Again, it is conjectured—“The first indications of the warm-blooded classes would appear, if introduced into the reptilian era, under the form of such small insectivorous mammals as are known at the present day to have a lower amount of respiration than the rest of the class; and the earliest discovered remains of mammalia,—as, for example, those in the Stonesfield oolite,—are actually the jaws of such species, with which are combined the characters of that order, Marsupialia, which is most nearly related to the oviparous vertebrata.” It has been seen that igneous and aqueous agents have remodeled, from time to time, the physical geography of the globe. Can it admit of a doubt that changes in the physical structure of the earth’s surface will be accompanied with other changes in the organic productions of extensive areas? Species, it is well known, both of plants and animals, are limited to particular localities of variable, and often of no great, extent. If marine, an alteration in the sea bottom will prove fatal to many. If terrestrial, an increase of altitude, the conversion of dry land into marshes and lakes, or of lakes and marshes into meadow and arid loamy soil, will completely alter the flora and fauna of the district in question. Look into any estuary or rocky pool along the shore of the ocean, swarming with testacea, and crustacea; every bowlder incrusted with corallines; the rocks carpeted all over with fuci, waving with every ripple their long graceful branches, or smoothing and polishing their sides in the violent currents; creeping things, too, innumerable shy stealthy creatures, darting amid the shingle, or burrowing in the sands; and the finny tribes, of all forms, glancing and sparkling like living gems in the dark green thickets. This is one description only of tens of thousands of such phenomena around the islands of Great Britain. An elevation of a few feet, and what myriads of animals, whose only habitat are these ocean caves, would perish, and their races be forever blotted from the things that were! These shores have witnessed many such upheavals. Not a plain, hill, or rock, in the whole continent of Europe, but once formed the bed of the sea. Even now, what a vast influence does mineralogical structure alone exercise over the economy of life, both as to the number of individuals and the character of species frequenting particular localities. Trees as well as plants have an adaptation to certain kinds of soil, and once firmly rooted, birds, insects, and creeping things, will also resort thither in quest of shelter or of food. Aquatic fowl, the waders and swimmers of our sea-shores, have their favorite haunt among the breakers or calm bays, whose submarine rocks furnish pasturage and shelter to molluscs, crustaceans, and fishes; while, again, over the marshy, the oozy, the sandy, the gravelly, or the rocky beach, other families, both terrestrial and marine, maintain their respective ascendency.