These facts teach us conclusively that the outlines of organic life on the globe have always been the same; that the great classes of animals and plants have always had their representatives, and that the variations which have been introduced, have been merely adaptations to the varying condition of the earth's surface. The higher and more complex natures, both of animals and plants, were not introduced at first, because the surface was not adapted to their existence; and they were brought in only as circumstances, favorable to their development, prepared the way. There is another fact of great interest on this subject. Even a cursory examination of the animals and plants now on the globe, shows such a gradation of their characters that they form a sort of chain, extending from the most to the least perfect species. But we see at once that the links of this chain are of very unequal length; or, rather, that there are in some instances wide intervals between the nearest species, as if one or more links had dropped out. How remarkable that some of these lost links should be found among the fossil species! I will refer to a few examples. Among existing animals no genera or tribes are more widely separated than those with thick skins, denominated pachydermata; such as the rhinoceros and the elephant. But among the fossil animals of the tertiary strata, this tribe of animals was much more common; and many of them fill up the blanks in the existing families, and thus render more perfect and uniform the great chain of being which binds together into one great system the present and past periods of organic life. A similar case occurs among fossil plants. In tropical climates we find a few species--not much over twenty--of a singular family of plants, the cycadeae connecting the great families of coniferae, or dicotyledons, with the palms, which are monocotyledonous, and the ferns, which are acotyledonous. The chasm, however, between those great and dissimilar classes of plants is but imperfectly filled by the few living species of cycadeae. But of the fossil species hitherto found above the coal formation, almost one half are cycadeae; so that here, too, the lost links of the chain are supplied. "Facts like these," says Dr. Buckland, "are inestimably precious to the natural theologian, for they identify, as it were, the Artificer, by details of manipulation throughout his works. They appeal to the physiologist, in language more commanding than human eloquence; the voice of very stocks and stones, that have been buried for countless ages in the deep recesses of the earth, proclaiming the universal agency of one all-directing, all-sustaining Creator, in whose will and power these harmonious systems originated, and by whose universal providence they are, and have at all times been, maintained."--_Bridgewater Treatise_, vol. i. p. 502.