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Show 102 EXTINCTION, [CHAP. IV. in our chapter on Geology ; but it must be here alluded to from being intimately connected with natural selection. Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure. But as from the high geometrical powers of increase of all organic beings, each area is already fully stocked with inhabitants, it follows that as each selected and favoured form increases in number, so will the less favoured forms decrease and become rare. Rarity, as geology tells us, is the precursor to extinction. We can, also, see that any forrn represented by few individuals will, during fluctuations in the seasons or in the number of its enemies, run a good chance of utter extinction. But we may go further than this ; for as ne"\v forms are continually and slowly being produced, unless we believe that the number of specific forms goes on perpetually and aln1ost indefinitely increasing, nun1bers inevitably must become extinct. That the nun;1ber of specific forms has not indefinitely increased, geology shows us plainly ; and indeed we can see reason why they should not have thus increased, for the number of places in the polity of nature is not indefinitely great,-not that we have any means of knowing that any one region has as yet got its maximum of species. Probably no region is as yet fully stocked, for at the Cape of Good Hope, where more species of plants are crowded together than in any other quarter of the world, some foreign plants have become naturalised, without causing, as far as we know, the ex-tinction of any natives. Furthermore, the species which are most numerous in individuals will have the best chance of producing within any given period favourable variations. We have evidence of this, in the facts given in the second chapter, showing that it is the common species which afford the greatest number of recorded varieties, or incipient species. Hence, rare species will be less quickly modified or improved within any given period, and they will consequently be beaten in the race for life by the modified descend-ants of the commoner species. From these several considerations I think it inevitably OHAP. IV.] DIVERGENCE OF CHARACTER. 103 follows, that as new species in the course of t' fonned through natural selection others will b rme are afd rarer, and finally extinct. The forms whi~jlo:: rdr~r c osest competition with those under . . n . 111 han d improv.e ment' will naturallh su.ua.e'gro rmngo smt odAifincda tion ave seen In the chapter on t e Stru le £ . . we that it is the rnost closely-allied forms gg ?r .Exrstence same.species! and species of the same gen::~~t~'f r~f :h~ genma,-whrch, from having 1 th a e constit~t~on, ar:d habits, gener~l1;r lome e i~~m~h structure, com· petition wrt.h each other· 0 onsequent1 y eea scehv erest ~~Jfdet{etn~~Jft;:~s~t!~e!~eoErftf~!~~!ti~~d~~dat!~~ en o. exterminate them. We see the ' extermination amon st d . same process of thr.;mgh ~he selection gof i~;~ov~df:~:tbd produo:ons, currous Instances could be iven h . y man. . any ~FY!:f~~~~!, ~~~:e~~ti!:{~~ ~ii£!::!~~;J~!~:!f black cattle were displaced b; ::~7n ttat the ancient these" were swept away by the short-hng- ~;ns, and that words of an ag-ricultural writer) " as itbns (I quote the ous pestilence!' ' Y some murder- Divergence of Character. -Th · · 1 . have designated by this term is o; Jrhc.Ip e, whiCh I my theory and ex lains I' b . g Importance on facts. In th :fi t p I ' as . . eheve, several important e rs p ace, varieties, even stron 1 k d ones, though having somewhat of th h g y-mar e -as is shown by the hopeless doubt e ~ aracter of species to rank them-yet certain! d'ffi f s In many cases how than do good and distinct y I . er roNm each other far less · t . species. evertheless ac ·d mgt" o my Vlew, varieties are species in the proce~s of~I - rna Ion, or are, as I have called th . . . or-b Row, then, does the lesser differen~~' ~~:~~e:t sp~cti~s. ecome augmented into the . varre res species? 'l'hat thI'S d oes h ab 'Igtureaalltye rh adpif ference betwe.e n fer from most of the innumer bl . pen, we must In-ture presenting well-marked d.iffie srpeenccreess; twhrhoeuregahso uvta rniea-- |