The story of the earth from the beginning of the Cambrian period to the present day was long ago divided by geologists into four great eras. The periods we have already covered—the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian—form the Primary or Palaeozoic Era, to which the earlier Archaean rocks were prefixed as a barren and less interesting introduction. The stretch of time on which we now enter, at the close of the Permian, is the Secondary or Mesozoic Era. It will be closed by a fresh upheaval of the earth and disturbance of life-conditions in the Chalk period, and followed by a Tertiary Era, in which the earth will approach its modern aspect. At its close there will be another series of upheavals, culminating in a great Ice-age, and the remaining stretch of the earth's story, in which we live, will form the Quaternary Era.
In point of duration these four eras differ enormously from each other. If the first be conceived as comprising sixteen million years—a very moderate estimate—the second will be found to cover less than eight million years, the third less than three million years, and the fourth, the Age of Man, much less than one million years; while the Archaean Age was probably as long as all these put together. But the division is rather based on certain gaps, or "unconformities," in the geological record; and, although the breaches are now partially filled, we saw that they correspond to certain profound and revolutionary disturbances in the face of the earth. We retain them, therefore, as convenient and logical divisions of the biological as well as the geological chronicle, and, instead of passing from one geological period to another, we may, for the rest of the story, take these three eras as wholes, and devote a few chapters to the chief advances made by living things in each era. The Mesozoic Era will be a protracted reaction between two revolutions: a period of low-lying land, great sea-invasions, and genial climate, between two upheavals of the earth. The Tertiary Era will represent a less sharply defined depression, with genial climate and luxuriant life, between two such upheavals.
The Mesozaic ("middle life") Era may very fitly be described as the Middle Ages of life on the earth. It by no means occupies a central position in the chronicle of life from the point of view of time or antiquity, just as the Middle Ages of Europe are by no means the centre of the chronicle of mankind, but its types of animals and plants are singularly transitional between the extinct ancient and the actual modern types. Life has been lifted to a higher level by the Permian revolution. Then, for some millions of years, the sterner process of selection relaxes, the warm bosom of the earth swarms again with a teeming and varied population, and a rich material is provided for the next great application of drastic selective agencies. To a poet it might seem that nature indulges each succeeding and imperfect type of living thing with a golden age before it is dismissed to make place for the higher.
The Mesozoic opens in the middle of the great revolution described in the last chapter. Its first section, the Triassic period, is at first a mere continuation of the Permian. A few hundred species of animals and hardy plants are scattered over a relatively bleak and inhospitable globe. Then the land begins to sink once more. The seas spread in great arms over the revelled continents, the plant world rejoices in the increasing warmth and moisture, and the animals increase in number and variety. We pass into the Jurassic period under conditions of great geniality. Warm seas are found as far north and south as our present polar regions, and the low-lying fertile lands are covered again with rich, if less gigantic, forests, in which hordes of stupendous animals find ample nourishment. The mammal and the bird are already on the stage, but their warm coats and warm blood offer no advantage in that perennial summer, and they await in obscurity the end of the golden age of the reptiles. At the end of the Jurassic the land begins to rise once more. The warm, shallow seas drain off into the deep oceans, and the moist, swampy lands are dried. The emergence continues throughout the Cretaceous (Chalk) period. Chains of vast mountains rise slowly into the air in many parts of the earth, and a new and comparatively rapid change in the vegetation—comparable to that at the close of the Carboniferous—announces the second great revolution. The Mesozoic closes with the dismissal of the great reptiles and the plants on which they fed, and the earth is prepared for its new monarchs, the flowering plants, the birds, and the mammals.
How far this repeated levelling of the land after its repeated upheavals is due to a real sinking of the crust we cannot as yet determine. The geologist of our time is disposed to restrict these mysterious rises and falls of the crust as much as possible. A much more obvious and intelligible agency has to be considered. The vast upheaval of nearly all parts of the land during the Permian period would naturally lead to a far more vigorous scouring of its surface by the rains and rivers. The higher the land, the more effectively it would be worn down. The cooler summits would condense the moisture, and the rains would sweep more energetically down the slopes of the elevated continents. There would thus be a natural process of levelling as long as the land stood out high above the water-line, but it seems probable that there was also a real sinking of the crust. Such subsidences have been known within historic times.
By the end of the Triassic—a period of at least two million years—the sea had reconquered a vast proportion of the territory wrested from it in the Permian revolution. Most of Europe, west of a line drawn from the tip of Norway to the Black Sea, was under water—generally open sea in the south and centre, and inland seas or lagoons in the west. The invasion of the sea continued, and reached its climax, in the Jurassic period. The greater part of Europe was converted into an archipelago. A small continent stood out in the Baltic region. Large areas remained above the sea-level in Austria, Germany, and France. Ireland, Wales, and much of Scotland were intact, and it is probable that a land bridge still connected the west of Europe with the east of America. Europe generally was a large cluster of islands and ridges, of various sizes, in a semi-tropical sea. Southern Asia was similarly revelled, and it is probable that the seas stretched, with little interruption, from the west of Europe to the Pacific. The southern continent had deep wedges of the sea driven into it. India, New Zealand, and Australia were successively detached from it, and by the end of the Mesozoic it was much as we find it to-day. The Arctic continent (north of Europe) was flooded, and there was a great interior sea in the western part of the North American continent.
This summary account of the levelling process which went on during the Triassic and Jurassic will prepare us to expect a return of warm climate and luxurious life, and this the record abundantly evinces. The enormous expansion of the sea—a great authority, Neumayr, believes that it was the greatest extension of the sea that is known in geology—and lowering of the land would of itself tend to produce this condition, and it may be that the very considerable volcanic activity, of which we find evidence in the Permian and Triassic, had discharged great volumes of carbon-dioxide into the atmosphere.
Whatever the causes were, the earth has returned to paradisiacal conditions. The vast ice-fields have gone, the scanty and scrubby vegetation is replaced by luscious forests of cycads, conifers, and ferns, and warmth-loving animals penetrate to what are now the Arctic and Antarctic regions. Greenland and Spitzbergen are fragments of a continent that then bore a luxuriant growth of ferns and cycads, and housed large reptiles that could not now live thousands of miles south of it. England, and a large part of Europe, was a tranquil blue coral-ocean, the fringes of its islands girt with reefs such as we find now only three thousand miles further south, with vast shoals of Ammonites, sometimes of gigantic size, preying upon its living population or evading its monstrous sharks; while the sunlit lands were covered with graceful, palmlike cycads and early yews and pines and cypresses, and quaint forms of reptiles throve on the warm earth or in the ample swamps, or rushed on outstretched wings through the purer air.
It was an evergreen world, a world, apparently, of perpetual summer. No trace is found until the next period of an alternation of summer and winter—no trees that shed their leaves annually, or show annual rings of growth in the wood—and there is little trace of zones of climate as yet. It is true that the sensitive Ammonites differ in the northern and the southern latitudes, but, as Professor Chamberlin says, it is not clear that the difference points to a diversity of climate. We may conclude that the absence of corals higher than the north of England implies a more temperate climate further north, but what Sir A. Geikie calls (with slight exaggeration) "the almost tropical aspect" of Greenland warns us to be cautious. The climate of the mid-Jurassic was very much warmer and more uniform than the climate of the earth to-day. It was an age of great vital expansion. And into this luxuriant world we shall presently find a fresh period of elevation, disturbance, and cold breaking with momentous evolutionary results. Meantime, we may take a closer look at these interesting inhabitants of the Middle Ages of the earth, before they pass away or are driven, in shrunken regiments, into the shelter of the narrowing tropics.
The principal change in the aspect of the earth, as the cold, arid plains and slopes of the Triassic slowly yield the moist and warm ow-lying lands of the Jurassic, to consists in the character of the vegetation. It is wholly intermediate in its forms between that of the primitive forests and that of the modern world. The great Cryptogams of the Carboniferous world—the giant Club-mosses and their kindred—have been slain by the long period of cold and drought. Smaller Horsetails (sometimes of a great size, but generally of the modern type) and Club-mosses remain, but are not a conspicuous feature in the landscape. On the other hand, there is as yet—apart from the Conifers—no trace of the familiar trees and flowers and grasses of the later world. The vast majority of the plants are of the cycad type. These—now confined to tropical and subtropical regions—with the surviving ferns, the new Conifers, and certain trees of the ginkgo type, form the characteristic Mesozoic vegetation.
A few words in the language of the modern botanist will show how this vegetation harmonises with the story of evolution. Plants are broadly divided into the lower kingdom of the Cryptogams (spore-bearing) and the upper kingdom of the Phanerogams (seed-bearing). As we saw, the Primary Era was predominantly the age of Cryptogams; the later periods witness the rise and supremacy of the Phanerogams. But these in turn are broadly divided into a less advanced group, the Gymnosperms, and a more advanced group, the Angiosperms or flowering plants. And, just as the Primary Era is the age of Cryptogams, the Secondary is the age of Gymnosperms, and the Tertiary (and present) is the age of Angiosperms. Of about 180,000 species of plants in nature to-day more than 100,000 are Angiosperms; yet up to the end of the Jurassic not a single true Angiosperm is found in the geological record.
This is a broad manifestation of evolution, but it is not quite an accurate statement, and its inexactness still more strongly confirms the theory of evolution. Though the Primary Era was predominantly the age of Cryptogams, we saw that a very large number of seed-bearing plants, with very mixed characters, appeared before its close. It thus prepares the way for the cycads and conifers and ginkgoes of the Mesozoic, which we may conceive as evolved from one or other branch of the mixed Carboniferous vegetation. We next find that the Mesozoic is by no means purely an age of Gymnosperms. I do not mean merely that the Angiosperms appear in force before its close, and were probably evolved much earlier. The fact is that the Gymnosperms of the Mesozoic are often of a curiously mixed character, and well illustrate the transition to the Angiosperms, though they may not be their actual ancestors. This will be clearer if we glance in succession at the various types of plant which adorned and enriched the Jurassic world.
The European or American landscape—indeed, the aspect of the earth generally, for there are no pronounced zones of climate—is still utterly different from any that we know to-day. No grass carpets the plains; none of the flowers or trees with which we are familiar, except conifers, are found in any region. Ferns grow in great abundance, and have now reached many of the forms with which we are acquainted. Thickets of bracken spread over the plains; clumps of Royal ferns and Hartstongues spring up in moister parts. The trees are conifers, cycads, and trees akin to the ginkgo, or Maidenhair Tree, of modern Japan. Cypresses, yews, firs, and araucarias (the Monkey Puzzle group) grow everywhere, though the species are more primitive than those of today. The broad, fan-like leaves and plum-like fruit of the ginkgoales, of which the temple-gardens of Japan have religiously preserved a solitary descendant, are found in the most distant regions. But the most frequent and characteristic tree of the Jurassic landscape is the cycad.
The cycads—the botanist would say Cycadophyta or Cycadales, to mark them off from the cycads of modern times—formed a third of the whole Jurassic vegetation, while to-day they number only about a hundred species in 180,000, and are confined to warm latitudes. All over the earth, from the Arctic to the Antarctic, their palm-like foliage showered from the top of their generally short stems in the Jurassic. But the most interesting point about them is that a very large branch of them (the Bennettiteae) went far beyond the modern Gymnosperm in their flowers and fruit, and approached the Angiosperms. Their fructifications "rivalled the largest flowers of the present day in structure and modelling" (Scott), and possibly already gave spots of sober colour to the monotonous primitive landscape. On the other hand, they approached the ferns so much more closely than modern cycads do that it is often impossible to say whether Jurassic remains must be classed as ferns or cycads.
We have here, therefore, a most interesting evolutionary group. The botanist finds even more difficulty than the zoologist in drawing up the pedigrees of his plants, but the general features of the larger groups which he finds in succession in the chronicle of the earth point very decisively to evolution. The seed-bearing ferns of the Coal-forest point upward to the later stage, and downward to a common origin with the ordinary spore-bearing ferns. Some of them are "altogether of a cycadean type" (Scott) in respect of the seed. On the other hand, the Bennettiteae of the Jurassic have the mixed characters of ferns, cycads, and flowering plants, and thus, in their turn, point downward to a lower ancestry and upward to the next great stage in plant-development. It is not suggested that the seed-ferns we know evolved into the cycads we know, and these in turn into our flowering plants. It is enough for the student of evolution to see in them so many stages in the evolution of plants up to the Angiosperm level. The gaps between the various groups are less rigid than scientific men used to think.
Taller than the cycads, firmer in the structure of the wood, and destined to survive in thousands of species when the cycads would be reduced to a hundred, were the pines and yews and other conifers of the Jurassic landscape. We saw them first appearing, in the stunted Walchias and Voltzias, during the severe conditions of the Permian period. Like the birds and mammals they await the coming of a fresh period of cold to give them a decided superiority over the cycads. Botanists look for their ancestors in some form related to the Cordaites of the Coal-forest. The ginkgo trees seem to be even more closely related to the Cordaites, and evolved from an early and generalised branch of that group. The Cordaites, we may recall, more or less united in one tree the characters of the conifer (in their wood) and the cycad (in their fruit).
So much for the evolutionary aspect of the Jurassic vegetation in itself. Slender as the connecting links are, it points clearly enough to a selection of higher types during the Permian revolution from the varied mass of the Carboniferous flora, and it offers in turn a singularly varied and rich group from which a fresh selection may choose yet higher types. We turn now to consider the animal population which, directly or indirectly, fed upon it, and grew with its growth. To the reptiles, the birds, and the mammals, we must devote special chapters. Here we may briefly survey the less conspicuous animals of the Mesozoic Epoch.
The insects would be one of the chief classes to benefit by the renewed luxuriance of the vegetation. The Hymenopters (butterflies) have not yet appeared. They will, naturally, come with the flowers in the next great phase of organic life. But all the other orders of insects are represented, and many of our modern genera are fully evolved. The giant insects of the Coal-forest, with their mixed patriarchal features, have given place to more definite types. Swarms of dragon-flies, may-flies, termites (with wings), crickets, and cockroaches, may be gathered from the preserved remains. The beetles (Coleopters) have come on the scene in the Triassic, and prospered exceedingly. In some strata three-fourths of the insects are beetles, and as we find that many of them are wood-eaters, we are not surprised. Flies (Dipters) and ants (Hymenopters) also are found, and, although it is useless to expect to find the intermediate forms of such frail creatures, the record is of some evolutionary interest. The ants are all winged. Apparently there is as yet none of the remarkable division of labour which we find in the ants to-day, and we may trust that some later period of change may throw light on its origin.
Just as the growth of the forests—for the Mesozoic vegetation has formed immense coal-beds in many parts of the world, even in Yorkshire and Scotland—explains this great development of the insects, they would in their turn supply a rich diet to the smaller land animals and flying animals of the time. We shall see this presently. Let us first glance at the advances among the inhabitants of the seas.
The most important and stimulating event in the seas is the arrival of the Ammonite. One branch of the early shell-fish, it will be remembered, retained the head of its naked ancestor, and lived at the open mouth of its shell, thus giving birth to the Cephalopods. The first form was a long, straight, tapering shell, sometimes several feet long. In the course of time new forms with curved shells appeared, and began to displace the straight-shelled. Then Cephalopods with close-coiled shells, like the nautilus, came, and—such a shell being an obvious advantage—displaced the curved shells. In the Permian, we saw, a new and more advanced type of the coiled-shell animal, the Ammonite, made its appearance, and in the Triassic and Jurassic it becomes the ogre or tyrant of the invertebrate world. Sometimes an inch or less in diameter, it often attained a width of three feet or more across the shell, at the aperture of which would be a monstrous and voracious mouth.
The Ammonites are not merely interesting as extinct monsters of the earth's Middle Ages, and stimulating terrors of the deep to the animals on which they fed. They have an especial interest for the evolutionist. The successive chambers which the animal adds, as it grows, to the habitation of its youth, leave the earlier chambers intact. By removing them in succession in the adult form we find an illustration of the evolution of the elaborate shell of the Jurassic Ammonite. It is an admirable testimony to the validity of the embryonic law we have often quoted—that the young animal is apt to reproduce the past stages of its ancestry—that the order of the building of the shell in the late Ammonite corresponds to the order we trace in its development in the geological chronicle. About a thousand species of Ammonites were developed in the Mesozoic, and none survived the Mesozoic. Like the Trilobites of the Primary Era, like the contemporary great reptiles on land, the Ammonites were an abortive growth, enjoying their hour of supremacy until sterner conditions bade them depart. The pretty nautilus is the only survivor to-day of the vast Mesozoic population of coiled-shell Cephalopods.
A rival to the Ammonite appeared in the Triassic seas, a formidable forerunner of the cuttle-fish type of Cephalopod. The animal now boldly discards the protecting and confining shell, or spreads over the outside of it, and becomes a "shell-fish" with the shell inside. The octopus of our own time has advanced still further, and become the most powerful of the invertebrates. The Belemnite, as the Mesozoic cuttle-fish is called, attained so large a size that the internal bone, or pen (the part generally preserved), is sometimes two feet in length. The ink-bags of the Belemnite also are sometimes preserved, and we see how it could balk a pursuer by darkening the waters. It was a compensating advantage for the loss of the shell.
In all the other classes of aquatic animals we find corresponding advances. In the remaining Molluscs the higher or more effective types are displacing the older. It is interesting to note that the oyster is fully developed, and has a very large kindred, in the Mesozoic seas. Among the Brachiopods the higher sloping-shoulder type displaces the square-shoulder shells. In the Crustacea the Trilobites and Eurypterids have entirely disappeared; prawns and lobsters abound, and the earliest crab makes its appearance in the English Jurassic rocks. This sudden arrival of a short-tailed Crustacean surprises us less when we learn that the crab has a long tail in its embryonic form, but the actual line of its descent is not clear. Among the Echinoderms we find that the Cystids and Blastoids have gone, and the sea-lilies reach their climax in beauty and organisation, to dwindle and almost disappear in the last part of the Mesozoic. One Jurassic sea-lily was found to have 600,000 distinct ossicles in its petrified frame. The free-moving Echinoderms are now in the ascendant, the sea-urchins being especially abundant. The Corals are, as we saw, extremely abundant, and a higher type (the Hexacoralla) is superseding the earlier and lower (Tetracoralla).
Finally, we find a continuous and conspicuous advance among the fishes. At the close of the Triassic and during the Jurassic they seem to undergo profound and comparatively rapid changes. The reason will, perhaps, be apparent in the next chapter, when we describe the gigantic reptiles which feed on them in the lakes and shore-waters. A greater terror than the shark had appeared in their environment. The Ganoids and Dipneusts dwindle, and give birth to their few modern representatives. The sharks with crushing teeth diminish in number, and the sharp-toothed modern shark attains the supremacy in its class, and evolves into forms far more terrible than any that we know to-day. Skates and rays of a more or less modern type, and ancestral gar-pikes and sturgeons, enter the arena. But the most interesting new departure is the first appearance, in the Jurassic, of bony-framed fishes (Teleosts). Their superiority in organisation soon makes itself felt, and they enter upon the rapid evolution which will, by the next period, give them the first place in the fish world.
Over the whole Mesozoic world, therefore, we find advance and the promise of greater advance. The Permian stress has selected the fittest types to survive from the older order; the Jurassic luxuriance is permitting a fresh and varied expansion of life, in preparation for the next great annihilation of the less fit and selection of the more fit. Life pauses before another leap. The Mesozoic earth—to apply to it the phrase which a geologist has given to its opening phase—welcomes the coming and speeds the parting guest. In the depths of the ocean a new movement is preparing, but we have yet to study the highest forms of Mesozoic life before we come to the Cretaceous disturbances.
From one point of view the advance of life on the earth seems to proceed not with the even flow of a river, but in the successive waves of an oncoming tide. It is true that we have detected a continuous advance behind all these rising and receding waves, yet their occurrence is a fact of some interest, and not a little speculation has been expended on it. When the great procession of life first emerges out of the darkness of Archaean times, it deploys into a spreading world of strange Crustaceans, and we have the Age of Trilobites. Later there is the Age of Fishes, then of Cryptogams and Amphibia, and then of Cycads and Reptiles, and there will afterwards be an Age of Birds and Mammals, and finally an Age of Man. But there is no ground for mystic speculation on this circumstance of a group of organisms fording the earth for a few million years, and then perishing or dwindling into insignificance. We shall see that a very plain and substantial process put an end to the Age of the Cycads, Ammonites, and Reptiles, and we have seen how the earlier dynasties ended.
The phrase, however, the Age of Reptiles, is a fitting and true description of the greater part of the Mesozoic Era, which lies, like a fertile valley, between the Permian and the Chalk upheavals. From the bleak heights of the Permian period, or—more probably—from its more sheltered regions, in which they have lingered with the ferns and cycads, the reptiles spread out over the earth, as the summer of the Triassic period advances. In the full warmth and luxuriance of the Jurassic they become the most singular and powerful army that ever trod the earth. They include small lizard-like creatures and monsters more than a hundred feet in length. They swim like whales in the shallow seas; they shrink into the shell of the giant turtle; they rear themselves on towering hind limbs, like colossal kangaroos; they even rise into the air, and fill it with the dragons of the fairy tale. They spread over the whole earth from Australia to the Arctic circle. Then the earth seems to grow impatient of their dominance, and they shrink towards the south, and struggle in a diminished territory. The colossal monsters and the formidable dragons go the way of all primitive life, and a ragged regiment of crocodiles, turtles, and serpents in the tropics, with a swarm of smaller creatures in the fringes of the warm zone, is all that remains, by the Tertiary Era, of the world-conquering army of the Mesozoic reptiles.
They had appeared, as we said, in the Permian period. Probably they had been developed during the later Carboniferous, since we find them already branched into three orders, with many sub-orders, in the Permian. The stimulating and selecting disturbances which culminated in the Permian revolution had begun in the Carboniferous. Their origin is not clear, as the intermediate forms between them and the amphibia are not found. This is not surprising, if we may suppose that some of the amphibia had, in the growing struggle, pushed inland, or that, as the land rose and the waters were drained in certain regions, they had gradually adopted a purely terrestrial life, as some of the frogs have since done. In the absence of water their frames would not be preserved and fossilised. We can, therefore, understand the gap in the record between the amphibia and the reptiles. From their structure we gather that they sprang from at least two different branches of the amphibia. Their remains fall into two great groups, which are known as the Diapsid and the Synapsid reptiles. The former seem to be more closely related to the Microsauria, or small salamander-like amphibia of the Coal-forest; the latter are nearer to the Labyrinthodonts. It is not suggested that these were their actual ancestors, but that they came from the same early amphibian root.
We find both these groups, in patriarchal forms, in Europe, North America, and South Africa during the Permian period. They are usually moderate in size, but in places they seem to have found good conditions and prospered. A few years ago a Permian bed in Russia yielded a most interesting series of remains of Synapsid reptiles. Some of them were large vegetarian animals, more than twelve feet in length; others were carnivores with very powerful heads and teeth as formidable as those of the tiger. Another branch of the same order lived on the southern continent, Gondwana Land, and has left numerous remains in South Africa. We shall see that they are connected by many authorities with the origin of the mammals. [*] The other branch, the Diapsids, are represented to-day by the curiously primitive lizard of New Zealand, the tuatara (Sphenodon, or Hatteria), of which I have seen specimens, nearly two feet in length, that one did not care to approach too closely. The Diapsids are chiefly interesting, however, as the reputed ancestors of the colossal reptiles of the Jurassic age and the birds.
The purified air of the Permian world favoured the reptiles' being lung-breathers, but the cold would check their expansion for a time. The reptile, it is important to remember' usually leaves its eggs to be hatched by the natural warmth of the ground. But as the cold of the Permian yielded to a genial climate and rich vegetation in the course of the Triassic, the reptiles entered upon their memorable development. The amphibia were now definitely ousted from their position of dominance. The increase of the waters had at first favoured them, and we find more than twenty genera, and some very large individuals, of the amphibia in the Triassic. One of them, the Mastodonsaurus, had a head three feet long and two feet wide. But the spread of the reptiles checked them, and they shrank rapidly into the poor and defenceless tribe which we find them in nature to-day.
To follow the prolific expansion of the reptiles in the semi-tropical conditions of the Jurassic age is a task that even the highest authorities approach with great diffidence. Science is not yet wholly agreed in the classification of the vast numbers of remains which the Mesozoic rocks have yielded, and the affinities of the various groups are very uncertain. We cannot be content, however, merely to throw on the screen, as it were, a few of the more quaint and monstrous types out of the teeming Mesozoic population, and describe their proportions and peculiarities. They fall into natural and intelligible groups or orders, and their features are closely related to the differing regions of the Jurassic world. While, therefore, we must abstain from drawing up settled genealogical trees, we may, as we review in succession the monsters of the land, the waters, and the air, glance at the most recent and substantial conjectures of scientific men as to their origin and connections.
The Deinosaurs (or "terrible reptiles"), the monarchs of the land and the swamps, are the central and outstanding family of the Mesozoic reptiles. As the name implies, this group includes most of the colossal animals, such as the Diplodocus, which the illustrated magazine has made familiar to most people. Fortunately the assiduous research of American geologists and their great skill and patience in restoring the dead forms enable us to form a very fair picture of this family of medieval giants and its remarkable ramifications. [*]
The Diapsid reptiles of the Permian had evolved a group with horny, parrot-like beaks, the Rhyncocephalia (or "beak-headed" reptiles), of which the tuatara of New Zealand is a lingering representative. New Zealand seems to have been cut off from the southern continent at the close of the Permian or beginning of the Triassic, and so preserved for us that very interesting relic of Permian life. From some primitive level of this group, it is generally believed, the great Deinosaurs arose. Two different orders seem to have arisen independently, or diverged rapidly from each other, in different parts of the world. One group seems to have evolved on the "lost Atlantis," the land between Western Europe and America, whence they spread westward to America, eastward over Europe, and southward to the continent which still united Africa and Australia. We find their remains in all these regions. Another stock is believed to have arisen in America.
Both these groups seem to have been more or less biped, rearing themselves on large and powerful hind limbs, and (in some cases, at least) probably using their small front limbs to hold or grasp their food. The first group was carnivorous, the second herbivorous; and, as the reptiles of the first group had four or five toes on each foot, they are known as the Theropods (or "beast-footed" ), while those of the second order, which had three toes, are called the Ornithopods (or "bird-footed"). Each of them then gave birth to an order of quadrupeds. In the spreading waters and rich swamps of the later Triassic some of the Theropods were attracted to return to an amphibious life, and became the vast, sprawling, ponderous Sauropods, the giants in a world of giants. On the other hand, a branch of the vegetarian Ornithopods developed heavy armour, for defence against the carnivores, and became, under the burden of its weight, the quadrupedal and monstrous Stegosauria and Ceratopsia. Taking this instructive general view of the spread of the Deinosaurs as the best interpretation of the material we have, we may now glance at each of the orders in succession.
The Theropods varied considerably in size and agility. The Compsognathus was a small, active, rabbit-like creature, standing about two feet high on its hind limbs, while the Megalosaurs stretched to a length of thirty feet, and had huge jaws armed with rows of formidable teeth. The Ceratosaur, a seventeen-foot-long reptile, had hollow bones, and we find this combination of lightness and strength in several members of the group. In many respects the group points more or less significantly toward the birds. The brain is relatively large, the neck long, and the fore limbs might be used for grasping, but had apparently ceased to serve as legs. Many of the Theropods were evidently leaping reptiles, like colossal kangaroos, twenty or more feet in length when they were erect. It is the general belief that the bird began its career as a leaping reptile, and the feathers, or expanded scales, on the front limbs helped at first to increase the leap. Some recent authorities hold, however, that the ancestor of the bird was an arboreal reptile.
To the order of the Sauropods belong most of the monsters whose discovery has attracted general attention in recent years. Feeding on vegetal matter in the luscious swamps, and having their vast bulk lightened by their aquatic life, they soon attained the most formidable proportions. The admirer of the enormous skeleton of Diplodocus (which ran to eighty feet) in the British Museum must wonder how even such massive limbs could sustain the mountain of flesh that must have covered those bones. It probably did not walk so firmly as the skeleton suggests, but sprawled in the swamps or swam like a hippopotamus. But the Diplodocus is neither the largest nor heaviest of its family. The Brontosaur, though only sixty feet long, probably weighed twenty tons. We have its footprints in the rocks to-day, each impression measuring about a square yard. Generally, it is the huge thigh-bones of these monsters that have survived, and give us an idea of their size. The largest living elephant has a femur scarcely four feet long, but the femur of the Atlantosaur measures more than seventy inches, and the femur of the Brachiosaur more than eighty. Many of these Deinosaurs must have measured more than a hundred feet from the tip of the snout to the end of the tail, and stood about thirty feet high from the ground. The European Sauropods did not, apparently, reach the size of their American cousins—so early did the inferiority of Europe begin—but our Ceteosaur seems to have been about fifty feet long and ten feet in height. Its thigh-bone was sixty-four inches long and twenty-seven inches in circumference at the shaft. And in this order of reptiles, it must be remembered, the bones are solid.
To complete the picture of the Sauropods, we must add that the whole class is characterised by the extraordinary smallness of the brain. The twenty-ton Brontosaur had a brain no larger than that of a new-born human infant. Quite commonly the brain of one of these enormous animals is no larger than a man's fist. It is true that, as far as the muscular and sexual labour was concerned, the brain was supplemented by a great enlargement of the spinal cord in the sacral region (at the top of the thighs). This inferior "brain" was from ten to twenty times as large as the brain in the skull. It would, however, be fully occupied with the movement of the monstrous limbs and tail, and the sex-life, and does not add in the least to the "mental" power of the Sauropods. They were stupid, sluggish, unwieldy creatures, swollen parasites upon a luxuriant vegetation, and we shall easily understand their disappearance at the end of the Mesozoic Era, when the age of brawn will yield to an age of brain.
The next order of the Deinosaurs is that of the biped vegetarians, the Ornithopods, which gradually became heavily armoured and quadrupedal. The familiar Iguanodon is the chief representative of this order in Europe. Walking on its three-toed hind limbs, its head would be fourteen or fifteen feet from the ground. The front part of its jaws was toothless and covered with horn. It had, in fact, a kind of beak, and it also approached the primitive bird in the structure of its pelvis and in having five toes on its small front limbs. Some of the Ornithopods, such as the Laosaur, were small (three or four feet in height) and active, but many of the American specimens attained a great size. The Camptosaur, which was closely related to the Iguanodon in structure, was thirty feet from the snout to the end of the tail, and the head probably stood eighteen feet from the ground. One of the last great representatives of the group in America, the Trachodon, about thirty feet in length, had a most extraordinary head. It was about three and a half feet in length, and had no less than 2000 teeth lining the mouth cavity. It is conjectured that it fed on vegetation containing a large proportion of silica.
In the course of the Jurassic, as we saw, a branch of these biped, bird-footed vegetarians developed heavy armour, and returned to the quadrupedal habit. We find them both in Europe and America, and must suppose that the highway across the North Atlantic still existed.
The Stegosaur is one of the most singular and most familiar representatives of the group in the Jurassic. It ran to a length of thirty feet, and had a row of bony plates, from two to three feet in height, standing up vertically along the ridge of its back, while its tail was armed with formidable spikes. The Scleidosaur, an earlier and smaller (twelve-foot) specimen, also had spines and bony plates to protect it. The Polacanthus and Ankylosaur developed a most effective armour-plating over the rear. As we regard their powerful armour, we seem to see the fierce-toothed Theropods springing from the rear upon the poor-mouthed vegetarians. The carnivores selected the vegetarians, and fitted them to survive. Before the end of the Mesozoic, in fact, the Ornithopods became aggressive as well as armoured. The Triceratops had not only an enormous skull with a great ridged collar round the neck, but a sharp beak, a stout horn on the nose, and two large and sharp horns on the top of the head. We will see something later of the development of horns. The skulls of members of the Ceratops family sometimes measured eight feet from the snout to the ridge of the collar. They were, however, sluggish and stupid monsters, with smaller brains even than the Sauropods.
Such, in broad outline, was the singular and powerful family of the Mesozoic Deinosaurs. Further geological research in all parts of the world will, no doubt, increase our knowledge of them, until we can fully understand them as a great family throwing out special branches to meet the different conditions of the crowded Jurassic age. Even now they afford a most interesting page in the story of evolution, and their total disappearance from the face of the earth in the next geological period will not be unintelligible. We turn from them to the remaining orders of the Jurassic reptiles.
In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are the typical representatives of that extinct race. The two animals, however, belong to very different branches of the reptile world, and are by no means the most formidable of the Mesozoic reptiles. Many orders of the land reptiles sent a branch into the waters in an age which, we saw, was predominantly one of water-surface. The Ichthyosauria ("fish-reptiles") and Thalattosauria ("sea-reptiles") invaded the waters at their first expansion in the later Triassic. The latter groups soon became extinct, but the former continued for some millions of years, and became remarkably adapted to marine life, like the whale at a later period.
The Ichthyosaur of the Jurassic is a remarkably fish-like animal. Its long tapering frame—sometimes forty feet in length, but generally less than half that length—ends in a dip of the vertebral column and an expansion of the flesh into a strong tail-fin. The terminal bones of the limbs depart more and more from the quadruped type, until at last they are merely rows of circular bony plates embedded in the broad paddle into which the limb has been converted. The head is drawn out, sometimes to a length of five feet, and the long narrow jaws are set with two formidable rows of teeth; one specimen has about two hundred teeth. In some genera the teeth degenerate in the course of time, but this merely indicates a change of diet. One fossilised Ichthyosaur of the weaker-toothed variety has been found with the remains of two hundred Belemnites in its stomach. It is a flash of light on the fierce struggle and carnage which some recent writers have vainly striven to attenuate. The eyes, again, which may in the larger animals be fifteen inches in diameter, are protected by a circle of radiating bony plates. In fine, the discovery of young developed skeletons inside the adult frames has taught us that the Ichthgosaur had become viviparous, like the mammal. Cutting its last connection with the land, on which it originated it ceased to lay eggs, and developed the young within its body.
The Ichthyosaur came of the reptile group which we have called the Diapsids. The Plesiosaur seems to belong to the Synapsid branch. In the earlier Mesozoic we find partially aquatic representatives of the line, like the Nothosaur, and in the later Plesiosaur the adaptation to a marine life is complete. The skin has lost its scales, and the front limbs are developed into powerful paddles, sometimes six feet in length. The neck is drawn out until, in some specimens, it is found to consist of seventy-six vertebrae: the longest neck in the animal world. It is now doubted, however, if the neck was very flexible, and, as the jaws were imperfectly joined, the common picture of the Plesiosaur darting its snake-like neck in all directions to seize its prey is probably wrong. It seems to have lived on small food, and been itself a rich diet to the larger carnivores. We find it in all the seas of the Mesozoic world, varying in length from six to forty feet, but it is one of the sluggish and unwieldy forms that are destined to perish in the coming crisis.
The last, and perhaps the most interesting, of the doomed monsters of the Mesozoic was the Pterosaur, or "flying reptile." It is not surprising that in the fierce struggle which is reflected in the arms and armour of the great reptiles, a branch of the family escaped into the upper region. We have seen that there were leaping reptiles with hollow bones, and although the intermediate forms are missing, there is little doubt that the Pterosaur developed from one or more of these leaping Deinosaurs. As it is at first small, when it appears in the early Jurassic—it is disputed in the late Triassic—it probably came from a small and agile Deinosaur, hunted by the carnivores, which relied on its leaping powers for escape. A flapperlike broadening of the fore limbs would help to lengthen the leap, and we must suppose that this membrane increased until the animal could sail through the air, like the flying-fish, and eventually sustain its weight in the air. The wing is, of course, not a feathery frame, as in the bird, but a special skin spreading between the fore limb and the side of the body. In the bat this skin is supported by four elongated fingers of the hand, but in the Pterosaur the fifth (or fourth) finger alone—which is enormously elongated and strengthened—forms its outer frame. It is as if, in flying experiments, a man were to have a web of silk stretching from his arm and an extension of his little finger to the side of his body.
From the small early specimens in the early Jurassic the flying reptiles grow larger and larger until the time of their extinction in the stresses of the Chalk upheaval. Small Pterosaurs continue throughout the period, but from these bat-like creatures we rise until we come to such dragons as the American Pteranodon, with a stretch of twenty-two feet between its extended wings and jaws about four feet long. There were long-tailed Pterosaurs (Ramphorhyncus), sometimes with a rudder-like expansion of the end of the tail, and short-tailed Pterosaurs (Pterodactyl), with compact bodies and keeled breasts, like the bird. In the earlier part of the period they all have the heavy jaws and numerous teeth of the reptile, with four or five well-developed fingers on the front limbs. In the course of time they lose the teeth—an advantage in the distribution of the weight of the body while flying—and develop horny beaks. In the gradual shaping of the breast-bone and head, also, they illustrate the evolution of the bird-form.
But the birds were meantime developing from a quite different stock, and would replace the Pterosaurs at the first change in the environment. There is ground for thinking that these flying reptiles were warm-blooded like the birds. Their hollow bones seem to point to the effective breathing of a warm-blooded animal, and the great vitality they would need in flying points toward the same conclusion. Their brain, too, approached that of the bird, and was much superior to that of the other reptiles. But they had no warm coats to retain their heat, no clavicle to give strength to the wing machinery, and, especially in the later period, they became very weak in the hind limbs (and therefore weak or slow in starting their flight). The coming selection will therefore dismiss them from the scene, with the Deinosaurs and Ammonites, and retain the better organised bird as the lord of the air.
There remain one or two groups of the Mesozoic reptiles which are still represented in nature. The turtle-group (Chelonia) makes its appearance in the Triassic and thrives in the Jurassic. Its members are extinct and primitive forms of the thick-shelled reptiles, but true turtles, both of marine and fresh water, abound before the close of the Mesozoic. The sea-turtles attain an enormous size. Archelon, one of the primitive types, measured about twelve feet across the shell. Another was thirteen feet long and fifteen feet from one outstretched flipper to the other. In the Chalk period they form more than a third of the reptile remains in some regions. They are extremely interesting in that they show, to some extent, the evolution of their characteristic shell. In some of the larger specimens the ribs have not yet entirely coalesced.
The Crocodilians also appear in the later Triassic, abound in the Jurassic, and give way before the later types, the true Crocodiles, in the Cretaceous. They were marine animals with naked skin, a head and neck something like that of the Ichthyosaur, and paddles like those of the Plesiosaur. Their back limbs, however, were not much changed after their adaptation to life in the sea, and it is concluded that they visited the land to lay their eggs. The Teleosaur was a formidable narrow-spouted reptile, somewhat resembling the crocodiles of the Ganges in the external form of the jaws. The modern crocodiles, which replaced this ancient race of sea-crocodiles, have a great advantage over them in the fact that their nostrils open into the mouth in its lower depths. They can therefore close their teeth on their prey under water and breathe through the nose.
Snakes are not found until the close of the Mesozoic, and do not figure in its characteristic reptile population. We will consider them later. But there was a large group of reptiles in the later Mesozoic seas which more or less correspond to the legendary idea of a sea-serpent. These Dolichosaurs ("long reptiles") appear at the beginning of the Chalk period, and develop into a group, the Mososaurians, which must have added considerably to the terrors of the shore-waters. Their slender scale-covered bodies were commonly twenty to thirty feet in length. The supreme representative of the order, the Mososaur, of which about forty species are known, was sometimes seventy-five feet long. It had two pairs of paddles—so that the name of sea-serpent is very imperfectly applicable—and four rows of formidable teeth on the roof of its mouth. Like the Deinosaurs and Pterosaurs, the order was doomed to be entirely extinguished after a brief supremacy in its environment.
From this short and summary catalogue the reader will be able to form some conception of the living inhabitants of the Mesozoic world. It is assuredly the Age of Reptiles. Worms, snails, and spiders were, we may assume, abundant enough, and a great variety of insects flitted from tree to tree or sheltered in the fern brakes. But the characteristic life, in water and on land, was the vast and diversified family of the reptiles. In the western and the eastern continent, and along the narrowing bridge that still united them, in the northern hemisphere and the southern, and along every ridge of land that connected them, these sluggish but formidable monsters filled the stage. Every conceivable device in the way of arms and armour, brute strength and means of escape, seemed to be adopted in their development, as if they were the final and indestructible outcome of the life-principle. And within a single geological period the overwhelming majority of them, especially the larger and more formidable of them, were ruthlessly slain, leaving not a single descendant on the earth. Let us see what types of animals were thus preferred to them in the next great application of selective processes.