the origin of species(物种起源)-第43部分

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natural　selection。　But　I　am　bound　to　confess；　that；　with　all　my　faith　in　this　principle；　I　should　never　have　anticipated　that　natural　selection　could　have　been　efficient　in　so　high　a　degree；　had　not　the　case　of　these　neuter　insects　convinced　me　of　the　fact。　I　have；　therefore；　discussed　this　case；　at　some　little　but　wholly　insufficient　length；　in　order　to　show　the　power　of　natural　selection；　and　likewise　because　this　is　by　far　the　most　serious　special　difficulty；　which　my　theory　has　encountered。　The　case；　also；　is　very　interesting；　as　it　proves　that　with　animals；　as　with　plants；　any　amount　of　modification　in　structure　can　be　effected　by　the　accumulation　of　numerous；　slight；　and　as　we　must　call　them　accidental；　variations；　which　are　in　any　manner　profitable；　without　exercise　or　habit　having　come　into　play。　For　no　amount　of　exercise；　or　habit；　or　volition；　in　the　utterly　sterile　members　of　a　community　could　possibly　have　affected　the　structure　or　instincts　of　the　fertile　members；　which　alone　leave　descendants。　I　am　surprised　that　no　one　has　advanced　this　demonstrative　case　of　neuter　insects；　against　the　well…known　doctrine　of　Lamarck。　
Summary。　I　have　endeavoured　briefly　in　this　chapter　to　show　that　the　mental　qualities　of　our　domestic　animals　vary；　and　that　the　variations　are　inherited。　Still　more　briefly　I　have　attempted　to　show　that　instincts　vary　slightly　in　a　state　of　nature。　No　one　will　dispute　that　instincts　are　of　the　highest　importance　to　each　animal。　Therefore　I　can　see　no　difficulty；　under　changing　conditions　of　life；　in　natural　selection　accumulating　slight　modifications　of　instinct　to　any　extent；　in　any　useful　direction。　In　some　cases　habit　or　use　and　disuse　have　probably　come　into　play。　I　do　not　pretend　that　the　facts　given　in　this　chapter　strengthen　in　any　great　degree　my　theory；　but　none　of　the　cases　of　difficulty；　to　the　best　of　my　judgment；　annihilate　it。　On　the　other　hand；　the　fact　that　instincts　are　not　always　absolutely　perfect　and　are　liable　to　mistakes；　that　no　instinct　has　been　produced　for　the　exclusive　good　of　other　animals；　but　that　each　animal　takes　advantage　of　the　instincts　of　others；　that　the　canon　in　natural　history；　of　'natura　non　facit　saltum'　is　applicable　to　instincts　as　well　as　to　corporeal　structure；　and　is　plainly　explicable　on　the　foregoing　views；　but　is　otherwise　inexplicable；　all　tend　to　corroborate　the　theory　of　natural　selection。　
This　theory　is；　also；　strengthened　by　some　few　other　facts　in　regard　to　instincts；　as　by　that　common　case　of　closely　allied；　but　certainly　distinct；　species；　when　inhabiting　distant　parts　of　the　world　and　living　under　considerably　different　conditions　of　life；　yet　often　retaining　nearly　the　same　instincts。　For　instance；　we　can　understand　on　the　principle　of　inheritance；　how　it　is　that　the　thrush　of　South　America　lines　its　nest　with　mud；　in　the　same　peculiar　manner　as　does　our　British　thrush：　how　it　is　that　the　male　wrens　（Troglodytes）　of　North　America；　build　'cock…nests；'　to　roost　in；　like　the　males　of　our　distinct　Kitty…wrens；　a　habit　wholly　unlike　that　of　any　other　known　bird。　Finally；　it　may　not　be　a　logical　deduction；　but　to　my　imagination　it　is　far　more　satisfactory　to　look　at　such　instincts　as　the　young　cuckoo　ejecting　its　foster…brothers；　ants　making　slaves；　　the　larvae　of　ichneumonidae　feeding　within　the　live　bodies　of　caterpillars；　not　as　specially　endowed　or　created　instincts；　but　as　small　consequences　of　one　general　law；　leading　to　the　advancement　of　all　organic　beings；　namely；　multiply；　vary；　let　the　strongest　live　and　the　weakest　die。　　

Chapter　8　…　Hybridism

THE　view　generally　entertained　by　naturalists　is　that　species；　when　intercrossed；　have　been　specially　endowed　with　the　quality　of　sterility；　in　order　to　prevent　the　confusion　of　all　organic　forms。　This　view　certainly　seems　at　first　probable；　for　species　within　the　same　country　could　hardly　have　kept　distinct　had　they　been　capable　of　crossing　freely。　The　importance　of　the　fact　that　hybrids　are　very　generally　sterile；　has；　I　think；　been　much　underrated　by　some　late　writers。　On　the　theory　of　natural　selection　the　case　is　especially　important；　inasmuch　as　the　sterility　of　hybrids　could　not　possibly　be　of　any　advantage　to　them；　and　therefore　could　not　have　been　acquired　by　the　continued　preservation　of　successive　profitable　degrees　of　sterility。　I　hope；　however；　to　be　able　to　show　that　sterility　is　not　a　specially　acquired　or　endowed　quality；　but　is　incidental　on　other　acquired　differences。　
In　treating　this　subject；　two　classes　of　facts；　to　a　large　extent　fundamentally　different；　have　generally　been　confounded　together；　namely；　the　sterility　of　two　species　when　first　crossed；　and　the　sterility　of　the　hybrids　produced　from　them。　
Pure　species　have　of　course　their　organs　of　reproduction　in　a　perfect　condition；　yet　when　intercrossed　they　produce　either　few　or　no　offspring。　Hybrids；　on　the　other　hand；　have　their　reproductive　organs　functionally　impotent；　as　may　be　clearly　seen　in　the　state　of　the　male　element　in　both　plants　and　animals；　though　the　organs　themselves　are　perfect　in　structure；　as　far　as　the　microscope　reveals。　In　the　first　case　the　two　sexual　elements　which　go　to　form　the　embryo　are　perfect；　in　the　second　case　they　are　either　not　at　all　developed；　or　are　imperfectly　developed。　This　distinction　is　important；　when　the　cause　of　the　sterility；　which　is　common　to　the　two　cases；　has　to　be　considered。　The　distinction　has　probably　been　slurred　over；　owing　to　the　sterility　in　both　cases　being　looked　on　as　a　special　endowment；　beyond　the　province　of　our　reasoning　powers。　
The　fertility　of　varieties；　that　is　of　the　forms　known　or　believed　to　have　descended　from　common　parents；　when　intercrossed；　and　likewise　the　fertility　of　their　mongrel　offspring；　is；　on　my　theory；　of　equal　importance　with　the　sterility　of　species；　for　it　seems　to　make　a　broad　and　clear　distinction　between　varieties　and　species。　
First；　for　the　sterility　of　species　when　crossed　and　of　their　hybrid　offspring。　It　is　impossible　to　study　the　several　memoirs　and　works　of　those　two　conscientious　and　admirable　observers；　K？lreuter　and　G？rtner；　who　almost　devoted　their　lives　to　this　subject；　without　being　deeply　impressed　with　the　high　generality　of　some　degree　of　sterility。　K？lreuter　makes　the　rule　universal；　but　then　he　cuts　the　knot；　for　in　ten　cases　in　which　he　found　two　forms；　considered　by　most　authors　as　distinct　species；　quite　fertile　together；　he　unhesitatingly　ranks　them　as　varieties。　G？rtner；　also；　makes　the　rule　equally　universal；　and　he　disputes　the　entire　fertility　of　K？lreuter's　ten　cases。　But　in　these　and　in　many　other　cases；　G？rtner　is　obliged　carefully　to　count　the　seeds；　in　order　to　show　that　there　is　any　degree　of　sterility。　He　always　compares　the　maximum　number　of　seeds　produced　by　two　species　when　crossed　and　by　their　hybrid　offspring；　with　the　average　number　produced　by　both　pure　parent…species　in　a　state　of　nature。　But　a　serious　cause　of　error　seems　to　me　to　be　here　introduced：　a　plant　to　be　hybridised　must　be　castrated；　and；　what　is　often　more　important；　must　be　secluded　in　order　to　prevent　pollen　being　brought　to　it　by　insects　from　other　plants。　Nearly　all　the　plants　experimentised　on　by　G？rtner　were　potted；　and　apparently　were　kept　in　a　chamber　in　his　house。　That　these　processes　are　often　injurious　to　the　fertility　of　a　plant　cannot　be　doubted；　for　G？rtner　gives　in　his　table　about　a　score　of　cases　of　plants　which　he　castrated；　and　artificially　fertilised　with　their　own　pollen；　and　（excluding　all　cases　such　as　the　Leguminosae；　in　which　there　is　an　acknowledged　difficulty　in　the　manipulation）　half　of　these　twenty　plants　had　their　fertility　in　some　degree　impaired。　Moreover；　as　G？rtner　during　several　years　repeatedly　crossed　the　primrose　and　cowslip；　which　we　have　such　good　reason　to　believe　to　be　varieties；　and　only　once　or　twice　succeeded　in　getting　fertile　seed；　as　he　found　the　common　red　and　blue　pimpernels　（Anagallis　arvensis　and　coerulea）；　which　the　best　botanists　rank　as　varieties；　absolutely　sterile　together；　and　as　he　came　to　the　same　conclusion　in　several　other　analogous　cases；　it　seems　to　me　that　we　may　well　be　permitted　to　doubt　whether　many　other　species　are　really　so　sterile；　when　intercrossed；　as　G？rtner　believes。　
It　is　certain；　on　the　one　hand；　that　the　sterility　of　various　species　when　crossed　is　so　different　in　degree　and　graduates　away　so　insensibly；　and；　on　the　other　hand；　that　the　fertility　of　pure　species　is　so　easily　affected　by　various　circumstances；　that　for　all　practical　purposes　it　is　most　difficult　to　say　where　perfect　fertility　ends　and　sterility　begins。　I　think　no　better　evidence　of　this　can　be　required　than　that　the　two　most　experienced　observers　who　have　ever　lived；　namely；　K？lreuter　and　G？rtner；　should　have　arrived　at　diametrically　opposite　conclusions　in　regard　to　the　very　same　species。　It　is　also　most　instructive　to　compare　but　I　have　not　space　here　to　enter　on　details　the　evidence　advanced　by　our　best　botanists　on　the　question　whether　certain　doubtful　forms　should　be　ranked　as　species　or　varieties；　with　the　evidence　from　fertility　adduced　by　different　hybridisers；　or　by　the　same　author；　from　experiments　made　during　different　years。　It　can　thus　be　shown　that　neither　sterility　nor　fertility　affords　any　clear　distinction　between　species　and　varieties；　but　that　the　evidence　from　this　source　graduates　away；　and　is　doubtful　in　the　same　degree　as　is　the　evidence　derived　from　other　constitutional　and　structural　differences。　
In　regard　to　the　sterility　of　hybrids　in　successive　generations；　though　G？rtner　was　enabled　to　rear　some　hybrids；　carefully　guarding　them　from　a　cross　with　either　pure　parent；　for　six　or　seven；　and　in　one　case　for　ten　generations；　yet　he　asserts　positively　that　their　fertility　never　increased；　but　generally　greatly　decreased。　I　do　not　doubt　that　this　is　usually　the　case；　and　that　the　fertility　often　suddenly　decreases　in　the　first　few　generations。　Nevertheless　I　believe　that　in　all　these　experiments　the　fertility　has　been　diminished　by　an　independent　cause；　namely；　from　close　interbreeding。　I　have　collected　so　large　a　body　of　facts；　showing　that　close　interbreeding　lessens　fertility；　and；　on　the　other　hand；　that　an　occasional　cross　with　a　distinct　individual　or　varie