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d¡¡the¡¡decrease¡¡of¡¡the¡¡song¡­thrush¡£¡¡How¡¡frequently¡¡we¡¡hear¡¡of¡¡one¡¡species¡¡of¡¡rat¡¡taking¡¡the¡¡place¡¡of¡¡another¡¡species¡¡under¡¡the¡¡most¡¡different¡¡climates£¡¡¡In¡¡Russia¡¡the¡¡small¡¡Asiatic¡¡cockroach¡¡has¡¡everywhere¡¡driven¡¡before¡¡it¡¡its¡¡great¡¡congener¡£¡¡One¡¡species¡¡of¡¡charlock¡¡will¡¡supplant¡¡another£»¡¡and¡¡so¡¡in¡¡other¡¡cases¡£¡¡We¡¡can¡¡dimly¡¡see¡¡why¡¡the¡¡competition¡¡should¡¡be¡¡most¡¡severe¡¡between¡¡allied¡¡forms£»¡¡which¡¡fill¡¡nearly¡¡the¡¡same¡¡place¡¡in¡¡the¡¡economy¡¡of¡¡nature£»¡¡but¡¡probably¡¡in¡¡no¡¡one¡¡case¡¡could¡¡we¡¡precisely¡¡say¡¡why¡¡one¡¡species¡¡has¡¡been¡¡victorious¡¡over¡¡another¡¡in¡¡the¡¡great¡¡battle¡¡of¡¡life¡£¡¡
A¡¡corollary¡¡of¡¡the¡¡highest¡¡importance¡¡may¡¡be¡¡deduced¡¡from¡¡the¡¡foregoing¡¡remarks£»¡¡namely£»¡¡that¡¡the¡¡structure¡¡of¡¡every¡¡organic¡¡being¡¡is¡¡related£»¡¡in¡¡the¡¡most¡¡essential¡¡yet¡¡often¡¡hidden¡¡manner£»¡¡to¡¡that¡¡of¡¡all¡¡other¡¡organic¡¡beings£»¡¡with¡¡which¡¡it¡¡comes¡¡into¡¡competition¡¡for¡¡food¡¡or¡¡residence£»¡¡or¡¡from¡¡which¡¡it¡¡has¡¡to¡¡escape£»¡¡or¡¡on¡¡which¡¡it¡¡preys¡£¡¡This¡¡is¡¡obvious¡¡in¡¡the¡¡structure¡¡of¡¡the¡¡teeth¡¡and¡¡talons¡¡of¡¡the¡¡tiger£»¡¡and¡¡in¡¡that¡¡of¡¡the¡¡legs¡¡and¡¡claws¡¡of¡¡the¡¡parasite¡¡which¡¡clings¡¡to¡¡the¡¡hair¡¡on¡¡the¡¡tiger's¡¡body¡£¡¡But¡¡in¡¡the¡¡beautifully¡¡plumed¡¡seed¡¡of¡¡the¡¡dandelion£»¡¡and¡¡in¡¡the¡¡flattened¡¡and¡¡fringed¡¡legs¡¡of¡¡the¡¡water¡­beetle£»¡¡the¡¡relation¡¡seems¡¡at¡¡first¡¡confined¡¡to¡¡the¡¡elements¡¡of¡¡air¡¡and¡¡water¡£¡¡Yet¡¡the¡¡advantage¡¡of¡¡plumed¡¡seeds¡¡no¡¡doubt¡¡stands¡¡in¡¡the¡¡closest¡¡relation¡¡to¡¡the¡¡land¡¡being¡¡already¡¡thickly¡¡clothed¡¡by¡¡other¡¡plants£»¡¡so¡¡that¡¡the¡¡seeds¡¡may¡¡be¡¡widely¡¡distributed¡¡and¡¡fall¡¡on¡¡unoccupied¡¡ground¡£¡¡In¡¡the¡¡water¡­beetle£»¡¡the¡¡structure¡¡of¡¡its¡¡legs£»¡¡so¡¡well¡¡adapted¡¡for¡¡diving£»¡¡allows¡¡it¡¡to¡¡compete¡¡with¡¡other¡¡aquatic¡¡insects£»¡¡to¡¡hunt¡¡for¡¡its¡¡own¡¡prey£»¡¡and¡¡to¡¡escape¡¡serving¡¡as¡¡prey¡¡to¡¡other¡¡animals¡£¡¡
The¡¡store¡¡of¡¡nutriment¡¡laid¡¡up¡¡within¡¡the¡¡seeds¡¡of¡¡many¡¡plants¡¡seems¡¡at¡¡first¡¡sight¡¡to¡¡have¡¡no¡¡sort¡¡of¡¡relation¡¡to¡¡other¡¡plants¡£¡¡But¡¡from¡¡the¡¡strong¡¡growth¡¡of¡¡young¡¡plants¡¡produced¡¡from¡¡such¡¡seeds¡¡£¨as¡¡peas¡¡and¡¡beans£©£»¡¡when¡¡sown¡¡in¡¡the¡¡midst¡¡of¡¡long¡¡grass£»¡¡I¡¡suspect¡¡that¡¡the¡¡chief¡¡use¡¡of¡¡the¡¡nutriment¡¡in¡¡the¡¡seed¡¡is¡¡to¡¡favour¡¡the¡¡growth¡¡of¡¡the¡¡young¡¡seedling£»¡¡whilst¡¡struggling¡¡with¡¡other¡¡plants¡¡growing¡¡vigorously¡¡all¡¡around¡£¡¡
Look¡¡at¡¡a¡¡plant¡¡in¡¡the¡¡midst¡¡of¡¡its¡¡range£»¡¡why¡¡does¡¡it¡¡not¡¡double¡¡or¡¡quadruple¡¡its¡¡numbers£¿¡¡We¡¡know¡¡that¡¡it¡¡can¡¡perfectly¡¡well¡¡withstand¡¡a¡¡little¡¡more¡¡heat¡¡or¡¡cold£»¡¡dampness¡¡or¡¡dryness£»¡¡for¡¡elsewhere¡¡it¡¡ranges¡¡into¡¡slightly¡¡hotter¡¡or¡¡colder£»¡¡damper¡¡or¡¡drier¡¡districts¡£¡¡In¡¡this¡¡case¡¡we¡¡can¡¡clearly¡¡see¡¡that¡¡if¡¡we¡¡wished¡¡in¡¡imagination¡¡to¡¡give¡¡the¡¡plant¡¡the¡¡power¡¡of¡¡increasing¡¡in¡¡number£»¡¡we¡¡should¡¡have¡¡to¡¡give¡¡it¡¡some¡¡advantage¡¡over¡¡its¡¡competitors£»¡¡or¡¡over¡¡the¡¡animals¡¡which¡¡preyed¡¡on¡¡it¡£¡¡On¡¡the¡¡confines¡¡of¡¡its¡¡geographical¡¡range£»¡¡a¡¡change¡¡of¡¡constitution¡¡with¡¡respect¡¡to¡¡climate¡¡would¡¡clearly¡¡be¡¡an¡¡advantage¡¡to¡¡our¡¡plant£»¡¡but¡¡we¡¡have¡¡reason¡¡to¡¡believe¡¡that¡¡only¡¡a¡¡few¡¡plants¡¡or¡¡animals¡¡range¡¡so¡¡far£»¡¡that¡¡they¡¡are¡¡destroyed¡¡by¡¡the¡¡rigour¡¡of¡¡the¡¡climate¡¡alone¡£¡¡Not¡¡until¡¡we¡¡reach¡¡the¡¡extreme¡¡confines¡¡of¡¡life£»¡¡in¡¡the¡¡arctic¡¡regions¡¡or¡¡on¡¡the¡¡borders¡¡of¡¡an¡¡utter¡¡desert£»¡¡will¡¡competition¡¡cease¡£¡¡The¡¡land¡¡may¡¡be¡¡extremely¡¡cold¡¡or¡¡dry£»¡¡yet¡¡there¡¡will¡¡be¡¡competition¡¡between¡¡some¡¡few¡¡species£»¡¡or¡¡between¡¡the¡¡individuals¡¡of¡¡the¡¡same¡¡species£»¡¡for¡¡the¡¡warmest¡¡or¡¡dampest¡¡spots¡£¡¡
Hence£»¡¡also£»¡¡we¡¡can¡¡see¡¡that¡¡when¡¡a¡¡plant¡¡or¡¡animal¡¡is¡¡placed¡¡in¡¡a¡¡new¡¡country¡¡amongst¡¡new¡¡competitors£»¡¡though¡¡the¡¡climate¡¡may¡¡be¡¡exactly¡¡the¡¡same¡¡as¡¡in¡¡its¡¡former¡¡home£»¡¡yet¡¡the¡¡conditions¡¡of¡¡its¡¡life¡¡will¡¡generally¡¡be¡¡changed¡¡in¡¡an¡¡essential¡¡manner¡£¡¡If¡¡we¡¡wished¡¡to¡¡increase¡¡its¡¡average¡¡numbers¡¡in¡¡its¡¡new¡¡home£»¡¡we¡¡should¡¡have¡¡to¡¡modify¡¡it¡¡in¡¡a¡¡different¡¡way¡¡to¡¡what¡¡we¡¡should¡¡have¡¡done¡¡in¡¡its¡¡native¡¡country£»¡¡for¡¡we¡¡should¡¡have¡¡to¡¡give¡¡it¡¡some¡¡advantage¡¡over¡¡a¡¡different¡¡set¡¡of¡¡competitors¡¡or¡¡enemies¡£¡¡
It¡¡is¡¡good¡¡thus¡¡to¡¡try¡¡in¡¡our¡¡imagination¡¡to¡¡give¡¡any¡¡form¡¡some¡¡advantage¡¡over¡¡another¡£¡¡Probably¡¡in¡¡no¡¡single¡¡instance¡¡should¡¡we¡¡know¡¡what¡¡to¡¡do£»¡¡so¡¡as¡¡to¡¡succeed¡£¡¡It¡¡will¡¡convince¡¡us¡¡of¡¡our¡¡ignorance¡¡on¡¡the¡¡mutual¡¡relations¡¡of¡¡all¡¡organic¡¡beings£»¡¡a¡¡conviction¡¡as¡¡necessary£»¡¡as¡¡it¡¡seems¡¡to¡¡be¡¡difficult¡¡to¡¡acquire¡£¡¡All¡¡that¡¡we¡¡can¡¡do£»¡¡is¡¡to¡¡keep¡¡steadily¡¡in¡¡mind¡¡that¡¡each¡¡organic¡¡being¡¡is¡¡striving¡¡to¡¡increase¡¡at¡¡a¡¡geometrical¡¡ratio£»¡¡that¡¡each¡¡at¡¡some¡¡period¡¡of¡¡its¡¡life£»¡¡during¡¡some¡¡season¡¡of¡¡the¡¡year£»¡¡during¡¡each¡¡generation¡¡or¡¡at¡¡intervals£»¡¡has¡¡to¡¡struggle¡¡for¡¡life£»¡¡and¡¡to¡¡suffer¡¡great¡¡destruction¡£¡¡When¡¡we¡¡reflect¡¡on¡¡this¡¡struggle£»¡¡we¡¡may¡¡console¡¡ourselves¡¡with¡¡the¡¡full¡¡belief£»¡¡that¡¡the¡¡war¡¡of¡¡nature¡¡is¡¡not¡¡incessant£»¡¡that¡¡no¡¡fear¡¡is¡¡felt£»¡¡that¡¡death¡¡is¡¡generally¡¡prompt£»¡¡and¡¡that¡¡the¡¡vigorous£»¡¡the¡¡healthy£»¡¡and¡¡the¡¡happy¡¡survive¡¡and¡¡multiply¡£¡¡¡¡
Chapter¡¡4¡¡¡­¡¡Natural¡¡Selection

How¡¡will¡¡the¡¡struggle¡¡for¡¡existence£»¡¡discussed¡¡too¡¡briefly¡¡in¡¡the¡¡last¡¡chapter£»¡¡act¡¡in¡¡regard¡¡to¡¡variation£¿¡¡Can¡¡the¡¡principle¡¡of¡¡selection£»¡¡which¡¡we¡¡have¡¡seen¡¡is¡¡so¡¡potent¡¡in¡¡the¡¡hands¡¡of¡¡man£»¡¡apply¡¡in¡¡nature£¿¡¡I¡¡think¡¡we¡¡shall¡¡see¡¡that¡¡it¡¡can¡¡act¡¡most¡¡effectually¡£¡¡Let¡¡it¡¡be¡¡borne¡¡in¡¡mind¡¡in¡¡what¡¡an¡¡endless¡¡number¡¡of¡¡strange¡¡peculiarities¡¡our¡¡domestic¡¡productions£»¡¡and£»¡¡in¡¡a¡¡lesser¡¡degree£»¡¡those¡¡under¡¡nature£»¡¡vary£»¡¡and¡¡how¡¡strong¡¡the¡¡hereditary¡¡tendency¡¡is¡£¡¡Under¡¡domestication£»¡¡it¡¡may¡¡be¡¡truly¡¡said¡¡that¡¡the£»¡¡whole¡¡organisation¡¡becomes¡¡in¡¡some¡¡degree¡¡plastic¡£¡¡Let¡¡it¡¡be¡¡borne¡¡in¡¡mind¡¡how¡¡infinitely¡¡complex¡¡and¡¡close¡­fitting¡¡are¡¡the¡¡mutual¡¡relations¡¡of¡¡all¡¡organic¡¡beings¡¡to¡¡each¡¡other¡¡and¡¡to¡¡their¡¡physical¡¡conditions¡¡of¡¡life¡£¡¡Can¡¡it£»¡¡then£»¡¡be¡¡thought¡¡improbable£»¡¡seeing¡¡that¡¡variations¡¡useful¡¡to¡¡man¡¡have¡¡undoubtedly¡¡occurred£»¡¡that¡¡other¡¡variations¡¡useful¡¡in¡¡some¡¡way¡¡to¡¡each¡¡being¡¡in¡¡the¡¡great¡¡and¡¡complex¡¡battle¡¡of¡¡life£»¡¡should¡¡sometimes¡¡occur¡¡in¡¡the¡¡course¡¡of¡¡thousands¡¡of¡¡generations£¿¡¡If¡¡such¡¡do¡¡occur£»¡¡can¡¡we¡¡doubt¡¡£¨remembering¡¡that¡¡many¡¡more¡¡individuals¡¡are¡¡born¡¡than¡¡can¡¡possibly¡¡survive£©¡¡that¡¡individuals¡¡having¡¡any¡¡advantage£»¡¡however¡¡slight£»¡¡over¡¡others£»¡¡would¡¡have¡¡the¡¡best¡¡chance¡¡of¡¡surviving¡¡and¡¡of¡¡procreating¡¡their¡¡kind£¿¡¡On¡¡the¡¡other¡¡hand£»¡¡we¡¡may¡¡feel¡¡sure¡¡that¡¡any¡¡variation¡¡in¡¡the¡¡least¡¡degree¡¡injurious¡¡would¡¡be¡¡rigidly¡¡destroyed¡£¡¡This¡¡preservation¡¡of¡¡favourable¡¡variations¡¡and¡¡the¡¡rejection¡¡of¡¡injurious¡¡variations£»¡¡I¡¡call¡¡Natural¡¡Selection¡£¡¡Variations¡¡neither¡¡useful¡¡nor¡¡injurious¡¡would¡¡not¡¡be¡¡affected¡¡by¡¡natural¡¡selection£»¡¡and¡¡would¡¡be¡¡left¡¡a¡¡fluctuating¡¡element£»¡¡as¡¡perhaps¡¡we¡¡see¡¡in¡¡the¡¡species¡¡called¡¡polymorphic¡£¡¡
We¡¡shall¡¡best¡¡understand¡¡the¡¡probable¡¡course¡¡of¡¡natural¡¡selection¡¡by¡¡taking¡¡the¡¡case¡¡of¡¡a¡¡country¡¡undergoing¡¡some¡¡physical¡¡change£»¡¡for¡¡instance£»¡¡of¡¡climate¡£¡¡The¡¡proportional¡¡numbers¡¡of¡¡its¡¡inhabitants¡¡would¡¡almost¡¡immediately¡¡undergo¡¡a¡¡change£»¡¡and¡¡some¡¡species¡¡might¡¡become¡¡extinct¡£¡¡We¡¡may¡¡conclude£»¡¡from¡¡what¡¡we¡¡have¡¡seen¡¡of¡¡the¡¡intimate¡¡and¡¡complex¡¡manner¡¡in¡¡which¡¡the¡¡inhabitants¡¡of¡¡each¡¡country¡¡are¡¡bound¡¡together£»¡¡that¡¡any¡¡change¡¡in¡¡the¡¡numerical¡¡proportions¡¡of¡¡some¡¡of¡¡the¡¡inhabitants£»¡¡independently¡¡of¡¡the¡¡change¡¡of¡¡climate¡¡itself£»¡¡would¡¡most¡¡seriously¡¡affect¡¡many¡¡of¡¡the¡¡others¡£¡¡If¡¡the¡¡country¡¡were¡¡open¡¡on¡¡its¡¡borders£»¡¡new¡¡forms¡¡would¡¡certainly¡¡immigrate£»¡¡and¡¡this¡¡also¡¡would¡¡seriously¡¡disturb¡¡the¡¡relations¡¡of¡¡some¡¡of¡¡the¡¡former¡¡inhabitants¡£¡¡Let¡¡it¡¡be¡¡remembered¡¡how¡¡powerful¡¡the¡¡influence¡¡of¡¡a¡¡single¡¡introduced¡¡tree¡¡or¡¡mammal¡¡has¡¡been¡¡shown¡¡to¡¡be¡£¡¡But¡¡in¡¡the¡¡case¡¡of¡¡an¡¡island£»¡¡or¡¡of¡¡a¡¡country¡¡partly¡¡surrounded¡¡by¡¡barriers£»¡¡into¡¡which¡¡new¡¡and¡¡better¡¡adapted¡¡forms¡¡could¡¡not¡¡freely¡¡enter£»¡¡we¡¡should¡¡then¡¡have¡¡places¡¡in¡¡the¡¡economy¡¡of¡¡nature¡¡which¡¡would¡¡assuredly¡¡be¡¡better¡¡filled¡¡up£»¡¡if¡¡some¡¡of¡¡the¡¡original¡¡inhabitants¡¡were¡¡in¡¡some¡¡manner¡¡modified£»¡¡for£»¡¡had¡¡the¡¡area¡¡been¡¡open¡¡to¡¡immigration£»¡¡these¡¡same¡¡places¡¡would¡¡have¡¡been¡¡seized¡¡on¡¡by¡¡intruders¡£¡¡In¡¡such¡¡case£»¡¡every¡¡slight¡¡modification£»¡¡which¡¡in¡¡the¡¡course¡¡of¡¡ages¡¡chanced¡¡to¡¡arise£»¡¡and¡¡which¡¡in¡¡any¡¡way¡¡favoured¡¡the¡¡individuals¡¡of¡¡any¡¡of¡¡the¡¡species£»¡¡by¡¡better¡¡adapting¡¡them¡¡to¡¡their¡¡altered¡¡conditions£»¡¡would¡¡tend¡¡to¡¡be¡¡preserved£»¡¡and¡¡natural¡¡selection¡¡would¡¡thus¡¡have¡¡free¡¡scope¡¡for¡¡the¡¡work¡¡of¡¡improvement¡£¡¡
We¡¡have¡¡reason¡¡to¡¡believe£»¡¡as¡¡stated¡¡in¡¡the¡¡first¡¡chapter£»¡¡that¡¡a¡¡change¡¡in¡¡the¡¡conditions¡¡of¡¡life£»¡¡by¡¡specially¡¡acting¡¡on¡¡the¡¡reproductive¡¡system£»¡¡causes¡¡or¡¡increases¡¡variability£»¡¡and¡¡in¡¡the¡¡foregoing¡¡case¡¡the¡¡conditions¡¡of¡¡life¡¡are¡¡supposed¡¡to¡¡have¡¡undergone¡¡a¡¡change£»¡¡and¡¡this¡¡would¡¡manifestly¡¡be¡¡favourable¡¡to¡¡natural¡¡selection£»¡¡by¡¡giving¡¡a¡¡better¡¡chance¡¡of¡¡profitable¡¡variations¡¡occurring£»¡¡and¡¡unless¡¡profitable¡¡variations¡¡do¡¡occur£»¡¡natural¡¡selection¡¡can¡¡do¡¡nothing¡£¡¡Not¡¡that£»¡¡as¡¡I¡¡believe£»¡¡any¡¡extreme¡¡amount¡¡of¡¡variability¡¡is¡¡necessary£»¡¡as¡¡man¡¡can¡¡certainly¡¡produce¡¡great¡¡results¡¡by¡¡adding¡¡up¡¡in¡¡any¡¡given¡¡direction¡¡mere¡¡individual¡¡differences£»¡¡so¡¡could¡¡Nature£»¡¡but¡¡far¡¡more¡¡easily£»¡¡from¡¡having¡¡incomparably¡¡longer¡¡time¡¡at¡¡her¡¡disposal¡£¡¡Nor¡¡do¡¡I¡¡believe¡¡that¡¡any¡¡great¡¡physical¡¡change£»¡¡as¡¡of¡¡climate£»¡¡or¡¡any¡¡unusual¡¡degree¡¡of¡¡isolation¡¡to¡¡check¡¡immigration£»¡¡is¡¡actually¡¡necessary¡¡to¡¡produce¡¡new¡¡and¡¡unoccupied¡¡places¡¡for¡¡natural¡¡selection¡¡to¡¡fill¡¡up¡¡by¡¡modifying¡¡and¡¡improving¡¡some¡¡of¡¡the¡¡varying¡¡inhabitants¡£¡¡For¡¡as¡¡all¡¡the¡¡inhabitants¡¡of¡¡each¡¡country¡¡are¡¡struggling¡¡together¡¡with¡¡nicely¡¡balanced¡¡forces£»¡¡extremely¡¡slight¡¡modifications¡¡in¡¡the¡¡structure¡¡or¡¡habits¡¡of¡¡one¡¡inhabitant¡¡would¡¡often¡¡give¡¡it¡¡an¡¡advantage¡¡over¡¡others£»¡¡and¡¡still¡¡further¡¡modifications¡¡of¡¡the¡¡same¡¡kind¡¡would¡¡often¡¡still¡¡further¡¡increase¡¡the¡¡advantage¡£¡¡No¡¡country¡¡can¡¡be¡¡named¡¡in¡¡which¡¡all¡¡the¡¡native¡¡inhabitants¡¡are¡¡now¡¡so¡¡perfectly¡¡adapted¡¡to¡¡each¡¡other¡¡and¡¡to¡¡the¡¡physical¡¡conditions¡¡under¡¡which¡¡they¡¡live£»¡¡that¡¡none¡¡of¡¡them

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