Just as the animal world is primarily divided into Invertebrates and Vertebrates, the plant world is primarily divided into a lower kingdom of spore-bearing plants (the Cryptogams) and an upper kingdom of seed-bearing plants (the Phanerogams). Again, just as the first half of the earth's story is the age of Invertebrate animals, so it is the age of Cryptogamous plants. So far evolution was always justified in the plant record. But there is a third parallel, of much greater interest. We saw that at one time the evolutionist was puzzled by the clean division of animals into Invertebrate and Vertebrate, and the sudden appearance of the backbone in the chronicle: he was just as much puzzled by the sharp division of our plants into Cryptogams and Phanerogams, and the sudden appearance of the latter on the earth during the Coal-forest period. And the issue has been a fresh and recent triumph for evolution.
Plants are so well preserved in the coal that many years of microscopic study of the remains, and patient putting-together of the crushed and scattered fragments, have shown the Carboniferous plants in quite a new light. Instead of the Coal-forest being a vast assemblage of Cryptogams, upon which the higher type of the Phanerogam is going suddenly to descend from the clouds, it is, to a very great extent, a world of plants that are struggling upward, along many paths, to the higher level. The characters of the Cryptogam and Phanerogam are so mixed up in it that, although the special lines of development are difficult to trace, it is one massive testimony to the evolution of the higher from the lower. The reproductive bodies of the great Lepidodendra are sometimes more like seeds than spores, while both the wood and the leaves of the Sigillaria have features which properly belong to the Phanerogam. In another group (called the Sphenophyllales) the characters of these giant Club-mosses are blended with the characters of the giant Horsetails, and there is ground to think that the three groups have descended from an earlier common ancestor.
Further, it is now believed that a large part of what were believed to be Conifers, suddenly entering from the unknown, are not Conifers at all, but Cordaites. The Cordaites is a very remarkable combination of features that are otherwise scattered among the Cryptogams, Cycads, and Conifers. On the other hand, a very large part of what the geologist had hitherto called "Ferns" have turned out to be seed-bearing plants, half Cycad and half Fern. Numbers of specimens of this interesting group—the Cycadofilices (cycad-ferns) or Pteridosperms (seed-ferns)—have been beautifully restored by our botanists. [*] They have afforded a new and very plausible ancestor for the higher trees which come on the scene toward the close of the Coal-forests, while their fern-like characters dispose botanists to think that they and the Ferns may be traced to a common ancestor. This earlier stage is lost in those primitive ages from which not a single leaf has survived in the rocks. We can only say that it is probable that the Mosses, Ferns, Lycopods, etc., arose independently from the primitive level. But the higher and more important development is now much clearer. The Coal-forest is not simply a kingdom of Cryptogams. It is a world of aspiring and mingled types. Let it be subjected to some searching test, some tremendous spell of adversity, and we shall understand the emergence of the higher types out of the luxuriant profusion and confusion of forms.
* See, especially, D. H. Scott, "Studies of Fossil Botany"
(2nd ed., 1908), and "The Evolution of Plants" (1910—small
popular manual).
CHAPTER IX. THE ANIMALS OF THE COAL-FOREST
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.