the origin of species(物种起源)-第58章

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special　endowment；　although　in　the　case　of　crossing；　the　difficulty　is　as　important　for　the　endurance　and　stability　of　specific　forms；　as　in　the　case　of　grafting　it　is　unimportant　for　their　welfare。　
Causes　of　the　Sterility　of　first　Crosses　and　of　Hybrids。　We　may　now　look　a　little　closer　at　the　probable　causes　of　the　sterility　of　first　crosses　and　of　hybrids。　These　two　cases　are　fundamentally　different；　for；　as　just　remarked；　in　the　union　of　two　pure　species　the　male　and　female　sexual　elements　are　perfect；　whereas　in　hybrids　they　are　imperfect。　Even　in　first　crosses；　the　greater　or　lesser　difficulty　in　effecting　a　union　apparently　depends　on　several　distinct　causes。　There　must　sometimes　be　a　physical　impossibility　in　the　male　element　reaching　the　ovule；　as　would　be　the　case　with　a　plant　having　a　pistil　too　long　for　the　pollen…tubes　to　reach　the　ovarium。　It　has　also　been　observed　that　when　pollen　of　one　species　is　placed　on　the　stigma　of　a　distantly　allied　species；　though　the　pollen…tubes　protrude；　they　do　not　penetrate　the　stigmatic　surface。　Again；　the　male　element　may　reach　the　female　element；　but　be　incapable　of　causing　an　embryo　to　be　developed；　as　seems　to　have　been　the　case　with　some　of　Thuret's　experiments　on　Fuci。　No　explanation　can　be　given　of　these　facts；　any　more　than　why　certain　trees　cannot　be　grafted　on　others。　Lastly；　an　embryo　may　be　developed；　and　then　perish　at　an　early　period。　This　latter　alternative　has　not　been　sufficiently　attended　to；　but　I　believe；　from　observations　communicated　to　me　by　Mr。　Hewitt；　who　has　had　great　experience　in　hybridising　gallinaceous　birds；　that　the　early　death　of　the　embryo　is　a　very　frequent　cause　of　sterility　in　first　crosses。　I　was　at　first　very　unwilling　to　believe　in　this　view；　as　hybrids；　when　once　born；　are　generally　healthy　and　long…lived；　as　we　see　in　the　case　of　the　common　mule。　Hybrids；　however；　are　differently　circumstanced　before　and　after　birth：　when　born　and　living　in　a　country　where　their　two　parents　can　live；　they　are　generally　placed　under　suitable　conditions　of　life。　But　a　hybrid　partakes　of　only　half　of　the　nature　and　constitution　of　its　mother；　and　therefore　before　birth；　as　long　as　it　is　nourished　within　its　mother's　womb　or　within　the　egg　or　seed　produced　by　the　mother；　it　may　be　exposed　to　conditions　in　some　degree　unsuitable；　and　consequently　be　liable　to　perish　at　an　early　period；　more　especially　as　all　very　young　beings　seem　eminently　sensitive　to　injurious　or　unnatural　conditions　of　life。　
In　regard　to　the　sterility　of　hybrids；　in　which　the　sexual　elements　are　imperfectly　developed；　the　case　is　very　different。　I　have　more　than　once　alluded　to　a　large　body　of　facts；　which　I　have　collected；　showing　that　when　animals　and　plants　are　removed　from　their　natural　conditions；　they　are　extremely　liable　to　have　their　reproductive　systems　seriously　affected。　This；　in　fact；　is　the　great　bar　to　the　domestication　of　animals。　Between　the　sterility　thus　superinduced　and　that　of　hybrids；　there　are　many　points　of　similarity。　In　both　cases　the　sterility　is　independent　of　general　health；　and　is　often　accompanied　by　excess　of　size　or　great　luxuriance。　In　both　cases；　the　sterility　occurs　in　various　degrees；　in　both；　the　male　element　is　the　most　liable　to　be　affected；　but　sometimes　the　female　more　than　the　male。　In　both；　the　tendency　goes　to　a　certain　extent　with　systematic　affinity；　or　whole　groups　of　animals　and　plants　are　rendered　impotent　by　the　same　unnatural　conditions；　and　whole　groups　of　species　tend　to　produce　sterile　hybrids。　On　the　other　hand；　one　species　in　a　group　will　sometimes　resist　great　changes　of　conditions　with　unimpaired　fertility；　and　certain　species　in　a　group　will　produce　unusually　fertile　hybrids。　No　one　can　tell；　till　he　tries；　whether　any　particular　animal　will　breed　under　confinement　or　any　plant　seed　freely　under　culture；　nor　can　he　tell；　till　he　tries；　whether　any　two　species　of　a　genus　will　produce　more　or　less　sterile　hybrids。　Lastly；　when　organic　beings　are　placed　during　several　generations　under　conditions　not　natural　to　them；　they　are　extremely　liable　to　vary；　which　is　due；　as　I　believe；　to　their　reproductive　systems　having　been　specially　affected；　though　in　a　lesser　degree　than　when　sterility　ensues。　So　it　is　with　hybrids；　for　hybrids　in　successive　generations　are　eminently　liable　to　vary；　as　every　experimentalist　has　observed。　
Thus　we　see　that　when　organic　beings　are　placed　under　new　and　unnatural　conditions；　and　when　hybrids　are　produced　by　the　unnatural　crossing　of　two　species；　the　reproductive　system；　independently　of　the　general　state　of　health；　is　affected　by　sterility　in　a　very　similar　manner。　In　the　one　case；　the　conditions　of　life　have　been　disturbed；　though　often　in　so　slight　a　degree　as　to　be　inappreciable　by　us；　in　the　other　case；　or　that　of　hybrids；the　external　conditions　have　remained　the　same；　but　the　organisation　has　been　disturbed　by　two　different　structures　and　constitutions　having　been　blended　into　one。　For　it　is　scarcely　possible　that　two　organisations　should　be　compounded　into　one；　without　some　disturbance　occurring　in　the　development；　or　periodical　action；　or　mutual　relation　of　the　different　parts　and　organs　one　to　another；　or　to　the　conditions　of　life。　When　hybrids　are　able　to　breed　inter　se；　they　transmit　to　their　offspring　from　generation　to　generation　the　same　compounded　organisation；　and　hence　we　need　not　be　surprised　that　their　sterility；　though　in　some　degree　variable；　rarely　diminishes。　
It　must；　however；　be　confessed　that　we　cannot　understand；　excepting　on　vague　hypotheses；　several　facts　with　respect　to　the　sterility　of　hybrids；　for　instance；　the　unequal　fertility　of　hybrids　produced　from　reciprocal　crosses；　or　the　increased　sterility　in　those　hybrids　which　occasionally　and　exceptionally　resemble　closely　either　pure　parent。　Nor　do　I　pretend　that　the　foregoing　remarks　go　to　the　root　of　the　matter：　no　explanation　is　offered　why　an　organism；　when　placed　under　unnatural　conditions；　is　rendered　sterile。　All　that　I　have　attempted　to　show；　is　that　in　two　cases；　in　some　respects　allied；　sterility　is　the　common　result；　in　the　one　case　from　the　conditions　of　life　having　been　disturbed；　in　the　other　case　from　the　organisation　having　been　disturbed　by　two　organisations　having　been　compounded　into　one。　
It　may　seem　fanciful；　but　I　suspect　that　a　similar　parallelism　extends　to　an　allied　yet　very　different　class　of　facts。　It　is　an　old　and　almost　universal　belief；　founded；　I　think；　on　a　considerable　body　of　evidence；　that　slight　changes　in　the　conditions　of　life　are　beneficial　to　all　living　things。　We　see　this　acted　on　by　farmers　and　gardeners　in　their　frequent　exchanges　of　seed；　tubers；　&c。；　from　one　soil　or　climate　to　another；　and　back　again。　During　the　convalescence　of　animals；　we　plainly　see　that　great　benefit　is　derived　from　almost　any　change　in　the　habits　of　life。　Again；　both　with　plants　and　animals；　there　is　abundant　evidence；　that　a　cross　between　very　distinct　individuals　of　the　same　species；　that　is　between　members　of　different　strains　or　sub…breeds；　gives　vigour　and　fertility　to　the　offspring。　I　believe；　indeed；　from　the　facts　alluded　to　in　our　fourth　chapter；　that　a　certain　amount　of　crossing　is　indispensable　even　with　hermaphrodites；　and　that　close　interbreeding　continued　during　several　generations　between　the　nearest　relations；　especially　if　these　be　kept　under　the　same　conditions　of　life；　always　induces　weakness　and　sterility　in　the　progeny。　
Hence　it　seems　that；　on　the　one　hand；　slight　changes　in　the　conditions　of　life　benefit　all　organic　beings；　and　on　the　other　hand；　that　slight　crosses；　that　is　crosses　between　the　males　and　females　of　the　same　species　which　have　varied　and　become　slightly　different；　give　vigour　and　fertility　to　the　offspring。　But　we　have　seen　that　greater　changes；　or　changes　of　a　particular　nature；　often　render　organic　beings　in　some　degree　sterile；　and　that　greater　crosses；　that　is　crosses　between　males　and　females　which　have　become　widely　or　specifically　different；　produce　hybrids　which　are　generally　sterile　in　some　degree。　I　cannot　persuade　myself　that　this　parallelism　is　an　accident　or　an　illusion。　Both　series　of　facts　seem　to　be　connected　together　by　some　common　but　unknown　bond；　which　is