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Show 75 GENERAL CONSIDERATIONS ltESPECTING THE ORIGIN OF THE ANIMATED TRIBES THus concludes the wondrous chapter of the earth's history which is told by geology. It takes up our globe at the period when its original incandescent state had near· ly ceased; conducts it through what we have every rea .. son to believe \V ere vast or at least very considerable s~a .. ces of tirne, in the course of which many superficial changes took place, and vegetable and animal life was gradually developed; and drops just at the point when ma_n was apparently about to enter on the scene. The con:pilation of such a history, from n1aterials of so extrao_rdtnary a character, and the powerful nature of the ev~dence which the3e materials afford, are calculated to exctte our admiration, and the result must be allowed to exalt the dignity of science, as a product of man's industry and his reason. If there is anything more than another 1m pressed on OU1' minds by the course of the geological history, it is, that the same laws and conditions of nature now apparent to us have existed throughout the whole time, though the operation of some of these lavvs may nmv be less co_n~picuou:) than in the early ages, from some of the condthons having come to a settlement and a close. That seas ha~e flowed and ebbed, and winds disturbed their surfaces In the time of the secondary rocks, we have proof on the :yet preserved surfaces of the sands which constituted margins of the seas in those days. Even the fall of the wind-sl.ated rain is evidenced on the same tablets. The washing down of detached matter from elevated grounds, w~ich \Ve see rivers constantly engaged in at the present hm~, and which is daily shallowing the seas adjacent to thelt mouths, only appear to have proceeded on a greater sc.ale in earlier epochs. The volcanic subterranean force, wh1ch we see belching forth lavas on the sides of mountains, and throwing up new elevations by land and sea, was only more powerfully operah ve in distant ages. To turn to organic nature, vegetation seems to have proceeded then t:Xactly as now. The very alternations of the sBasons has been read in unmistakeable characters in sections of the trees of those days, precisely as it might be read in a ser• ORIGIN OF 'l'ItE ANT1\1A'rED TR.lBES. '7"f non of ~ tr~e cut dow~ yesterday. The system of prey amongst an1mals flounshed throughout the whole of the pr?-hurnan period; and the adaptation of all plants and anuna]s to their respective sphere of existence was as perfect in those early ages as it is still. But, as has been observed, the operation of the laws may be modified by conditions. At one early age, if there was any dry land at all, it was perhaps en velo_?ed in an atmosphere unfit for the existence of terrestrial animals, and ~~ich bad to go through some changes before that cond1 twn was altered. In the carbonigenous era, dry land seems to have ~onsisted only of clusters of islands, and the temperature was much above what now obtains at the ~arne pl~ces. Volcanic forces, and perhaps also the disl~ tegratmg power, seem to have been on the decrease s!nce the first, or we have at least long enjoyed an exemption f~·om such paroxysms of the former, as appear to have prevailed at the close of .the coal formation in England, and throughout the tertiary era. The surface has also undergone a gradual progress by ·which it has becon1e always more and more variegated, and thereby fitted for the resider1ce of a higher class of animals. In pursuing the progress of the development of both plants and animals upon the globe, we have seen an advance in both cases, ~ong the line leading to the higher forms of organization. Among plants we have first seaweeds, afterward land plants: and among these the simpler ( C'.ellular and cryptogamic) before the more complex. In the department of zoology, we see zoophytes, radiata, mollusca, articulata, existing for ages before there were any higher forms. The first step forward gives fishes, the hurnblest class of the vertebrata; and, moreover, the earliest fishes partake of the character of the next lowest sub-kingdom, the articulata. Afterward come land aniDlals, of which the first are reptiles, universally allowed to be the type next in advance from fishes, and to be con· nected with these by the links of an insensible gradation. From reptiles we advance to birds, and thence to mammalia, which are commenced by marsupialia, acknow .. ledgedly low forms in their class. That there is thus a progress of some kind, the most superficial glance at the geological history is sufficient to convince us. Indeed the doctrine of the gradation of animal forms has received a remarkable support from the discoveries of this sc.ience, |