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.
* The civic authorities of Glasgow have wisely exposed and
protected this instructive piece of Coal-forest in one of
their parks. I noticed, however that in the admirable
printed information they supply to the public, they describe
the trees as "at least several hundred thousand years old."
There is no authority in the world who would grant less than
ten million years since the Coal-forest period.
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.