolved to deter browsing animals from eating them. The theory is thus only a different application of natural selection; Professor Henslow, on the other hand, stands alone in denying the selection, and believing that the insects directly developed the scents, honeys, colours, and shapes by mechanical irritation. The great majority of botanists adhere to the older view, and see in the wonderful Tertiary expansion of the flowers a manifold adaptation to the insect friends and insect foes which then became very abundant and varied.

Resisting the temptation to glance at the marvellous adaptations which we find to-day in our plant world—the insect-eating plants, the climbers, the parasites, the sensitive plants, the water-storing plants in dry regions, and so on—we must turn to the consideration of the insects themselves. We have already studied the evolution of the insect in general, and seen its earlier forms. The Tertiary Era not only witnessed a great deployment of the insects, but was singularly rich in means of preserving them. The "fly in amber" has ceased to be a puzzle even to the inexpert. Amber is the resin that exuded from pine-like trees, especially in the Baltic region, in the Eocene and Oligocene periods. Insects stuck in the resin, and were buried under fresh layers of it, and we find them embalmed in it as we pick up the resin on the shores of the Baltic to-day. The Tertiary lakes were also important cemeteries of insects. A great bed at Florissart, in Colorado, is described by one of the American experts who examined it as "a Tertiary Pompeii." It has yielded specimens of about a thousand species of Tertiary insects. Near the large ancient lake, of which it marks the site, was a volcano, and the fine ash yielded from the cone seems to have buried myriads of insects in the water. At Oeningen a similar lake-deposit has, although only a few feet thick, yielded 900 species of insects.

Yet these rich and numerous finds throw little light on the evolution of the insect, except in the general sense that they show species and even genera quite different from those of to-day. No new families of insects have appeared since the Eocene, and the ancient types had by that time disappeared. Since the Eocene, however, the species have been almost entirely changed, so that the insect record, from its commencement in the Primary Era, has the stamp of evolution on every page of it. Unfortunately, insects, especially the higher and later insects, are such frail structures that they are only preserved in very rare conditions. The most important event of the insect-world in the Tertiary is the arrival of the butterflies, which then appear for the first time. We may assume that they spread with great rapidity and abundance in the rich floral world of the mid-Jurassic. More than 13,000 species of Lepidoptera are known to-day, and there are probably twice that number yet to be classified by the entomologist. But so far the Tertiary deposits have yielded only the fragmentary remains of about twenty individual butterflies.

The evolutionary study of the insects is, therefore, not so much concerned with the various modifications of the three pairs of jaws, inherited from the primitive Tracheate, and the wings, which have given us our vast variety of species. It is directed rather to the more interesting questions of what are called the "instincts" of the insects, the remarkable metamorphosis by which the young of the higher orders attain the adult form, and the extraordinary colouring and marking of bees, wasps, and butterflies. Even these questions, however, are so large that only a few words can be said here on the tendencies of recent research.

In regard to the psychic powers of insects it may be said, in the first place, that it is seriously disputed among the modern authorities whether even the highest insects (the ant, bee, and wasp) have any degree whatever of the intelligence which an earlier generation generously bestowed on them. Wasmann and Bethe, two of the leading authorities on ants, take the negative view; Forel claims that they show occasional traces of intelligence. It is at all events clear that the enormous majority of, if not all, their activities—and especially those activities of the ant and the bee which chiefly impress the imagination—are not intelligent, but instinctive actions. And the second point to be noted is that the word "instinct," in the old sense of some innate power or faculty directing the life of an animal, has been struck out of the modern scientific dictionary. The ant or bee inherits a certain mechanism of nerves and muscles which will, in certain circumstances, act in the way we call "instinctive." The problem is to find how this mechanism and its remarkable actions were slowly evolved.

In view of the innumerable and infinitely varied forms of "instinct" in the insect world we must restrict ourselves to a single illustration—say, the social life of the ants and the bees. We are not without indications of the gradual development of this social life. In the case of the ant we find that the Tertiary specimens—and about a hundred species are found in Switzerland alone, whereas there are only fifty species in the whole of Europe to-day—all have wings and are, apparently, of the two sexes, not neutral. This seems to indicate that even in the mid-Tertiary some millions of years after the first appearance of the ant, the social life which we admire in the ants today had not yet been developed. The Tertiary bees, on the other hand, are said to show some traces of the division of labour (and modification of structure) which make the bees so interesting; but in this case the living bees, rising from a solitary life through increasing stages of social co-operation, give us some idea of the gradual development of this remarkable citizenship.

It seems to me that the great selective agency which has brought about these, and many other remarkable activities of the insects (such as the storing of food with their eggs by wasps), was probably the occurrence of periods of cold, and especially the beginning of a winter season in the Cretaceous or Tertiary age. In the periods of luxuriant life (the Carboniferous, the Jurassic, or the Oligocene), when insects swarmed and varied in every direction, some would vary in the direction of a more effective placing of the eggs; and the supervening period of cold and scarcity would favour them. When a regular winter season set in, this tendency would be enormously increased. It is a parallel case to the evolution of the birds and mammals from the reptiles. Those that varied most in the direction of care for the egg and the young would have the largest share in the next generation. When we further reflect that since the Tertiary the insect world has passed through the drastic disturbance of the climate in the great Ice-Age, we seem to have an illuminating clue to one of the most remarkable features of higher insect life.

The origin of the colour marks' and patterns on so many of the higher insects, with which we may join the origin of the stick-insects, leaf-insects, etc., is a subject of lively controversy in science to-day. The protective value of the appearance of insects which look almost exactly like dried twigs or decaying leaves, and of an arrangement of the colours of the wings of butterflies which makes them almost invisible when at rest, is so obvious that natural selection was confidently invoked to explain them. In other cases certain colours or marks seemed to have a value as "warning colours," advertising the nauseousness of their possessors to the bird, which had learned to recognise them; in other cases these colours and marks seemed to be borrowed by palatable species, whose unconscious "mimicry" led to their survival; in other cases, again, the patterns and spots were regarded as "recognition marks," by which the male could find his mate.

Science is just now passing through a phase of acute criticism—as the reader will have realised by this time—and many of the positions confidently adopted in the earlier constructive stage are challenged. This applies to the protective colours, warning colours, mimicry, etc., of insects. Probably some of the affirmations of the older generation of evolutionists were too rigid and extensive; and probably the denials of the new generation are equally exaggerated. When all sound criticism has been met, there remains a vast amount of protective colouring, shaping, and marking in the insect world of which natural selection gives us the one plausible explanation. But the doctrine of natural selection does not mean that every feature of an animal shall have a certain utility. It will destroy animals with injurious variations and favour animals with useful variations; but there may be a large amount of variation, especially in colour, to which it is quite indifferent. In this way much colour-marking may develop, either from ordinary embryonic variations or (as experiment on butterflies shows) from the direct influence of surroundings which has no vital significance. In this way, too, small variations of no selective value may gradually increase until they chance to have a value to the animal. [*]

* For a strong statement of the new critical position see Dewar and Finn's "Making of Species," 1909, ch. vi.

The origin of the metamorphosis, or pupa-stage, of the higher insects, with all its wonderful protective devices, is so obscure and controverted that we must pass over it. Some authorities think that the sleep-stage has been evolved for the protection of the helpless transforming insect; some believe that it occurs because movement would be injurious to the insect in that stage; some say that the muscular system is actually dissolved in its connections; and some recent experts suggest that it is a reminiscence of the fact that the ancestors of the metamorphosing insects were addicted to internal parasitism in their youth. It is one of the problems of the future. At present we have no fossil pupa-remains (though we have one caterpillar) to guide us. We must leave these fascinating but difficult problems of insect life, and glance at the evolution of the birds.

To the student of nature whose interest is confined to one branch of science the record of life is a mysterious Succession of waves. A comprehensive view of nature, living and non-living, past and present, discovers scores of illuminating connections, and even sees at times the inevitable sequence of events. Thus if the rise of the Angiospermous vegetation on the ruins of the Mesozoic world is understood in the light of geological and climatic changes, and the consequent deploying of the insects, especially the suctorial insects, is a natural result, the simultaneous triumph of the birds is not unintelligible. The grains and fruits of the Angiosperms and the vast swarms of insects provided immense stores of food; the annihilation of the Pterosaurs left a whole stratum of the earth free for their occupation.

We saw that a primitive bird, with very striking reptilian features, was found in the Jurassic rocks, suggesting very clearly the evolution of the bird from the reptile in the cold of the Permian or Triassic period. In the Cretaceous we found the birds distributed in a number of genera, but of two leading types. The Ichthyornis type was a tern-like flying bird, with socketed teeth and biconcave vertebrae like the reptile, but otherwise fully evolved into a bird. Its line is believed to survive in the gannets, cormorants, pelicans, and frigate-birds of to-day. The less numerous Hesperornis group were large and powerful divers. Then there is a blank in the record, representing the Cretaceous upheaval, and it unfortunately conceals the first great ramification of the bird world. When the light falls again on the Eocene period we find great numbers of our familiar types quite developed. Primitive types of gulls, herons, pelicans, quails, ibises, flamingoes, albatrosses, buzzards, hornbills, falcons, eagles, owls, plovers, and woodcocks are found in the Eocene beds; the Oligocene beds add parrots, trogons, cranes, marabouts, secretary-birds, grouse, swallows, and woodpeckers. We cannot suppose that every type has been preserved, but we see that our bird-world was virtually created in the early part of the Tertiary Era.

With these more or less familiar types were large ostrich-like survivors of the older order. In the bed of the sea which covered the site of London in the Eocene are found the remains of a toothed bird (Odontopteryx), though the teeth are merely sharp outgrowths of the edge of the bill. Another bird of the same period and region (Gastornis) stood about ten feet high, and must have looked something like a wading ostrich. Other large waders, even more ostrich-like in structure, lived in North America; and in Patagonia the remains have been found of a massive bird, about eight feet high, with a head larger than that of any living animal except the elephant, rhinoceros, and hippopotamus (Chamberlin).

The absence of early Eocene remains prevents us from tracing the lines of our vast and varied bird-kingdom to their Mesozoic beginnings. And when we appeal to the zoologist to supply the missing links of relationship, by a comparison of the structures of living birds, we receive only uncertain and very general suggestions. [*] He tells us that the ostrich-group (especially the emus and cassowaries) are one of the most primitive stocks of the bird world, and that the ancient Dinornis group and the recently extinct moas seem to be offshoots of that stock. The remaining many thousand species of Carinate birds (or flying birds with a keel [carina]-shaped breast-bone for the attachment of the flying muscles) are then gathered into two great branches, which are "traceable to a common stock" (Pycraft), and branch in their turn along the later lines of development. One of these lines—the pelicans, cormorants, etc.—seems to be a continuation of the Ichthyornis type of the Cretaceous, with the Odontopteryx as an Eocene offshoot; the divers, penguins, grebes, and petrels represent another ancient stock, which may be related to the Hesperornis group of the Cretaceous. Dr. Chalmers Mitchell thinks that the "screamers" of South America are the nearest representatives of the common ancestor of the keel-breasted birds. But even to give the broader divisions of the 19,000 species of living birds would be of little interest to the general reader.

* The best treatment of the subject will be found in W. P. Pycraft's History of Birds, 1910.

The special problems of bird-evolution are as numerous and unsettled as those of the insects. There is the same dispute as to "protective colours" and "recognition marks", the same uncertainty as to the origin of such instinctive practices as migration and nesting. The general feeling is that the annual migration had its origin in the overcrowding of the regions in which birds could live all the year round. They therefore pushed northward in the spring and remained north until the winter impoverishment drove them south again. On this view each group would be returning to its ancestral home, led by the older birds, in the great migration flights. The curious paths they follow are believed by some authorities to mark the original lines of their spread, preserved from generation to generation through the annual lead of the older birds. If we recollect the Ice-Age which drove the vast majority of the birds south at the end of the Tertiary, and imagine them later following the northward retreat of the ice, from their narrowed and overcrowded southern territory, we may not be far from the secret of the annual migration.

A more important controversy is conducted in regard to the gorgeous plumage and other decorations and weapons of the male birds. Darwin, as is known, advanced a theory of "sexual selection" to explain these. The male peacock, to take a concrete instance, would have developed its beautiful tail because, through tens of thousands of generations, the female selected the more finely tailed male among the various suitors. Dr. Wallace and other authorities always disputed this aesthetic sentiment and choice on the part of the female. The general opinion today is that Darwin's theory could not be sustained in the range and precise sense he gave to it. Some kind of display by the male in the breeding season would be an advantage, but to suppose that the females of any species of birds or mammals had the definite and uniform taste necessary for the creation of male characters by sexual selection is more than difficult. They seem to be connected in origin rather with the higher vitality of the male, but the lines on which they were selected are not yet understood.

This general sketch of the enrichment of the earth with flowering plants, insects, and birds in the Tertiary Era is all that the limits of the present work permit us to give. It is an age of exuberant life and abundant food; the teeming populations overflow their primitive boundaries, and, in adapting themselves to every form of diet, every phase of environment, and every device of capture or escape, the spreading organisms are moulded into tens of thousands of species. We shall see this more clearly in the evolution of the mammals. What we chiefly learn from the present chapter is the vital interconnection of the various parts of nature. Geological changes favour the spread of a certain type of vegetation. Insects are attracted to its nutritious seed-organs, and an age of this form of parasitism leads to a signal modification of the jaws of the insects themselves and to the lavish variety and brilliance of the flowers. Birds are attracted to the nutritious matter enclosing the seeds, and, as it is an advantage to the plant that its seeds be scattered beyond the already populated area, by passing through the alimentary canal of the bird, and being discharged with its excrements, a fresh line of evolution leads to the appearance of the large and coloured fruits. The birds, again, turn upon the swarming insects, and the steady selection they exercise leads to the zigzag flight and the protective colour of the butterfly, the concealment of the grub and the pupa, the marking of the caterpillar, and so on. We can understand the living nature of to-day as the outcome of that teeming, striving, changing world of the Tertiary Era, just as it in turn was the natural outcome of the ages that had gone before.



CHAPTER XVII. THE ORIGIN OF OUR MAMMALS

In our study of the evolution of the plant, the insect, and the bird we were seriously thwarted by the circumstance that their frames, somewhat frail in themselves, were rarely likely to be entombed in good conditions for preservation. Earlier critics of evolution used, when they were imperfectly acquainted with the conditions of fossilisation, to insinuate that this fragmentary nature of the geological record was a very convenient refuge for the evolutionist who was pressed for positive evidence. The complaint is no longer found in any serious work. Where we find excellent conditions for preservation, and animals suitable for preservation living in the midst of them, the record is quite satisfactory. We saw how the chalk has yielded remains of sea-urchins in the actual and gradual process of evolution. Tertiary beds which represent the muddy bottoms of tranquil lakes are sometimes equally instructive in their fossils, especially of shell-fish. The Paludina of a certain Slavonian lake-deposit is a classical example. It changes so greatly in the successive levels of the deposit that, if the intermediate forms were not preserved, we should divide it into several different species. The Planorbis is another well-known example. In this case we have a species evolving along several distinct lines into forms which differ remarkably from each other.

The Tertiary mammals, living generally on the land and only coming by accident into deposits suitable for preservation, cannot be expected to reveal anything like this sensible advance from form to form. They were, however, so numerous in the mid-Tertiary, and their bones are so well calculated to survive when they do fall into suitable conditions, that we can follow their development much more easily than that of the birds. We find a number of strange patriarchal beasts entering the scene in the early Eocene, and spreading into a great variety of forms in the genial conditions of the Oligocene and Miocene. As some of these forms advance, we begin to descry in them the features, remote and shadowy at first, of the horse, the deer, the elephant, the whale, the tiger, and our other familiar mammals. In some instances we can trace the evolution with a wonderful fullness, considering the remoteness of the period and the conditions of preservation. Then, one by one, the abortive, the inelastic, the ill-fitted types are destroyed by changing conditions or powerful carnivores, and the field is left to the mammals which filled it when man in turn began his destructive career.

The first point of interest is the origin of these Tertiary mammals. Their distinctive advantage over the mammals of the Mesozoic Era was-the possession by the mother of a placenta (the "after-birth" of the higher mammals), or structure in the womb by which the blood-vessels of the mother are brought into such association with those of the foetus that her blood passes into its arteries, and it is fully developed within the warm shelter of her womb. The mammals of the Mesozoic had been small and primitive animals, rarely larger than a rat, and never rising above the marsupial stage in organisation. They not only continued to exist, and give rise to their modern representatives (the opossum, etc.) during the Tertiary Era, but they shared the general prosperity. In Australia, where they were protected from the higher carnivorous mammals, they gave rise to huge elephant-like wombats (Diprotodon), with skulls two or three feet in length. Over the earth generally, however, they were superseded by the placental mammals, which suddenly break into the geological record in the early Tertiary, and spread with great vigour and rapidity over the four continents.

Were they a progressive offshoot from the Mesozoic Marsupials, or Monotremes, or do they represent a separate stock from the primitive half-reptile and half-mammal family? The point is disputed; nor does the scantiness of the record permit us to tell the place of their origin. The placental structure would be so great an advantage in a cold and unfavourable environment that some writers look to the northern land, connecting Europe and America, for their development. We saw, however, that this northern region was singularly warm until long after the spread of the mammals. Other experts, impressed by the parallel development of the mammals and the flowering plants, look to the elevated parts of eastern North America.

Such evidence as there is seems rather to suggest that South Africa was the cradle of the placental mammals. We shall find that many of our mammals originated in Africa; there, too, is found to-day the most primitive representative of the Tertiary mammals, the hyrax; and there we find in especial abundance the remains of the mammal-like reptiles (Theromorphs) which are regarded as their progenitors. Further search in the unexplored geological treasures and dense forests of Africa is needed. We may provisionally conceive the placental mammals as a group of the South African early mammals which developed a fortunate variation in womb-structure during the severe conditions of the early Mesozoic. In this new structure they would have no preponderant advantage as long as the genial Jurassic age favoured the great reptiles, and they may have remained as small and insignificant as the Marsupials. But with the fresh upheaval and climatic disturbance at the end of the Jurassic, and during the Cretaceous, they spread northward, and replaced the dying reptiles, as the Angiosperms replaced the dying cycads. When they met the spread of the Angiosperm vegetation they would receive another great stimulus to development.

They appear in Europe and North America in the earliest Cretaceous. The rise of the land had connected many hitherto isolated regions, and they seem to have poured over every bridge into all parts of the four continents. The obscurity of their origin is richly compensated by their intense evolutionary interest from the moment they enter the geological record. We have seen this in the case of every important group of plants and animals, and can easily understand it. The ancestral group was small and local; the descendants are widely spread. While, therefore, we discover remains of the later phases of development in our casual cuttings and quarries, the ancestral tomb may remain for ages in some unexplored province of the geological world. If this region is, as we suspect, in Africa, our failure to discover it as yet is all the more intelligible.

But these mammals of the early Tertiary are still of such a patriarchal or ancestral character that the student of evolution can dispense with their earlier phase. They combine in their primitive frames, in an elementary way, the features which we now find distributed in widely removed groups of their descendants. Most of them fall into two large orders: the Condylarthra, the ancestral herbivores from which we shall find our horses, oxen, deer, elephants, and hogs gradually issuing, and the Creodonta, the patriarchal carnivores, which will give birth to our lions and tigers, wolves and foxes, and their various cousins. As yet even the two general types of herbivore and carnivore are so imperfectly separated that it is not always possible to distinguish between them. Nearly all of them have the five-toed foot of the reptile ancestor; and the flat nails on their toes are the common material out of which the hoof of the ungulate and the claw of the carnivore will be presently fashioned. Nearly all have forty-four simply constructed teeth, from which will be evolved the grinders and tusks of the elephant or the canines of the tiger. They answer in every respect to the theory that some primitive local group was the common source of all our great mammals. With them are ancestral forms of Edentates (sloths, etc.) and Insectivores (moles, etc.), side-branches developing according to their special habits; and before the end of the Eocene we find primitive Rodents (squirrels, etc.) and Cheiroptera (bats).