By parity of reasoning it may be inferred, that as great changes would be effected in the waters of the globe as in the constituents of the atmosphere; and, while thus preparing for the introduction of new families of animals, the destruction of already existing tribes may be as conclusively imagined. The various calcareous deposits in the mountain limestone, magnesian, oolite, and chalk periods, would imply very different qualities in the condition of the ocean; an infusion or abstraction of ingredients as favorable to the existence of one kind of animal life as they would be destructive of another. A period of great plutonic action, too, when vast masses of melted matter, charged with metallic and other substances, were poured over the bed of the sea, could not fail to have considerable influence upon many of the inhabitants of the deep; and while providence was making arrangements for an increase, or diversity, or for higher types of animal life, the existence of other forms and classes was ordained to terminate. The introduction of new and higher races upon the earth has thus been accounted for. “Through such a medium as the air,” says the authority quoted above, “approaching in a corresponding degree to the physical properties of water, a cold-blooded animal might even rise above the surface, and wing its heavy flight, since this would demand less energetic muscular actions than are now requisite for such a kind of locomotion: and thus we may conceive why the atmosphere of our planet, during the earlier oolite periods, may have been traversed by creatures of no higher organization than saurians. If we may presume to conjecture that atmospheric pressure has been diminished, by a change in the composition, as well as by a diminution of the general mass of the air, the beautiful adaptation of the structure of birds to a medium thus rendered both lighter and more invigorating, by the abstraction of carbon and an increase of oxygen, must be appreciable by every physiologist. And it is not without interest to observe, that the period when such change would be thus indicated by the first appearance of birds in the Wealden strata, is likewise characterized by the prevalence of those dinosaurian reptiles, which in structure most nearly approach mammalia, and which in all probability, from their correspondence with crocodiles in the anatomy of the thorax, enjoyed a circulation as complete as that of the crocodile when breathing freely on dry land.”