The Story of Evolution/Chapter IX

We have next to see that when this period of searching adversity comes—as it will in the next chapter—the animal world also offers a luxuriant variety of forms from which the higher types may be selected. This, it need hardly be said, is just what we find in the geological record. The fruitful, steaming, rich-laden earth now offered tens of millions of square miles of pasture to vegetal feeders; the waters, on the other hand, teemed with gigantic sharks, huge Cephalopods, large scorpion-like and lobster-like animals, and shoals of armour-plated, hard-toothed fishes. Successive swarms of vegetarians—Worms, Molluscs, etc.—followed the plant on to the land; and swarms of carnivores followed the vegetarians, and assumed strange, new forms in adaptation to land-life. The migration had probably proceeded throughout the Devonian period, especially from the calmer shores of the inland seas. By the middle of the Coal-forest period there was a very large and varied animal population on the land. Like the plants, moreover, these animals were of an intermediate and advancing nature. No bird or butterfly yet flits from tree to tree; no mammal rears its young in the shelter of the ferns. But among the swarming population are many types that show a beginning of higher organisation, and there is a rich and varied material provided for the coming selection.

The monarch of the Carboniferous forest is the Amphibian. In that age of spreading swamps and "dim, watery woodlands," the stupid and sluggish Amphibian finds his golden age, and, except perhaps the scorpion, there is no other land animal competent to dispute his rule. Even the scorpion, moreover, would not find the Carboniferous Amphibian very vulnerable. We must not think of the smooth-skinned frogs and toads and innocent newts which to-day represent the fallen race of the Amphibia. They were then heavily armoured, powerfully armed, and sometimes as large as alligators or young crocodiles. It is a characteristic of advancing life that a new type of organism has its period of triumph, grows to enormous proportions, and spreads into many different types, until the next higher stage of life is reached, and it is dethroned by the new-comers.

The first indication—apart from certain disputed impressions in the Devonian—of the land-vertebrate is the footprint of an Amphibian on an early Carboniferous mud-flat. Hardened by the sun, and then covered with a fresh deposit when it sank beneath the waters, it remains to-day to witness the arrival of the five-toed quadruped who was to rule the earth. As the period proceeds, remains are found in great abundance, and we see that there must have been a vast and varied population of the Amphibia on the shores of the Carboniferous lagoons and swamps. There were at least twenty genera of them living in what is now the island of Britain, and was then part of the British-Scandinavian continent. Some of them were short and stumpy creatures, a few inches long, with weak limbs and short tails, and broad, crescent-shaped heads, their bodies clothed in the fine scaly armour of their fish-ancestor (the Branchiosaurs). Some (the Aistopods) were long, snake-like creatures, with shrunken limbs and bodies drawn out until, in some cases, the backbone had 150 vertebrae. They seem to have taken to the thickets, in the growing competition, as the serpents did later, and lost the use of their limbs, which would be merely an encumbrance in winding among the roots and branches. Some (the Microsaurs) were agile little salamander-like organisms, with strong, bony frames and relatively long and useful legs; they look as if they may even have climbed the trees in pursuit of snails and insects. A fourth and more formidable sub-order, the Labyrinthodonts—which take their name from the labyrinthine folds of the enamel in their strong teeth—were commonly several feet in length. Some of them attained a length of seven or eight feet, and had plates of bone over their heads and bellies, while the jaws in their enormous heads were loaded with their strong, labyrinthine teeth. Life on land was becoming as eventful and stimulating as life in the waters.

The general characteristic of these early Amphibia is that they very clearly retain the marks of their fish ancestry. All of them have tails; all of them have either scales or (like many of the fishes) plates of bone protecting the body. In some of the younger specimens the gills can still be clearly traced, but no doubt they were mainly lung-animals. We have seen how the fish obtained its lungs, and need add only that this change in the method of obtaining oxygen for the blood involved certain further changes of a very important nature. Following the fossil record, we do not observe the changes which are taking place in the soft internal organs, but we must not lose sight of them. The heart, for instance, which began as a simple muscular expansion or distension of one of the blood-vessels of some primitive worm, then doubled and became a two-chambered pump in the fish, now develops a partition in the auricle (upper chamber), so that the aerated blood is to some extent separated from the venous blood. This approach toward the warm-blooded type begins in the "mud-fish," and is connected with the development of the lungs. Corresponding changes take place in the arteries, and we shall find that this change in structure is of very great importance in the evolution of the higher types of land-life. The heart of the higher land-animals, we may add, passes through these stages in its embryonic development.

Externally the chief change in the Amphibian is the appearance of definite legs. The broad paddle of the fin is now useless, and its main stem is converted into a jointed, bony limb, with a five-toed foot, spreading into a paddle, at the end. But the legs are still feeble, sprawling supports, letting the heavy body down almost to the ground. The Amphibian is an imperfect, but necessary, stage in evolution. It is an improvement on the Dipneust fish, which now begins to dwindle very considerably in the geological record, but it is itself doomed to give way speedily before one of its more advanced descendants, the Reptile. Probably the giant salamander of modern Japan affords the best suggestion of the large and primitive salamanders of the Coal-forest, while the Caecilia—snake-like Amphibia with scaly skins, which live underground in South America—may not impossibly be degenerate survivors of the curious Aistopods.

Our modern tailless Amphibia, frogs and toads, appear much later in the story of the earth, but they are not without interest here on account of the remarkable capacity which they show to adapt themselves to different surroundings. There are frogs, like the tree-frog of Martinique, and others in regions where water is scarce, which never pass through the tadpole stage; or, to be quite accurate, they lose the gills and tail in the egg, as higher land-animals do. On the other hand, there is a modern Amphibian, the axolotl of Mexico, which retains the gills throughout life, and never lives on land. Dr. Gadow has shown that the lake in which it lives is so rich in food that it has little inducement to leave it for the land. Transferred to a different environment, it may pass to the land, and lose its gills. These adaptations help us to understand the rich variety of Amphibian forms that appeared in the changing conditions of the Carboniferous world.

When we think of the diet of the Amphibia we are reminded of the other prominent representatives of land life at the time. Snails, spiders, and myriapods crept over the ground or along the stalks of the trees, and a vast population of insects filled the air. We find a few stray wings in the Silurian, and a large number of wings and fragments in the Devonian, but it is in the Coal-forest that we find the first great expansion of insect life, with a considerable development of myriapods, spiders, and scorpions. Food was enormously abundant, and the insect at least had no rival in the air, for neither bird nor flying reptile had yet appeared. Hence we find the same generous growth as amongst the Amphibia. Large primitive "may-flies" had wings four or five inches long; great locust-like creatures had fat bodies sometimes twenty inches in length, and soared on wings of remarkable breadth, or crawled on their six long, sprawling legs. More than a thousand species of insects, and nearly a hundred species of spiders and fifty of myriapods, are found in the remains of the Coal-forests.

From the evolutionary point of view these new classes are as obscure in their origin, yet as manifestly undergoing evolution when they do fully appear, as the earlier classes we have considered. All are of a primitive and generalised character; that is to say, characters which are to-day distributed among widely different groups were then concentrated and mingled in one common ancestor, out of which the later groups will develop. All belong to the lowest orders of their class. No Hymenopters (ants, bees, and wasps) or Coleopters (beetles) are found in the Coal-forest; and it will be many millions of years before the graceful butterfly enlivens the landscapes of the earth. The early insects nearly all belong to the lower orders of the Orthopters (cockroaches, crickets, locusts, etc.) and Neuropters (dragon-flies, may-flies, etc.). A few traces of Hemipters (now mainly represented by the degenerate bugs) are found, but nine-tenths of the Carboniferous insects belong to the lowest orders of their class, the Orthopters and Neuropters. In fact, they are such primitive and generalised insects, and so frequently mingle the characteristics of the two orders, that one of the highest authorities, Scudder, groups them in a special and extinct order, the Palmodictyoptera; though this view is not now generally adopted. We shall find the higher orders of insects making their appearance in succession as the story proceeds.

Thus far, then, the insects of the Coal-forest are in entire harmony with the principle of evolution, but when we try to trace their origin and earlier relations our task is beset with difficulties. It goes without saying that such delicate frames as those of the earlier insects had very little chance of being preserved in the rocks until the special conditions of the forest-age set in. We are, therefore, quite prepared to hear that the geologist cannot give us the slenderest information. He finds the wing of what he calls "the primitive bug" (Protocimex), an Hemipterous insect, in the later Ordovician, and the wing of a "primitive cockroach" (Palaeoblattina) in the Silurian. From these we can merely conclude that insects were already numerous and varied. But we have already, in similar difficulties, received assistance from the science of zoology, and we now obtain from that science a most important clue to the evolution of the insect.

In South America, South Africa, and Australasia, which were at one time connected by a great southern continent, we find a little caterpillar-like creature which the zoologist regards with profound interest. It is so curious that he has been obliged to create a special class for it alone—a distinction which will be appreciated when I mention that the neighbouring class of the insects contains more than a quarter of a million living species. This valuable little animal, with its tiny head, round, elongated body, and many pairs of caterpillar-like legs, was until a few decades ago regarded as an Annelid (like the earth-worm). It has, in point of fact, the peculiar kidney-structures (nephridia) and other features of the Annelid, but a closer study discovered in it a character that separated it far from any worm-group. It was found to breathe the air by means of tracheae (little tubes running inward from the surface of the body), as the myriapods, spiders, and insects do. It was, in other words, "a kind of half-way animal between the Arthropods and the Annelids" ("Cambridge Natural History," iv, p. 5), a surviving kink in the lost chain of the ancestry of the insect. Through millions of years it has preserved a primitive frame that really belongs to the Cambrian, if not an earlier, age. It is one of the most instructive "living fossils" in the museum of nature.

Peripatus, as the little animal is called, points very clearly to an Annelid ancestor of all the Tracheates (the myriapods, spiders, and insects), or all the animals that breathe by means of trachere. To understand its significance we must glance once more at an early chapter in the story of life. We saw that a vast and varied wormlike population must have filled the Archaean ocean, and that all the higher lines of animal development start from one or other point in this broad kingdom. The Annelids, in which the body consists of a long series of connected rings or segments, as in the earth-worm, are one of the highest groups of these worm-like creatures, and some branch of them developed a pair of feet (as in the caterpillar) on each segment of the body and a tough, chitinous coat. Thus arose the early Arthropods, on tough-coated, jointed, articulated animals. Some of these remained in the water, breathing by means of gills, and became the Crustacea. Some, however, migrated to the land and developed what we may almost call "lungs"—little tubes entering the body at the skin and branching internally, to bring the air into contact with the blood, the tracheae.

In Peripatus we have a strange survivor of these primitive Annelid-Tracheates of many million years ago. The simple nature of its breathing apparatus suggests that the trachere were developed out of glands in the skin; just as the fish, when it came on land, probably developed lungs from its swimming bladders. The primitive Tracheates, delivered from the increasing carnivores of the waters, grew into a large and varied family, as all such new types do in favourable surroundings. From them in the course of time were evolved the three great classes of the Myriapods (millipedes and centipedes), the Arachnids (scorpions, spiders, and mites), and the Insects. I will not enter into the much-disputed and Obscure question of their nearer relationship. Some derive the Insects from the Myriapods, some the Myriapods from the Insects, and some think they evolved independently; while the rise of the spiders and scorpions is even more obscure.

But how can we see any trace of an Annelid ancestor in the vastly different frames of these animals which are said to descend from it? It is not so difficult as it seems to be at first sight. In the Myriapod we still have the elongated body and successive pairs of legs. In the Arachnid the legs are reduced in number and lengthened, while the various segments of the body are fused in two distinct body-halves, the thorax and the abdomen. In the Insect we have a similar concentration of the primitive long body. The abdomen is composed of a large number (usually nine or ten) of segments which have lost their legs and fused together. In the thorax three segments are still distinctly traceable, with three pairs of legs—now long jointed limbs—as in the caterpillar ancestor; in the Carboniferous insect these three joints in the thorax are particularly clear. In the head four or five segments are fused together. Their limbs have been modified into the jaws or other mouth-appendages, and their separate nerve-centres have combined to form the large ring of nerve-matter round the gullet which represents the brain of the insect.

How, then, do we account for the wings of the insect? Here we can offer nothing more than speculation, but the speculation is not without interest. It may be laid down in principle that the flying animal begins as a leaping animal. The "flying fish" may serve to suggest an early stage in the development of wings; it is a leaping fish, its extended fins merely buoying it, like the surfaces of an aeroplane, and so prolonging its leap away from its pursuer. But the great difficulty is to imagine any part of the smooth-coated primitive insect, apart from the limbs (and the wings of the insect are not developed from legs, like those of the bird), which might have even an initial usefulness in buoying the body as it leaped. It has been suggested, therefore, that the primitive insect returned to the water, as the whale and seal did in the struggle for life of a later period. The fact that the mayfly and dragon-fly spend their youth in the water is thought to confirm this. Returning to the water, the primitive insects would develop gills, like the Crustacea. After a time the stress of life in the water drove them back to the land, and the gills became useless. But the folds or scales of the tough coat, which had covered the gills, would remain as projecting planes, and are thought to have been the rudiment from which a long period of selection evolved the huge wings of the early dragon-flies and mayflies. It is generally believed that the wingless order of insects (Aptera) have not lost, but had never developed, wings, and that the insects with only one or two pairs all descend from an ancestor with three pairs.

The early date of their origin, the delicacy of their structure, and the peculiar form which their larval development has generally assumed, combine to obscure the evolution of the insect, and we must be content for the present with these general indications. The vast unexplored regions of Africa, South America, and Central Australia, may yet yield further clues, and the riddle of insect-metamorphosis may some day betray the secrets which it must hold. For the moment the Carboniferous insects interest us as a rich material for the operation of a coming natural selection. On them, as on all other Carboniferous life, a great trial is about to fall. A very small proportion of them will survive that trial, and they trill be the better organised to maintain themselves and rear their young in the new earth.

The remaining land-life of the Coal-forest is confined to worm-like organisms whose remains are not preserved, and land-snails which do not call for further discussion. We may, in conclusion, glance at the progress of life in the waters. Apart from the appearance of the great fishes and Crustacea, the Carboniferous period was one of great stimulation to aquatic life. Constant changes were taking place in the level and the distribution of land and water. The aspect of our coal seams to-day, alternating between thick layers of sand and mud, shows a remarkable oscillation of the land. Many recent authorities have questioned whether the trees grew on the sites where we find them to-day, and were not rather washed down into the lagoons and shallow waters from higher ground. In that case we could not too readily imagine the forest-clad region sinking below the waves, being buried under the deposits of the rivers, and then emerging, thousands of years later, to receive once more the thick mantle of sombre vegetation. Probably there was less rising and falling of the crust than earlier geologists imagined. But, as one of the most recent and most critical authorities, Professor Chamberlin, observes, the comparative purity of the coal, the fairly uniform thickness of the seams, the bed of clay representing soil at their base, the frequency with which the stumps are still found growing upright (as in the remarkable exposed Coal-forest surface in Glasgow, at the present ground-level), the perfectly preserved fronds and the general mixture of flora, make it highly probable that the coal-seam generally marks the actual site of a Coal-forest, and there were considerable vicissitudes in the distribution of land and water. Great areas of land repeatedly passed beneath the waters, instead of a re-elevation of the land, however, we may suppose that the shallow water was gradually filled with silt and debris from the land, and a fresh forest grew over it.

These changes are reflected in the progress of marine life, though their influence is probably less than that of the great carnivorous monsters which now fill the waters. The heavy Arthrodirans languish and disappear. The "pavement-toothed" sharks, which at first represent three-fourths of the Elasmobranchs, dwindle in turn, and in the formidable spines which develop on them we may see evidence of the great struggle with the sharp-toothed sharks which are displacing them. The Ostracoderms die out in the presence of these competitors. The smaller fishes (generally Crossopterygii) seem to live mainly in the inland and shore waters, and advance steadily toward the modern types, but none of our modern bony fishes have yet appeared.

More evident still is the effect of the new conditions upon the Crustacea. The Trilobite, once the master of the seas, slowly yields to the stronger competitors, and the latter part of the Carboniferous period sees the last genus of Trilobites finally extinguished. The Eurypterids (large scorpion-like Crustacea, several feet long) suffer equally, and are represented by a few lingering species. The stress favours the development of new and more highly organised Crustacea. One is the Limulus or "king-crab," which seems to be a descendant, or near relative, of the Trilobite, and has survived until modern times. Others announce the coming of the long-tailed Crustacea, of the lobster and shrimp type. They had primitive representatives in the earlier periods, but seem to have been overshadowed by the Trilobites and Eurypterids. As these in turn are crushed, the more highly organised Malacostraca take the lead, and primitive specimens of the shrimp and lobster make their appearance.

The Echinoderms are still mainly represented by the sea-lilies. The rocks which are composed of their remains show that vast areas of the sea-floor must have been covered with groves of sea-lilies, bending on their long, flexible stalks and waving their great flower-like arms in the water to attract food. With them there is now a new experiment in the stalked Echinoderm, the Blastoid, an armless type; but it seems to have been a failure. Sea-urchins are now found in the deposits, and, although their remains are not common, we may conclude that the star-fishes were scattered over the floor of the sea.

For the rest we need only observe that progress and rich diversity of forms characterise the other groups of animals. The Corals now form great reefs, and the finer Corals are gaining upon the coarser. The Foraminifers (the chalk-shelled, one-celled animals) begin to form thick rocks with their dead skeletons; the Radiolaria (the flinty-shelled microbes) are so abundant that more than twenty genera of them have been distinguished in Cornwall and Devonshire. The Brachiopods and Molluscs still abound, but the Molluscs begin to outnumber the lower type of shell-fish. In the Cephalopods we find an increasing complication of the structure of the great spiral-shelled types.

Such is the life of the Carboniferous period. The world rejoices in a tropical luxuriance. Semi-tropical vegetation is found in Spitzbergen and the Antarctic, as well as in North Europe, Asia, and America, and in Australasia; corals and sea-lilies flourish at any part of the earth's surface. Warm, dank, low-lying lands, bathed by warm oceans and steeped in their vapours, are the picture suggested—as we shall see more closely—to the minds of all geologists. In those happy conditions the primitive life of the earth erupts into an abundance and variety that are fitly illustrated in the well-preserved vegetation of the forest. And when the earth has at length flooded its surface with this seething tide of life; when the air is filled with a thousand species of insects, and the forest-floor feels the heavy tread of the giant salamander and the light feet of spiders, scorpions, centipedes, and snails, and the lagoons and shores teem with animals, the Golden Age begins to close, and all the semi-tropical luxuriance is banished. A great doom is pronounced on the swarming life of the Coal-forest period, and from every hundred species of its animals and plants only two or three will survive the searching test.