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Show 298 GEOLOGICAL SUCCESSION. [CHAP. X. other old species and the other whole gen~ra having become utterly extinct. In failing orders, With the ~enera and species decreasing in numbers~ as aJ?parently IS the case of the Edentata of South America, stlll fewer genera and species will have left modified blood-descendants. Summa;ry of the preceding and present Chapters.-1 have attempted to show that the geologic~l record is extrenlely imperfect ; that only a small port1on of the globe has been geologically explored with care; that only certain classes of organic beings have been largely preserved in a fossil state ; that the number both of specimens and of species, preserved in our museums, is absolutely as nothing compared with the incalculable number of generations which must have passed away even during a single formation ; that, owing to subsidence being necessary for the accu1nulation of fossiliferous deposits thick enough to resist future degradation, enormous intervals of time have elapsed between the successive formations; < that there has probably been more extinction during the periods of subsidence, and more variation during the periods of elevation, and during the latter the record will have been least perfectly kept ; that each single formation has not been continuously deposited ; that the duration of each formation is, perhaps, short compared with the average duration of specific forms; that migration has played an ilnportant part in the first appearance of new forms in any one area and formation; that widely ranging species are those which have varied most, and have oftenest given rise to new species ; and that varieties have at first often been local. All these causes taken conjointly, must have tended to make the geological record extremely imperfect, and will to a large extent explain why we do not find interminable varieties, connecting together all the extinct and existing forms of life by the finest graduated steps. He who rejects these views on the nature of the geological record, will rightly reject my whole theory. Foi· he may ask in vain where are the numberless transitional links which must formerly have connected the closely allied or representative species, found in the several stages 0BAP. X.] SUMMARY. 299 of the same great forination H . enormous intetvals of time ~h· h hay disbelieve in the our consecutive formations . he ave elatsed between portant a part migration mu'st he rna) over ook how immations of any one great regio ale P ayed, when the forare considered ; he rna ur : ~hone, as that of Europe, false~y apparent, suddeny co~n i~ afparent, but often speCies. He may ask where ar! th o w~ole groups of finitely numerous or anisms whi e remmns of. those inbefore the first bed gf the 8.1 . ch must have ex1sted long I c~n answer this latter qu~s~~~n ;JJ.sthm was d~posited : saying that as far as we can h y ypothetlcally, by extend they have for an eno see, w er~ our oceans now· where our oscillating conti rm~us period extended, and stood ever since the Silurian ~eno~h~ow stand they have that period the world h P ' but that long before ferent aspe~t; and that~le cl~e prese~ted a wholly difformations older than any kno e~ continents, formed of a metamorphosed condition o;n o ul~' mbay. now all be in ocean. ' may Ie uried under the leal:;}~~ts fi~~ai~e~!to~~g· culties, tll the . other great low on the theor of d y se.em o rne Simply to fol-natural selectiotl.. y We ce:~e~~u:~hd modification .t~rough new species come in slowl d n ers_tand how It Is that of different classes do not yn:n su~fesshvely ; how species at the ~arne rate, or in the sa::s~rl y c .ange _together, or run that all undergo mod'fi f egree' yet In the long extinction of old forms i: ca Ion to so~e ~xtent. The quence of the production of the almost IneVItable consestand why when a . ne'v orms. We can under-reappears. Group:~f~Ipe!J:: .once di~appeared it never and endure for u . Increase In numbers slowly of modification isn~~~:;s!rf[Iods of time ; for the proces~ complex contingencies 1.\slo;, a~d depends on many larger dominant . e ominant species of the scendants and fh.oups tend to leave many modified de f~rmed . . '.As theseu~r:~:n!~~-groups RJ?d groups ar~ VIgorous groups from th . . ni ~he. sp~cies . of the less common progenitor te d :ubi erioritY. Inherited from a ' n o ecome extmct together, and |