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ng¡¡a¡¡large¡¡amount¡¡of¡¡crossing¡¡with¡¡inferior¡¡animals¡£¡¡Thus¡¡it¡¡will¡¡be¡¡in¡¡nature£»¡¡for¡¡within¡¡a¡¡confined¡¡area£»¡¡with¡¡some¡¡place¡¡in¡¡its¡¡polity¡¡not¡¡so¡¡perfectly¡¡occupied¡¡as¡¡might¡¡be£»¡¡natural¡¡selection¡¡will¡¡always¡¡tend¡¡to¡¡preserve¡¡all¡¡the¡¡individuals¡¡varying¡¡in¡¡the¡¡right¡¡direction£»¡¡though¡¡in¡¡different¡¡degrees£»¡¡so¡¡as¡¡better¡¡to¡¡fill¡¡up¡¡the¡¡unoccupied¡¡place¡£¡¡But¡¡if¡¡the¡¡area¡¡be¡¡large£»¡¡its¡¡several¡¡districts¡¡will¡¡almost¡¡certainly¡¡present¡¡different¡¡conditions¡¡of¡¡life£»¡¡and¡¡then¡¡if¡¡natural¡¡selection¡¡be¡¡modifying¡¡and¡¡improving¡¡a¡¡species¡¡in¡¡the¡¡several¡¡districts£»¡¡there¡¡will¡¡be¡¡intercrossing¡¡with¡¡the¡¡other¡¡individuals¡¡of¡¡the¡¡same¡¡species¡¡on¡¡the¡¡confines¡¡of¡¡each¡£¡¡And¡¡in¡¡this¡¡case¡¡the¡¡effects¡¡of¡¡intercrossing¡¡can¡¡hardly¡¡be¡¡counterbalanced¡¡by¡¡natural¡¡selection¡¡always¡¡tending¡¡to¡¡modify¡¡all¡¡the¡¡individuals¡¡in¡¡each¡¡district¡¡in¡¡exactly¡¡the¡¡same¡¡manner¡¡to¡¡the¡¡conditions¡¡of¡¡each£»¡¡for¡¡in¡¡a¡¡continuous¡¡area£»¡¡the¡¡conditions¡¡will¡¡generally¡¡graduate¡¡away¡¡insensibly¡¡from¡¡one¡¡district¡¡to¡¡another¡£¡¡The¡¡intercrossing¡¡will¡¡most¡¡affect¡¡those¡¡animals¡¡which¡¡unite¡¡for¡¡each¡¡birth£»¡¡which¡¡wander¡¡much£»¡¡and¡¡which¡¡do¡¡not¡¡breed¡¡at¡¡a¡¡very¡¡quick¡¡rate¡£¡¡Hence¡¡in¡¡animals¡¡of¡¡this¡¡nature£»¡¡for¡¡instance¡¡in¡¡birds£»¡¡varieties¡¡will¡¡generally¡¡be¡¡confined¡¡to¡¡separated¡¡countries£»¡¡and¡¡this¡¡I¡¡believe¡¡to¡¡be¡¡the¡¡case¡£¡¡In¡¡hermaphrodite¡¡organisms¡¡which¡¡cross¡¡only¡¡occasionally£»¡¡and¡¡likewise¡¡in¡¡animals¡¡which¡¡unite¡¡for¡¡each¡¡birth£»¡¡but¡¡which¡¡wander¡¡little¡¡and¡¡which¡¡can¡¡increase¡¡at¡¡a¡¡very¡¡rapid¡¡rate£»¡¡a¡¡new¡¡and¡¡improved¡¡variety¡¡might¡¡be¡¡quickly¡¡formed¡¡on¡¡any¡¡one¡¡spot£»¡¡and¡¡might¡¡there¡¡maintain¡¡itself¡¡in¡¡a¡¡body£»¡¡so¡¡that¡¡whatever¡¡intercrossing¡¡took¡¡place¡¡would¡¡be¡¡chiefly¡¡between¡¡the¡¡individuals¡¡of¡¡the¡¡same¡¡new¡¡variety¡£¡¡A¡¡local¡¡variety¡¡when¡¡once¡¡thus¡¡formed¡¡might¡¡subsequently¡¡slowly¡¡spread¡¡to¡¡other¡¡districts¡£¡¡On¡¡the¡¡above¡¡principle£»¡¡nurserymen¡¡always¡¡prefer¡¡getting¡¡seed¡¡from¡¡a¡¡large¡¡body¡¡of¡¡plants¡¡of¡¡the¡¡same¡¡variety£»¡¡as¡¡the¡¡chance¡¡of¡¡intercrossing¡¡with¡¡other¡¡varieties¡¡is¡¡thus¡¡lessened¡£¡¡
Even¡¡in¡¡the¡¡case¡¡of¡¡slow¡­breeding¡¡animals£»¡¡which¡¡unite¡¡for¡¡each¡¡birth£»¡¡we¡¡must¡¡not¡¡overrate¡¡the¡¡effects¡¡of¡¡intercrosses¡¡in¡¡retarding¡¡natural¡¡selection£»¡¡for¡¡I¡¡can¡¡bring¡¡a¡¡considerable¡¡catalogue¡¡of¡¡facts£»¡¡showing¡¡that¡¡within¡¡the¡¡same¡¡area£»¡¡varieties¡¡of¡¡the¡¡same¡¡animal¡¡can¡¡long¡¡remain¡¡distinct£»¡¡from¡¡haunting¡¡different¡¡stations£»¡¡from¡¡breeding¡¡at¡¡slightly¡¡different¡¡seasons£»¡¡or¡¡from¡¡varieties¡¡of¡¡the¡¡same¡¡kind¡¡preferring¡¡to¡¡pair¡¡together¡£¡¡
Intercrossing¡¡plays¡¡a¡¡very¡¡important¡¡part¡¡in¡¡nature¡¡in¡¡keeping¡¡the¡¡individuals¡¡of¡¡the¡¡same¡¡species£»¡¡or¡¡of¡¡the¡¡same¡¡variety£»¡¡true¡¡and¡¡uniform¡¡in¡¡character¡£¡¡It¡¡will¡¡obviously¡¡thus¡¡act¡¡far¡¡more¡¡efficiently¡¡with¡¡those¡¡animals¡¡which¡¡unite¡¡for¡¡each¡¡birth£»¡¡but¡¡I¡¡have¡¡already¡¡attempted¡¡to¡¡show¡¡that¡¡we¡¡have¡¡reason¡¡to¡¡believe¡¡that¡¡occasional¡¡intercrosses¡¡take¡¡place¡¡with¡¡all¡¡animals¡¡and¡¡with¡¡all¡¡plants¡£¡¡Even¡¡if¡¡these¡¡take¡¡place¡¡only¡¡at¡¡long¡¡intervals£»¡¡I¡¡am¡¡convinced¡¡that¡¡the¡¡young¡¡thus¡¡produced¡¡will¡¡gain¡¡so¡¡much¡¡in¡¡vigour¡¡and¡¡fertility¡¡over¡¡the¡¡offspring¡¡from¡¡long¡­continued¡¡self¡­fertilisation£»¡¡that¡¡they¡¡will¡¡have¡¡a¡¡better¡¡chance¡¡of¡¡surviving¡¡and¡¡propagating¡¡their¡¡kind£»¡¡and¡¡thus£»¡¡in¡¡the¡¡long¡¡run£»¡¡the¡¡influence¡¡of¡¡intercrosses£»¡¡even¡¡at¡¡rare¡¡intervals£»¡¡will¡¡be¡¡great¡£¡¡If¡¡there¡¡exist¡¡organic¡¡beings¡¡which¡¡never¡¡intercross£»¡¡uniformity¡¡of¡¡character¡¡can¡¡be¡¡retained¡¡amongst¡¡them£»¡¡as¡¡long¡¡as¡¡their¡¡conditions¡¡of¡¡life¡¡remain¡¡the¡¡same£»¡¡only¡¡through¡¡the¡¡principle¡¡of¡¡inheritance£»¡¡and¡¡through¡¡natural¡¡selection¡¡destroying¡¡any¡¡which¡¡depart¡¡from¡¡the¡¡proper¡¡type£»¡¡but¡¡if¡¡their¡¡conditions¡¡of¡¡life¡¡change¡¡and¡¡they¡¡undergo¡¡modification£»¡¡uniformity¡¡of¡¡character¡¡can¡¡be¡¡given¡¡to¡¡their¡¡modified¡¡offspring£»¡¡solely¡¡by¡¡natural¡¡selection¡¡preserving¡¡the¡¡same¡¡favourable¡¡variations¡£¡¡
Isolation£»¡¡also£»¡¡is¡¡an¡¡important¡¡element¡¡in¡¡the¡¡process¡¡of¡¡natural¡¡selection¡£¡¡In¡¡a¡¡confined¡¡or¡¡isolated¡¡area£»¡¡if¡¡not¡¡very¡¡large£»¡¡the¡¡organic¡¡and¡¡inorganic¡¡conditions¡¡of¡¡life¡¡will¡¡generally¡¡be¡¡in¡¡a¡¡great¡¡degree¡¡uniform£»¡¡so¡¡that¡¡natural¡¡selection¡¡will¡¡tend¡¡to¡¡modify¡¡all¡¡the¡¡individuals¡¡of¡¡a¡¡varying¡¡species¡¡throughout¡¡the¡¡area¡¡in¡¡the¡¡same¡¡manner¡¡in¡¡relation¡¡to¡¡the¡¡same¡¡conditions¡£¡¡Intercrosses£»¡¡also£»¡¡with¡¡the¡¡individuals¡¡of¡¡the¡¡same¡¡species£»¡¡which¡¡otherwise¡¡would¡¡have¡¡inhabited¡¡the¡¡surrounding¡¡and¡¡differently¡¡circumstanced¡¡districts£»¡¡will¡¡be¡¡prevented¡£¡¡But¡¡isolation¡¡probably¡¡acts¡¡more¡¡efficiently¡¡in¡¡checking¡¡the¡¡immigration¡¡of¡¡better¡¡adapted¡¡organisms£»¡¡after¡¡any¡¡physical¡¡change£»¡¡such¡¡as¡¡of¡¡climate¡¡or¡¡elevation¡¡of¡¡the¡¡land£»¡¡&c¡££»¡¡and¡¡thus¡¡new¡¡places¡¡in¡¡the¡¡natural¡¡economy¡¡of¡¡the¡¡country¡¡are¡¡left¡¡open¡¡for¡¡the¡¡old¡¡inhabitants¡¡to¡¡struggle¡¡for£»¡¡and¡¡become¡¡adapted¡¡to£»¡¡through¡¡modifications¡¡in¡¡their¡¡structure¡¡and¡¡constitution¡£¡¡Lastly£»¡¡isolation£»¡¡by¡¡checking¡¡immigration¡¡and¡¡consequently¡¡competition£»¡¡will¡¡give¡¡time¡¡for¡¡any¡¡new¡¡variety¡¡to¡¡be¡¡slowly¡¡improved£»¡¡and¡¡this¡¡may¡¡sometimes¡¡be¡¡of¡¡importance¡¡in¡¡the¡¡production¡¡of¡¡new¡¡species¡£¡¡If£»¡¡however£»¡¡an¡¡isolated¡¡area¡¡be¡¡very¡¡small£»¡¡either¡¡from¡¡being¡¡surrounded¡¡by¡¡barriers£»¡¡or¡¡from¡¡having¡¡very¡¡peculiar¡¡physical¡¡conditions£»¡¡the¡¡total¡¡number¡¡of¡¡the¡¡individuals¡¡supported¡¡on¡¡it¡¡will¡¡necessarily¡¡be¡¡very¡¡small£»¡¡and¡¡fewness¡¡of¡¡individuals¡¡will¡¡greatly¡¡retard¡¡the¡¡production¡¡of¡¡new¡¡species¡¡through¡¡natural¡¡selection£»¡¡by¡¡decreasing¡¡the¡¡chance¡¡of¡¡the¡¡appearance¡¡of¡¡favourable¡¡variations¡£¡¡
If¡¡we¡¡turn¡¡to¡¡nature¡¡to¡¡test¡¡the¡¡truth¡¡of¡¡these¡¡remarks£»¡¡and¡¡look¡¡at¡¡any¡¡small¡¡isolated¡¡area£»¡¡such¡¡as¡¡an¡¡oceanic¡¡island£»¡¡although¡¡the¡¡total¡¡number¡¡of¡¡the¡¡species¡¡inhabiting¡¡it£»¡¡will¡¡be¡¡found¡¡to¡¡be¡¡small£»¡¡as¡¡we¡¡shall¡¡see¡¡in¡¡our¡¡chapter¡¡on¡¡geographical¡¡distribution£»¡¡yet¡¡of¡¡these¡¡species¡¡a¡¡very¡¡large¡¡proportion¡¡are¡¡endemic£»¡¡that¡¡is£»¡¡have¡¡been¡¡produced¡¡there£»¡¡and¡¡nowhere¡¡else¡£¡¡Hence¡¡an¡¡oceanic¡¡island¡¡at¡¡first¡¡sight¡¡seems¡¡to¡¡have¡¡been¡¡highly¡¡favourable¡¡for¡¡the¡¡production¡¡of¡¡new¡¡species¡£¡¡But¡¡we¡¡may¡¡thus¡¡greatly¡¡deceive¡¡ourselves£»¡¡for¡¡to¡¡ascertain¡¡whether¡¡a¡¡small¡¡isolated¡¡area£»¡¡or¡¡a¡¡large¡¡open¡¡area¡¡like¡¡a¡¡continent£»¡¡has¡¡been¡¡most¡¡favourable¡¡for¡¡the¡¡production¡¡of¡¡new¡¡organic¡¡forms£»¡¡we¡¡ought¡¡to¡¡make¡¡the¡¡comparison¡¡within¡¡equal¡¡times£»¡¡and¡¡this¡¡we¡¡are¡¡incapable¡¡of¡¡doing¡£¡¡
Although¡¡I¡¡do¡¡not¡¡doubt¡¡that¡¡isolation¡¡is¡¡of¡¡considerable¡¡importance¡¡in¡¡the¡¡production¡¡of¡¡new¡¡species£»¡¡on¡¡the¡¡whole¡¡I¡¡am¡¡inclined¡¡to¡¡believe¡¡that¡¡largeness¡¡of¡¡area¡¡is¡¡of¡¡more¡¡importance£»¡¡more¡¡especially¡¡in¡¡the¡¡production¡¡of¡¡species£»¡¡which¡¡will¡¡prove¡¡capable¡¡of¡¡enduring¡¡for¡¡a¡¡long¡¡period£»¡¡and¡¡of¡¡spreading¡¡widely¡£¡¡Throughout¡¡a¡¡great¡¡and¡¡open¡¡area£»¡¡not¡¡only¡¡will¡¡there¡¡be¡¡a¡¡better¡¡chance¡¡of¡¡favourable¡¡variations¡¡arising¡¡from¡¡the¡¡large¡¡number¡¡of¡¡individuals¡¡of¡¡the¡¡same¡¡species¡¡there¡¡supported£»¡¡but¡¡the¡¡conditions¡¡of¡¡life¡¡are¡¡infinitely¡¡complex¡¡from¡¡the¡¡large¡¡number¡¡of¡¡already¡¡existing¡¡species£»¡¡and¡¡if¡¡some¡¡of¡¡these¡¡many¡¡species¡¡become¡¡modified¡¡and¡¡improved£»¡¡others¡¡will¡¡have¡¡to¡¡be¡¡improved¡¡in¡¡a¡¡corresponding¡¡degree¡¡or¡¡they¡¡will¡¡be¡¡exterminated¡£¡¡Each¡¡new¡¡form£»¡¡also£»¡¡as¡¡soon¡¡as¡¡it¡¡has¡¡been¡¡much¡¡improved£»¡¡will¡¡be¡¡able¡¡to¡¡spread¡¡over¡¡the¡¡open¡¡and¡¡continuous¡¡area£»¡¡and¡¡will¡¡thus¡¡come¡¡into¡¡competition¡¡with¡¡many¡¡others¡£¡¡Hence¡¡more¡¡new¡¡places¡¡will¡¡be¡¡formed£»¡¡and¡¡the¡¡competition¡¡to¡¡fill¡¡them¡¡will¡¡be¡¡more¡¡severe£»¡¡on¡¡a¡¡large¡¡than¡¡on¡¡a¡¡small¡¡and¡¡isolated¡¡area¡£¡¡Moreover£»¡¡great¡¡areas£»¡¡though¡¡now¡¡continuous£»¡¡owing¡¡to¡¡oscillations¡¡of¡¡level£»¡¡will¡¡often¡¡have¡¡recently¡¡existed¡¡in¡¡a¡¡broken¡¡condition£»¡¡so¡¡that¡¡the¡¡good¡¡effects¡¡of¡¡isolation¡¡will¡¡generally£»¡¡to¡¡a¡¡certain¡¡extent£»¡¡have¡¡concurred¡£¡¡Finally£»¡¡I¡¡conclude¡¡that£»¡¡although¡¡small¡¡isolated¡¡areas¡¡probably¡¡have¡¡been¡¡in¡¡some¡¡respects¡¡highly¡¡favourable¡¡for¡¡the¡¡production¡¡of¡¡new¡¡species£»¡¡yet¡¡that¡¡the¡¡course¡¡of¡¡modification¡¡will¡¡generally¡¡have¡¡been¡¡more¡¡rapid¡¡on¡¡large¡¡areas£»¡¡and¡¡what¡¡is¡¡more¡¡important£»¡¡that¡¡the¡¡new¡¡forms¡¡produced¡¡on¡¡large¡¡areas£»¡¡which¡¡already¡¡have¡¡been¡¡victorious¡¡over¡¡many¡¡competitors£»¡¡will¡¡be¡¡those¡¡that¡¡will¡¡spread¡¡most¡¡widely£»¡¡will¡¡give¡¡rise¡¡to¡¡most¡¡new¡¡varieties¡¡and¡¡species£»¡¡and¡¡will¡¡thus¡¡play¡¡an¡¡important¡¡part¡¡in¡¡the¡¡changing¡¡history¡¡of¡¡the¡¡organic¡¡world¡£¡¡
We¡¡can£»¡¡perhaps£»¡¡on¡¡these¡¡views£»¡¡understand¡¡some¡¡facts¡¡which¡¡will¡¡be¡¡again¡¡alluded¡¡to¡¡in¡¡our¡¡chapter¡¡on¡¡geographical¡¡distribution£»¡¡for¡¡instance£»¡¡that¡¡the¡¡productions¡¡of¡¡the¡¡smaller¡¡continent¡¡of¡¡Australia¡¡have¡¡formerly¡¡yielded£»¡¡and¡¡apparently¡¡are¡¡now¡¡yielding£»¡¡before¡¡those¡¡of¡¡the¡¡larger¡¡Europaeo¡­Asiatic¡¡area¡£¡¡Thus£»¡¡also£»¡¡it¡¡is¡¡that¡¡continental¡¡productions¡¡have¡¡everywhere¡¡become¡¡so¡¡largely¡¡naturalised¡¡on¡¡islands¡£¡¡On¡¡a¡¡small¡¡island£»¡¡the¡¡race¡¡for¡¡life¡¡will¡¡have¡¡been¡¡less¡¡severe£»¡¡and¡¡there¡¡will¡¡have¡¡been¡¡less¡¡modification¡¡and¡¡less¡¡extermination¡£¡¡Hence£»¡¡perhaps£»¡¡it¡¡comes¡¡that¡¡the¡¡flora¡¡of¡¡Madeira£»¡¡according¡¡to¡¡Oswald¡¡Heer£»¡¡resembles¡¡the¡¡extinct¡¡tertiary¡¡flora¡¡of¡¡Europe¡£¡¡All¡¡fresh¡­water¡¡basins£»¡¡taken¡¡together£»¡¡make¡¡a¡¡small¡¡area¡¡compared¡¡with¡¡that¡¡of¡¡the¡¡sea¡¡or¡¡of¡¡the¡¡land£»¡¡and£»¡¡consequently£»¡¡the¡¡competition¡¡between¡¡fresh¡­water¡¡productions¡¡will¡¡have¡¡been¡¡less¡¡severe¡¡than¡¡elsewhere£»¡¡new¡¡forms¡¡will¡¡have¡¡been¡¡more¡¡slowly¡¡formed£»¡¡and¡¡old¡¡forms¡¡more¡¡slowly¡¡exterminated¡£¡¡And¡¡it¡¡is¡¡in¡¡fresh¡¡water¡¡that¡¡we¡¡find¡¡seven¡¡genera¡¡of¡¡Ganoid¡¡fishes£»¡¡remnants¡¡of¡¡a¡¡once¡¡preponderant¡¡order£º¡¡and¡¡in¡¡fresh¡¡water¡¡we¡¡find¡¡some¡¡of¡¡the¡¡most¡¡anomalous¡¡forms¡¡now¡¡known¡¡in¡¡the¡¡world£»¡¡as¡¡the¡¡Ornithorhynchus¡¡and¡¡Lepidosiren£»¡¡which£»¡¡like¡¡fossils£»¡¡connect¡¡to¡¡a¡¡certain¡¡extent¡¡orders¡¡now¡¡widely¡¡separated¡¡in¡¡the¡¡natural¡¡scale¡£¡¡These¡¡anomalous¡¡forms¡¡may¡¡almost¡¡be¡¡called¡¡living¡¡fossils£»¡¡they¡¡have¡¡endured¡¡to¡¡the¡¡present¡¡day£»¡¡from¡¡having¡¡inhabited¡¡a¡¡confined¡¡area£»¡¡and¡¡from¡¡having¡¡thus¡¡been¡¡exposed¡¡to¡¡less¡¡severe¡¡competition¡£¡¡
To¡¡sum¡¡up¡¡the¡¡circumstances¡¡favourable¡¡and¡¡unfavou

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