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Show CHAPTER XI. . t' tion of specJ.C s con sistcnt with their limitecl geo. Theory of the snccesswe ex me. . tl opinions of botanists respecting · · Tl e cl1scordance 111 Je . gr.tphical distnbubon- 1 cl'ff1 1 may arise from changes Ill 1 . h !ants have been • use' . the centres from w uc 'P . . f living spec1es-Whether there h :bsequent to the or•gm 0 . . physical geograp Y su f t' to time of certam ammals and • " : r that the loHs rom une are grounds for lll 1erxmg . . f new species ?-Whether any t d b the mtrocluchon o plants is compensa e Y ·t d within the hist01·ical era, even f s coultl be expec e eviuence of such new crea 1011 f t' tion :>-The question whether f t ·as cases o ex me . if they had been as requen cl . uccession can only be clecicled by the existing species have been create m s reference to geological monuments. WE have pom. ted out m. t11 e precedin(o)' chapters the stric. t dep'e ndence of each speci.e s of animal and plant£ on cerdta m . l d't' l·n the state of the earth's sur ace, an on PhysiCa con I wns . b' . the number an d attri'b u t es of other orcor anic beings mhah Jtm1g1 ' the same region. ·xvr c have also endeavoured to show.t at a1 these condi.t i.O ns at.e 1.1 1 a s tate of continual flu.c tuation, . t Je . crneous and aqueous agents remodelling, from tm~e t~ time, ltbh physt. cal creocrt •a p1 1 Y of the globe ' and the migratiOns of :cies causin~ n:w relations to spring up successively between sp o. b . gs We have deduced as a corollary, different orgamc em · ' · · tl that the speci.e s exi· st·m g a t any P~ rticular p1e riodI m. ust", mT he1ye f aO'eS become extinct one after t 1C ot Jet. course o o ' · f n Buffon must dI. e ou t ," to borrow an emphatical expresswn rm ' . 1 , ,, because Time fights agamst t Jem. . If the views which we have taken are just, there will be ~o difficulty in explaining why t lJ e h a bI' ta t'I on s of so . m.a ny sp,e cie.S . now restram. ed w.l t1n .n excee d'm g1 y narrow. hmits. Ev.ery 1 • ai e t 1 ted 111 the preceumg local revolution, such as those con emp a . ·hile chapter, tends to circumscn' be th e r ancre of some species, " o b l d to infer it enlarcres that of ot h ers .' an d as we have een e t . quire tl at newo species Ol'.J O. 'Inate m• one spo t only ' each mus Ie . 'n ti1m e to diffuse itseol f over a Wl' de area. The recent origi ' Ch. XI.] CENTRES OF VEGETATION, 17'7 therefore, of some species, and the high antiquity of others, may be equally consistent with the general fact of their limited distribution, some being local, because they have not existed long enough to admit of theit· wide dissemination; others, because circumstances in the animate or inanimate world have occurred to restrict the range which they may once have obtained. As considerable modifications in the relative levels of land and sea have taken place, in certain regions, since the existing species were in being, we can feel no surprise that the zoologist and botanist have hitherto found it difficult to refer the geographical distribution of species to any clear and determinate principles, since they have usually speculated on the phenomena, upon the assumption that the physical geography of the globe had undergone no material alteration since the introduction of the species now living. So long as this assumption was made, the facts relating to the geography of plants and animals appeared capricious in the extreme, and by many the subject was pronounced to be so full of mystery and anomalies, that the establishment of a satisfactory theory was hopeless. Some botanists conceived, in accordance with the hypothesis of Willdenow, that mountains were the centres of creation from which the plants now inhabiting large continents have radiated, to which Decandolle and others, with much reason, objected, that mountains, on the contrary, are often the barriers between two provinces of distinct vegetation. The geologist who is acquainted with the extensive modifications which the surface of the earth has undergone in very recent geological epochs, may be able,· perhaps, to reconcile both these theories in their application to different regions. A lofty range of mountains, which is so ancient as to date from a period when the species of animals and plants differed from those now living, will naturally form a barrier between contiguous provinces; but a chain which has been raised, in great part, within the epoch of existing species, and around which new lands have arisen from the sea within that period, will be a centre of peculiar vegetation. Vot, II. N |