OCR Text |
Show lEA PLANTS, CORALS, 1L TC. conditions, one in which organic life has probably playe<J a part. It is not easy to suppose that, at ·l nis period, carbon was adopted directly in its gaseous form into rocks; for, if so, why should it not have been -taken into earlier ones also? But we know that plants take it in, and transform it into substance; and we also know that there are classes of animals (marine polypes) which are capable of appropriating it, in connection with lime ( carbcmate of lime) from the waters of the ocean, provided it be there in solution; and this substance do these anirnals deposit in masses (coral reefs) equal in extent to many sti:ata. It has even been suggested, on strong grounds of probability, that a elass of limestone beds are simply these reefs subjected to subsequent heat and pressure. The appearance, then, of limestone beds in the early part of_ th~ stratified series, may be presumed to be connected w1th the fact of the commencement of organic life upon our planet, and, indeed, a consequent and a symptom of it. It may not be out of place here to remark, that carbon is p1·esu1ned to exist largely in the interior of the earth, from the fact of such considerable quantities of it issuinoat this day, in the form of carbonic acid gas, from fissure~ and springs. The primeval ~d subsequent history of this element is worthy of much attention, and we shall have to revert to it as a matter greatly concerning our subject Delabeche estimates the quantity of carbonic acid gas locked up in every cubic yard of limestone, at 16,000 cubic feet. The quantity locl{ed up in coal, in which it forms from 64 to 75 p.er cent., must also be enormous. I[ all this were disengaged in a gaseus form, the constitution of the atmosphere would undergo a change, of which the first effe~t would be the extinction of life in all land animals. But a large proportion of it must have at one time been in the atmosphere. The atmosphere would th~n, of com·~e,_ be incapable of supporting life in land antmals. It Is Important, however, to observe that such an atmosphere would not be inconsistent with a luxuriant, land vegetation; for experiment has proved that plants will flourish in air containing one-twelfth of this gas, or 166 times more than the present charge of our atmosphere. T~e results \Vhich. we observe are perfectly consistent wtth, anri :r:nay b? said to presuppose an atmosphere highly charged \Vlth this gas, from about the close of th~ prima· COMMENCEMENT OF ORGANIC LIFE. 33 ry non-fossiliferous rocks to the termination of the carb~ ni_ferous series, for the.re we ~ee vast ~epo~Jts (coal) containing carbon as a large mgredient, whtle at the same time tlte leaves of the Stone Book present no record of the contemporaneous existence of land animals. The hypothesis of the connection of the first limestone beds with the commencement of organic life upon our planet is supported by the fact, that in these beds we find the first remains of the bodies of animated creatures. My hypothesis tnay indeed be unsound; but, whether or not, it is clear, taking organic remains as upon the whole a faithful chronicle, that the deposition of these limestone beds was coeval with the existence of the earliest, or all but the earliest, living creatures upon earth. And what were those creatures? It might well be with a kind of awe that the ·uninstructed inquirer would wait for an answer to this question. But nature is simpler thdn · man's wit would make her, and behoJd, the interrogation only brings before us the unpretending forms of various zoophytes and ·polypes, together with a few single and double-valved shell-fish, (mollusks,) all of them creatures of the sea. It is rather surprising to find these before any vegetable forms, considering that vegetables appear 'to us as f01·min~ the necessary first link in the chain of nutrition; but It is probable that there were sea plants, and also some simpler forms of animal life, before this period, although of too slight a substance to leave any fossil trace of their existence. 'r: he exact point in the ascending stratified series at whiCh the first traces of organic life are to be found is not clearly determined. Dr. M'Culloch states that he found foss~l orthocerata (a kind of shell-fish) so early as the gneiss tract of Loch Eribol, in Suther lana; but Messrs. Sedgwick and Murchison, on a subsequent search, could not .verjfy the _discovery. It has also been .stated, that the gneiss and mica tract of Bohemia contains s01ne seams of grawacke, in which are m·o-anic remains; but British geologists have not as yet att~ched much importance to this statement. We have to look a little higher in the series for indubitable traces of organic life. Above the gneiss and mica slate system, or group of strata, is the Clay Slate and Grawacke Slate System; that is to say, it is higher in the order of suprapqsit_i~n, though very often it rests immediately on the prlmihve |