After this era of desolation and gloom, we enter upon what is technically termed the "Transition period," and here we begin to mark the gradual preparation of the globe for the reception of its destined inhabitants. The change is, however, at first very slight, and there is evidence of frequent convulsions and of a high degree of temperature; but the action of fire appears to have declined in force, and aqueous agencies are exerting themselves. The earlier portion of this formation is rendered peculiarly interesting by the fact, that during it the most ancient forms of life sprang into existence. It is true that merely a few species of shell-fish, with some corals, inhabited the depths of the ocean, while the dry land still remained untenanted; nevertheless, humble and scanty as they were, we can not fail to look with interest on the earliest types of that existence, which has subsequently reached such perfection in ourselves. The presence of corals shows, that although the transition seas had lost their high temperature, yet they retained a sufficient degree of heat to encourage the development of animals requiring warmth. These minute animals possess the remarkable property of extracting from the elementary bodies held in solution in the waters, the materials for forming new rocks. To the coral animalcule or polype we owe much of the vast limestone beds which are found in every part of the world, and many a vessel laden with the riches and productions of the earth finds a grave on the sunken reefs that are the fruit of its labors. As ages elapsed, and the universe became better adapted for the reception of life, the waters swarmed with zoophytes and corals, and in the silurian strata we find organic remains abundant; shell-fish are numerous and distinct in form, and in some instances display a very interesting anatomical construction. As an instance we may mention the Trilobite, an animal of the crustacean order; the front part of its body formed a large crescent-shaped shield, while the hinder portion consisted of a broad triangular tail, composed of segments folding over each other like the tail of a lobster; its most peculiar organ, however, was the eye, which was composed of four hundred minute spherical lenses placed in separate compartments, and so situated, that in the animal's usual place at the bottom of the ocean it could see every thing around. This kind of eye is also common to the existing butterfly and dragon-fly, the former of which has 35,000, and the latter 14,000 lenses.