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Show 256 IMPERFECTION OF THE (CHAP. IX. shallow parts, which are the most fayourable to life. Still less could this have happened. dunng the alternate periods of elevation· or to speak more accurately, the beds which were th~n ~ccumulated will. h~ve beer~ ~estroyed by being upraised and brought Within the hmits of the coast-action. . Thus the geological record will almost necessa~Ily be rendered interinittent. I feel much confidence In the truth of these views for they are in strict accordance with the general principl~s inc~llcated b~ S~r C. Lyell;_ and E. Forbes independently arnved at a sn:nlar co~clusion .. One remark is here worth a pass1ng notice. Dunng periods of elevation the are~ of the. land and of the adjoining shoal parts of the sea will be In.creased, and new stations will often be formed ;-all Circumstances n1ost favourable, as previously explained,. for the for~ation of new varieties and species; but durmg 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 t~ere ~ill be much extin~t~on, fe:wer new varieties or spemes will be forn1ed; and It IS dunng these very periods of subsidence, that our great deposits rich in fossils have been accumulated. Nature may ahnost 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 a~d 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 indis· futably required a vast number of years for its deposition, can see several reasons why each should not include a CHAP. IX.] GEOLOGICAL RECORD. 251 graduated series of links between the species which then lived; but I can by no Ineans pretend to assign due proportional weight to the following considerations. Although each forn;ation n1ay mark a v_ery long lapse of years, each perhaps Is short compared With the period requisite to change one species into another. I am aware that two palreontologists, 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~ vVhen :ve S?e a species :first aJ?pearing in the middle of any formation, 1t 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 through~ out E~rope b?en co;related with. perfect accuracy. vV1th manne an1mals of alllnnds, we may safely infer a large amount of migration during climatal and other chang~s; and when w~ _see .a speci~s :first appearing i?- any formati?n, the probability Is that It only then :first Immigrated Into that area. It is well known, for instance, that several. species appea::ed somew:hat earlier in the palreozoic bed~ of North Amenca than In those of Europe ; time havmg ap.Parently been required for their migration from the AmeriCa? to the ~uropean seas. In examining the latest deposits of various quarters of the world, it has eyerywhere been noted, that some few still existinO' spe~ Ies are. common in the deposit, but have become e~tinct 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 excell~ nt lesson .to re~ect on the ascertai:z:ed amount of migratiO~ of the Inhabitants of Europe during the Glacial period, wh1c~ for:r_ns only a part of one whole geological period; and likewise to reflect on the great changes of level, on |