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Show CHAPTER V NATURAL SELJWTION liY VARIATION AND SURVIVAL OF Tim F'ITTEST Effect of struggle for ?xistcnce lm<lcr unchanrrcd conditions-TJ fl' L U~tder change of conuitious- Divcrgcncc of character- I~! in s~~~ ~ P(' bu·ds-In mammalia- Divcrgcncc leads to a maxilllum of' lifoc~ :.; In area-:-~loscly allied species inhabit di -tinct ;rcas- Ada ih '.H <'a('h conchtwns at various periods of lifc- Tl . . , L ' Ltoll 1 o forms of life- Ext' t' f 1 . te contmued cxJ stcncc of low me IOn o ow types amoncr t1J l . l . Circumstances favoura' ble to the on .g m. of , new os pcc1c0 s ngp t.o ru a bml mal·s . of ~he dippers-The importance of isolation- On the ·a-: . J o a fc ongl!l satwn b , t 1 1 . · uv,wce o orr•au1- , y na ura sc ectwn-Summary of the Jirst five chapters. o IN tho preceding chapters we have accumulated b fac.ts an~ arfguments .which will enable us now to dc~l w~~;; t hoef very co1e o our subJect-the f .. · . of natural selection W h ormation of spccws by means strucrcrl f . . . o av~ seen how tremendous is the • bo c 01 oxis_tcnce always gomg on in n~ture owin ()' t tl gr cat powers of mcrcaso of all .. . b o lC tho fact of variabilit t . d' o~gamsms; we have a. ·cmta.iJ te(l of which V .· . r OX on mg to every part and Ol'giLil, cadt cntly. anc~l:: ~Imu tancOl~_JY an_d for ~he_ ~lost part in dependits '1U:ount . avo s.cen t a.t this vanabihty is both hrgc in affc~ts a co~~Jerr~~~twn to t!Ic size of _ca~h yart, and wma.lly and l . . proportiOn of tho mdi Yld uals in the la rue .· . <.f ommant spc.c lCs : And. ' l as tl y, we 11 avc . con how simil:a-, r :~~~a10~~l oc;urr_mg m cultivated plants and domestic animals sclcc~i~n t~lf t{em~ perpetuated _and accumulated by artifi<.:ial of our f ' "t ft ey avo resulted m all the wonderful varieties househorldm s, t owcrs ' and vc()'ctable d · . b s, our omostlc ammals and . pc s, many of which differ from each other far more m external characters, habits, and instincts than do species in CHAP. V NATURAL 'l~LI~ 'TION 103 a state of nature. We have now to in(1uirc whether there is any analo<~ous proce.·s in na.tnre, by which wild animals and pbnts can be permanently modified and new races or new species produced. Effect of Stntggle for Existence under Unchanged Conditions. Let ns first 'consider what will be the effect of the struggle for existence upon the animals and plants which we sec around us, under conditions which do not p rceptibly vary from year to year or from century to century. W c have soon that every species is exposed to numerous ~mel varied dangers throughout its entire existence, and that it is only hy means of tho exact adaptation of its organisation- inclndi1w its instincts and ha.bits -to its surroundings tlmt it is enabled to live till it produces offspring whieh may take its pla.ce when it cea ·c · to exist. We ha,ve seen aJso that, of the whole annual i11creasc only a very sm:Lll fraction survives; and thouuh the urvival in individual cases may sometimes be due rn.thcr to accident than to any real superiority, yet we cn.nnot donbt that, in the long run, those survive which arc best :fitted by their perf 'Ct organi ation to escape the dangers that surround them. This" survival of the :fittc t" is what Darwin termed "natural selection," because it loads to tho same results in nature as are produced by man's selection among domestic animals and cultivated plants. It prima.ry cfl'cct will, clearly, be to keep each species in tho mo t perfect health and vigour, with every part of its organisation in full harmony with tho conditions of its existence. It prevents any pos. iblc dctcriora.tion in the organic world, a.nll produces th:1t a.ppcarance of cxuhcmnt life and enjoyment, of health and beauty, that affords u so much pleasure, and which might load a. superficial oh ervcr to suppose that peace and quietude reigned throughout nature. The Effect under chang d Conditions . But tho very same process which, so long as conditions rcmai n substantially the same, secures the continuance of each species of animal or plant in its full perfection, will usually, under changed conditions, brin()' about whatever chan()'c of structure or habits may be nccessita.tcd by them. Tho changed conditions to which we refer arc. uch as we know have occurred |