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Show 292 IMPERFECTION OF THE CHAP. IX. . b .e th y had time to decay. preserve the remains e1ore e h On the other hand, as long as the bed of t e sea re-mained stationary, thick deposits coul~ not have been ul t d . the shallow parts which are the most accum a e In ' . h d f: bl t li·.ee Still less could this have appene a voura e o 1' · . 1 d . the alternate periods of elevation ; or, to spea r muorrien ga ccurat ely, the beds which we.r e then .a ccumu- 1a te d m' ll have been destroyed by being. upraised and brought within the limits of the. coast-action. . Thus the geological record will almost necessa~Ily be rendered intermittent. I feel much confidence In t.he truth of these views, for they are in strict accordance WJth the general principles inculcated by ~ir. C. Lyell ; ~nd E. Forbes independently arrived at ~ simil~r conclus1?n. One remark is here worth a passing notice. Dunng periods of elevation the area of the land and of the adjoining shoal parts of the sea will be i~creased, and new stations will often be formed ; -all cucumstances most favourable, as previously explained, for t~e formation of new varieties and species ; but dunng 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 dec~ease (excepting the productions on the s~ores 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 accu~ulated. Nature may almost be said to have guarded against the frequent discovery of her t~ansit~onal. or linking forms. From the foregoing consideratiOns It cannot be do~bted that the geological record, viewed as a whole, ~s extremely imperfect; but if we confine our attentiOn to any one formation, it becomes more difficult to under- CHAP. IX, GEOLOGICAL RECORD. 293 sta~d,. 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 ~ecord 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.' Al~ though each formation has indisputably required a vast number of years for its deposition, l can see several rea~ons why each should not include a graduated series of hnks between the species which then lived; but I can by no means. preten~ to assign due proportional weight to the followmg considerations. Although each form~tion may mark a very long lapse of y~a~s, each perhaps Is short compared with the period reqmsite to change one species into another. I am aware that two palmontologists, whose opinions are worthy of much deference, namely Bronn and Woodward, have concluded that the average duration of each 'form~tion is tw~ce or thrice as long as the average d~atwn of spem:fic forms. But insuperable difficulties, as I~ seems t~ me, prevent us coming to any just conclusi? n ?n this ~ead. When we see a species first ap~ earing In the middle of any formation, it would be rash I~ the ext:eme to infer that it had not elsewhere prevwu~ ly existed. So again when we find a species disap~ earing before the uppermost layers have been deposited, It would be equally rash to suppose that it then became wholl! extinct. vV e forget how small the area of Eu~ ope Is compared with the rest of the world; nor have he several stages of the same formation throughout Europe been .correl~ted with perfect accuracy. . ~Ith manne arumals of all kinds, we may safely In ~r a large amount of migration during climatal :n ot~er .changes; and when we see a species first ppeanng In any formation, the probability is that it |
