The Story of Evolution Part 11

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 gra.s.ses, and so on.

On the other hand, the facts we have already seen seem to justify the att.i.tude 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 pa.s.sing 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 cla.s.s 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 a.s.sault. 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 p.r.o.nounced." This is the outcome of the Cretaceous revolution. Southern Europe and Southern Asia have risen, and shaken the last ma.s.ses 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 sh.o.r.e. 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 Thalamoph.o.r.es, the Nummulites, spreads with such portentous abundance that its sh.e.l.ls--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 sh.e.l.l is, however, no longer a microscopic grain. It sometimes forms wonderful sh.e.l.ls, 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 j.a.pan.

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 lat.i.tude 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 gra.s.ses 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 ma.s.ses 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 sh.e.l.ls 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 cla.s.sification 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 cla.s.sification, 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 ma.s.s; 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 ill.u.s.trations 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 sh.e.l.ls were commonly ten inches long; the double sh.e.l.l 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-sh.e.l.l 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 pa.s.s 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 Jura.s.sic 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-Tria.s.sic cold; that, instead of descending to the warm moist levels with the rest at the end of the Tria.s.sic, 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 Jura.s.sic 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, gra.s.s, etc., were well developed before the end of the Cretaceous period. The botanist divides the Angiosperms into two leading groups, the Monocotyledons (palms, gra.s.ses, 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 gra.s.ses, 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 ma.s.s of Ferns, Conifers, Ginkgoales, Monocotyledons, and Dicotyledons. Here is a casual list of plants that then grew in the lat.i.tude 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 Jura.s.sic 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 gra.s.ses, 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 gra.s.s. The interest of the evolution of the gra.s.ses 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 gra.s.ses 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 gra.s.ses. 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 coa.r.s.er 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 Tria.s.sic and the Jura.s.sic 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 gra.s.ses, 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 gra.s.s and sedge, and, as the lakes dried up in the vicissitudes of climate, large areas of gra.s.s 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 gra.s.ses 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 coa.r.s.er gra.s.ses give opportunity to the smaller and finer, and lead in time to the development of the gra.s.sy 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 pa.s.sing 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.