Our next may be that of an incandescent globe, molten and glowing, and surrounded by a vast vaporous envelope, but tending by degrees to a condition in which it shall have a solid crust, on which the greater part of the watery vapour suspended in its atmosphere is to be condensed into a heated ocean. Our third picture may represent the world of what geologists call the archæan, or eozoic period, when the crust had been furrowed up into ridges of land, and corresponding but wider depressions occupied by the sea. Into the latter the rains falling on the land are carrying sediment derived from the wasting rocks, though the waters are still warm and the thinner parts of the crust are still welling out rocky material, either molten or dissolved in heated water. In this period there were probably low forms of animal life in the waters and plants on the land, though we know little of their exact nature. A fourth picture may represent that great and long-continued palæozoic period in which the waters swarmed with many forms of life, when fishes were introduced into the sea, and when the land became covered with dense forests of plants allied to the modern club-mosses, ferns, mares'-tails and pines; while insects, scorpions and snails, and some of the humbler forms of reptiles, found place on the land. Returning after an interval, we should see a fifth picture, that of the mesozoic world. This was the age of reptiles, when animals of that class attained their highest and most gigantic forms, and occupied in the sea, on the land, and in the air the places now held by the mammals and the birds; while the continents were covered with a flora distinct alike from that of the previous and succeeding periods, replaced, however, as time went on by forests very like those of the modern world. In this age the earliest mammals or ordinary quadrupeds were introduced, few at first, small and of low rank in their class. Birds also made their appearance, and toward the close of the period fishes of modern types swarmed for the first time in the sea. Lastly, we might see in the cenozoic, or tertiary age, the newest of all, quadrupeds dominant on the land and modern types of animal life in the sea. In this period our continents finally assumed their present forms. Toward its close and after many vicissitudes of geography and climate, and several successive dynasties of mammalian life, man and the land animals now his contemporaries occupied the world, and thus the cenozoic passes into the _anthropic_, or modern period, called by some, but without good reason, 'quaternary,' since it is in all respects a proper continuation of the tertiary, or cenozoic.[3] [3] It will be seen that our six pictures are in some degree parallel with the 'days' of creation. This is not an intentional reconciliation. It merely expresses the fact of the case, whatever its significance.