按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
subsidence。 Since publishing my views on this subject in 1845； I have watched the progress of Geology； and have been surprised to note how author after author； in treating of this or that great formation； has come to the conclusion that it was accumulated during subsidence。 I may add； that the only ancient tertiary formation on the west coast of South America； which has been bulky enough to resist such degradation as it has as yet suffered； but which will hardly last to a distant geological age； was certainly deposited during a downward oscillation of level； and thus gained considerable thickness。
All geological facts tell us plainly that each area has undergone numerous slow oscillations of level； and apparently these oscillations have affected wide spaces。 Consequently formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation； may have been formed over wide spaces during periods of subsidence； but only where the supply of sediment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to decay。 On the other hand； as long as the bed of the sea remained stationary； thick deposits could not have been accumulated in the shallow parts； which are the most favourable to life。 Still less could this have happened during the alternate periods of elevation； or； to speak more accurately； the beds which were then accumulated will have been destroyed by being upraised and brought within the limits of the coast…action。
Thus the geological record will almost necessarily be rendered intermittent。 I feel much confidence in the truth of these views； for they are in strict accordance with the general principles inculcated by Sir C。 Lyell； and E。 Forbes independently arrived at a similar conclusion。
One remark is here worth a passing notice。 During periods of elevation the area of the land and of the adjoining shoal parts of the sea will be increased； and new stations will often be formed； all circumstances most favourable； as previously explained； for the formation of new varieties and species； but during such periods there will generally be a blank in the geological record。 On the other hand； during subsidence； the inhabited area and number of inhabitants will decrease （excepting the productions on the shores of a continent when first broken up into an archipelago）； and consequently during subsidence； though there will be much extinction； fewer new varieties or species will be formed； and it is during these very periods of subsidence； that our great deposits rich in fossils have been accumulated。 Nature may almost be said to have guarded against the frequent discovery of her transitional or linking forms。
From the foregoing considerations it cannot be doubted that the geological record； viewed as a whole； is extremely imperfect； but if we confine our attention to any one formation； it becomes more difficult to understand； why we do not therein find closely graduated varieties between the allied species which lived at its commencement and at its close。 Some cases are on record of the same species presenting distinct varieties in the upper and lower parts of the same formation； but； as they are rare； they may be here passed over。 Although each formation has indisputably required a vast number of years for its deposition； I can see several reasons why each should not include a graduated series of links between the species which then lived； but I can by no means pretend to assign due proportional weight to the following considerations。
Although each formation may mark a very long lapse of years； each perhaps is short compared with the period requisite to change one species into another。 I am aware that two palaeontologists； whose opinions are worthy of much deference； namely Bronn and Woodward； have concluded that the average duration of each formation is twice or thrice as long as the average duration of specific forms。 But insuperable difficulties； as it seems to me； prevent us coming to any just conclusion on this head。 When we see a species first appearing in the middle of any formation； it would be rash in the extreme to infer that it had not elsewhere previously existed。 So again when we find a species disappearing before the uppermost layers have been deposited； it would be equally rash to suppose that it then became wholly extinct。 We forget how small the area of Europe is compared with the rest of the world； nor have the several stages of the same formation throughout Europe been correlated with perfect accuracy。
With marine animals of all kinds； we may safely infer a large amount of migration during climatal and other changes； and when we see a species first appearing in any formation； the probability is that it only then first immigrated into that area。 It is well known； for instance； that several species appeared somewhat earlier in the palaeozoic beds of North America than in those of Europe； time having apparently been required for their migration from the American to the European seas。 In examining the latest deposits of various quarters of the world； it has everywhere been noted； that some few still existing species are common in the deposit； but have become extinct in the immediately surrounding sea； or； conversely； that some are now abundant in the neighbouring sea； but are rare or absent in this particular deposit。 It is an excellent lesson to reflect on the ascertained amount of migration of the inhabitants of Europe during the Glacial period； which forms only a part of one whole geological period； and likewise to reflect on the great changes of level； on the inordinately great change of climate； on the prodigious lapse of time； all included within this same glacial period。 Yet it may be doubted whether in any quarter of the world； sedimentary deposits； including fossil remains； have gone on accumulating within the same area during the whole of this period。 It is not； for instance； probable that sediment was deposited during the whole of the glacial period near the mouth of the Mississippi； within that limit of depth at which marine animals can flourish； for we know what vast geographical changes occurred in other parts of America during this space of time。 When such beds as were deposited in shallow water near the mouth of the Mississippi during some part of the glacial period shall have been upraised； organic remains will probably first appear and disappear at different levels； owing to the migration of species and to geographical changes。 And in the distant future； a geologist examining these beds； might be tempted to conclude that the average duration of life of the embedded fossils had been less than that of the glacial period； instead of having been really far greater； that is extending from before the glacial epoch to the present day。
In order to get a perfect gradation between two forms in the upper and lower parts of the same formation； the deposit must have gone on accumulating for a very long period； in order to have given sufficient time for the slow process of variation； hence the deposit will generally have to be a very thick one； and the species undergoing modification will have had to live on the same area throughout this