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Show 30G IMPERFECTIO~ OF THE CHAP. TX. here they would remain confined, unt.il some of the species became adapted to a cooler climate, and were enabled to double the southern capes of Africa or Australia, and thus reach other and distant seas. From these and similar considerations, but chiefly from our ignorance of the geology of other countries beyond the confines of Europe and the United States ; and from the revolution in our palooontological ideas on many points, which the discoveries of even the last dozen years have effected, it seems to me to be about as rash in us to dogmatize on the succession of organic beings throughout the world., as it would be for a naturalist to land for five minutes on some one barren point in Australia, and then to discuss the number and range of its productions. On the sudden appearance of groups of Allied Speaies in the lowest known fossiliferous strata.-There is another and allied difficulty, which is much graver. I allude to the manner in which numbers of species of the same group, suddenly appear in the lowest known fossiliferous rocks. Most of the arguments which have convinced me that all the existing species of the same group have descended from one progenitor, apply with nearly equal force to the earliest known species. For instance, I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age, and which probably differed greatly from any known animal. Some of the most ancient Silurian animals, as the N au til us, Lingula, &c., do not differ much from living species; and it cannot on my theory be supposed, that these old species were the progenitors of all the species of the orders to which they belong, for they do not present characters in any degree intermediate between them. CHAP. IX. GEOLOGICAL RECORD. 307 If, moreover, they had been the progenitors of these orders, they would almost certainly have been long ago supplanted and exterminated by their numerous and improved descendants. Consequently, lf_!!!Y theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer th~n, the whole interval from the Silurian age to the present day ; and that during these vast, yet quite un- l J known, periods of time, the world swarmed with living. creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer, Several of the most eminent geologists, with Sir R. Murchison at their head, are convinced that we see in the organic remains of the lowest Silurian stratum the dawn of life on this planet. Other highly competent judges, as Lyell and the late E. Forbes, dispute this conclusion. We should not forget that only a small portion of the world is known with accuracy. M. Barrande has lately added another and lower stage to the Silurian system, abounding with new and peculiar species. Traces of life have been detected in the Longmynd beds beneath Barrande's so-called primordial zone. The presence of phosphatic nodules and bituminous matter in some of the lowest azoic rocks, probably indicates the former e11fistence of life at these periods. But the difficulty of understanding the ab- 1 sence of vast piles of fossiliferous strata, which on my theory no doubt were somewhere accumulated b.efon~ ' the ""Silurian epoch, is -yery great. If these most ancient beds had been wholly worn away .by denudation, or obliterated by metamorphic action1 we ought to find only small remnants of the formations next succeeding them in .age, and these ought to be very generally in |