the origin of species(物种起源)-第40章

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　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　variety　increases　fertility；　that　I　cannot　doubt　the　correctness　of　this　almost　universal　belief　amongst　breeders。　Hybrids　are　seldom　raised　by　experimentalists　in　great　numbers；　and　as　the　parent…species；　or　other　allied　hybrids；　generally　grow　in　the　same　garden；　the　visits　of　insects　must　be　carefully　prevented　during　the　flowering　season：　hence　hybrids　will　generally　be　fertilised　during　each　generation　by　their　own　individual　pollen；　and　I　am　convinced　that　this　would　be　injurious　to　their　fertility；　already　lessened　by　their　hybrid　origin。　I　am　strengthened　in　this　conviction　by　a　remarkable　statement　repeatedly　made　by　G？rtner；　namely；　that　if　even　the　less　fertile　hybrids　be　artificially　fertilised　with　hybrid　pollen　of　the　same　kind；　their　fertility；　notwithstanding　the　frequent　ill　effects　of　manipulation；　sometimes　decidedly　increases；　and　goes　on　increasing。　Now；　in　artificial　fertilisation　pollen　is　as　often　taken　by　chance　（as　I　know　from　my　own　experience）　from　the　anthers　of　another　flower；　as　from　the　anthers　of　the　flower　itself　which　is　to　be　fertilised；　so　that　a　cross　between　two　flowers；　though　probably　on　the　same　plant；　would　be　thus　effected。　Moreover；　whenever　complicated　experiments　are　in　progress；　so　careful　an　observer　as　G？rtner　would　have　castrated　his　hybrids；　and　this　would　have　insured　in　each　generation　a　cross　with　the　pollen　from　a　distinct　flower；　either　from　the　same　plant　or　from　another　plant　of　the　same　hybrid　nature。　And　thus；　the　strange　fact　of　the　increase　of　fertility　in　the　successive　generations　of　artificially　fertilised　hybrids　may；　I　believe；　be　accounted　for　by　close　interbreeding　having　been　avoided。　
Now　let　us　turn　to　the　results　arrived　at　by　the　third　most　experienced　hybridiser；　namely；　the　Hon。　and　Rev。　W。　Herbert。　He　is　as　emphatic　in　his　conclusion　that　some　hybrids　are　perfectly　fertile　as　fertile　as　the　pure　parent…species　as　are　K？lreuter　and　G？rtner　that　some　degree　of　sterility　between　distinct　species　is　a　universal　law　of　nature。　He　experimentised　on　some　of　the　very　same　species　as　did　G？rtner。　The　difference　in　their　results　may；　I　think；　be　in　part　accounted　for　by　Herbert's　great　horticultural　skill；　and　by　his　having　hothouses　at　his　command。　Of　his　many　important　statements　I　will　here　give　only　a　single　one　as　an　example；　namely；　that　'every　ovule　in　a　pod　of　Crinum　capense　fertilised　by　C。　revolutum　produced　a　plant；　which　（he　says）　I　never　saw　to　occur　in　a　case　of　its　natural　fecundation。'　So　that　we　here　have　perfect；　or　even　more　than　commonly　perfect；　fertility　in　a　first　cross　between　two　distinct　species。　
This　case　of　the　Crinum　leads　me　to　refer　to　a　most　singular　fact；　namely；　that　there　are　individual　plants；　as　with　certain　species　of　Lobelia；　and　with　all　the　species　of　the　genus　Hippeastrum；　which　can　be　far　more　easily　fertilised　by　the　pollen　of　another　and　distinct　species；　than　by　their　own　pollen。　For　these　plants　have　been　found　to　yield　seed　to　the　pollen　of　a　distinct　species；　though　quite　sterile　with　their　own　pollen；　notwithstanding　that　their　own　pollen　was　found　to　be　perfectly　good；　for　it　fertilised　distinct　species。　So　that　certain　individual　plants　and　all　the　individuals　of　certain　species　can　actually　be　hybridised　much　more　readily　than　they　can　be　self…fertilised！　For　instance；　a　bulb　of　Hippeastrum　aulicum　produced　four　flowers；　three　were　fertilised　by　Herbert　with　their　own　pollen；　and　the　fourth　was　subsequently　fertilised　by　the　pollen　of　a　compound　hybrid　descended　from　three　other　and　distinct　species：　the　result　was　that　'the　ovaries　of　the　three　first　flowers　soon　ceased　to　grow；　and　after　a　few　days　perished　entirely；　whereas　the　pod　impregnated　by　the　pollen　of　the　hybrid　made　vigorous　growth　and　rapid　progress　to　maturity；　and　bore　good　seed；　which　vegetated　freely。'　In　a　letter　to　me；　in　1839；　Mr　Herbert　told　me　that　he　had　then　tried　the　experiment　during　five　years；　and　he　continued　to　try　it　during　several　subsequent　years；　and　always　with　the　same　result。　This　result　has；　also；　been　confirmed　by　other　observers　in　the　case　of　Hippeastrum　with　its　sub…genera；　and　in　the　case　of　some　other　genera；　as　Lobelia；　Passiflora　and　Verbascum。　Although　the　plants　in　these　experiments　appeared　perfectly　healthy；　and　although　both　the　ovules　and　pollen　of　the　same　flower　were　perfectly　good　with　respect　to　other　species；　yet　as　they　were　functionally　imperfect　in　their　mutual　self…action；　we　must　infer　that　the　plants　were　in　an　unnatural　state。　Nevertheless　these　facts　show　on　what　slight　and　mysterious　causes　the　lesser　or　greater　fertility　of　species　when　crossed；　in　comparison　with　the　same　species　when　self…fertilised；　sometimes　depends。　
The　practical　experiments　of　horticulturists；　though　not　made　with　scientific　precision；　deserve　some　notice。　It　is　notorious　in　how　complicated　a　manner　the　species　of　Pelargonium；　Fuchsia；　Calceolaria；　Petunia；　Rhododendron；　&c。；　have　been　crossed；　yet　many　of　these　hybrids　seed　freely。　For　instance；　Herbert　asserts　that　a　hybrid　from　Calceolaria　integrifolia　and　plantaginea；　species　most　widely　dissimilar　in　general　habit；　'reproduced　itself　as　perfectly　as　if　it　had　been　a　natural　species　from　the　mountains　of　Chile。'　I　have　taken　some　pains　to　ascertain　the　degree　of　fertility　of　some　of　the　complex　crosses　of　Rhododendrons；　and　I　am　assured　that　many　of　them　are　perfectly　fertile。　Mr　C。　Noble；　for　instance；　informs　me　that　he　raises　stocks　for　grafting　from　a　hybrid　between　Rhod。　Ponticum　and　Catawbiense；　and　that　this　hybrid　'seeds　as　freely　as　it　is　possible　to　imagine。'　Had　hybrids；　when　fairly　treated；　gone　on　decreasing　in　fertility　in　each　successive　generation；　as　G？rtner　believes　to　be　the　case；　the　fact　would　have　been　notorious　to　nurserymen。　Horticulturists　raise　large　beds　of　the　same　hybrids；　and　such　alone　are　fairly　treated；　for　by　insect　agency　the　several　individuals　of　the　same　hybrid　variety　are　allowed　to　freely　cross　with　each　other；　and　the　injurious　influence　of　close　interbreeding　is　thus　prevented。　Any　one　may　readily　convince　himself　of　the　efficiency　of　insect…agency　by　examining　the　flowers　of　the　more　sterile　kinds　of　hybrid　rhododendrons；　which　produce　no　pollen；　for　he　will　find　on　their　stigmas　plenty　of　pollen　brought　from　other　flowers。　
In　regard　to　animals；　much　fewer　experiments　have　been　carefully　tried　than　with　plants。　If　our　systematic　arrangements　can　be　trusted；　that　is　if　the　genera　of　animals　are　as　distinct　from　each　other；　as　are　the　genera　of　plants；　then　we　may　infer　that　animals　more　widely　separated　in　the　scale　of　nature　can　be　more　easily　crossed　than　in　the　case　of　plants；　but　the　hybrids　themselves　are；　I　think；　more　sterile。　I　doubt　whether　any　case　of　a　perfectly　fertile　hybrid　animal　can　be　considered　as　thoroughly　well　authenticated。　It　should；　however；　be　borne　in　mind　that；　owing　to　few　animals　breeding　freely　under　confinement；　few　experiments　have　been　fairly　tried：　for　instance；　the　canary…bird　has　been　crossed　with　nine　other　finches；　but　as　not　one　of　these　nine　species　breeds　freely　in　confinement；　we　have　no　right　to　expect　that　the　first　crosses　between　them　and　the　canary；　or　that　their　hybrids；　should　be　perfectly　fertile。　Again；　with　respect　to　the　fertility　in　successive　generations　of　the　more　fertile　hybrid　animals；　I　hardly　know　of　an　instance　in　which　two　families　of　the　same　hybrid　have　been　raised　at　the　same　time　from　different　parents；　so　as　to　avoid　the　ill　effects　of　close　interbreeding。　On　the　contrary；　brothers　and　sisters　have　usually　been　crossed　in　each　successive　generation；　in　opposition　to　the　constantly　repeated　admonition　of　every　breeder。　And　in　this　case；　it　is　not　at　all　surprising　that　the　inherent　sterility　in　the　hybrids　should　have　gone　on　increasing。　If　we　were　to　act　thus；　and　pair　brothers　and　sisters　in　the　case　of　any　pure　animal；　which　from　any　cause　had　the　least　tendency　to　sterility；　the　breed　would　assuredly　be　lost　in　a　very　few　generations。　
Although　I　do　not　know　of　any　thoroughly　well…authenticated　cases　of　perfectly　fertile　hybrid　animals；　I　have　some　reason　to　believe　that　the　hybrids　from　Cervulus　vaginalis　and　Reevesii；　and　from　Phasianus　colchicus　with　p。　torquatus　and　with　p。　versicolor　are　perfectly　fertile。　The　hybrids　from　the　common　and　Chinese　geese　（A。　cygnoides）；　species　which　are　so　different　that　they　are　generally　ranked　in　distinct　genera；　have　often　bred　in　this　country　with　either　pure　parent；　and　in　one　single　instance　they　have　bred　inter　se。　This　was　effected　by　Mr　Eyton；　who　raised　two　hybrids　from　the　same　parents　but　from　different　hatches；　and　from　these　two　birds　he　raised　no　less　than　eight　hybrids　（grandchildren　of　the　pure　geese）　from　one　nest。　In　India；　however；　these　cross…bred　geese　must　be　far　more　fertile；　for　I　am　assured　by　two　eminently　capable　judges；　namely　Mr　Blyth　and　Capt。　Hutton；　that　whole　flocks　of　these　crossed　geese　are　kept　in　various　parts　of　the　country；　and　as　they　are　kept　for　profit；　where　neither　pure　parent…species　exists；　they　must　certainly　be　highly　fertile。　
A　doctrine　which　originated　with　Pallas；　has　been　largely　accepted　by　modern　naturalists；　namely；　that　most　of　our　domestic　animals　have　descended　from　two　or　more　aboriginal　species；　since　commingled　by　intercrossing。　On　this　view；　the　aboriginal　species　must　either　at　first　have　produced　quite　fertile　hybrids；　or　the　hybrids　must　have　become　in　subsequent　generations　quite　fertile　under　domestication。　This　latter　alternative　seems　to　me　the　most　probable；　and　I　am　inclined　to　believe　in　its　truth；　although　its　rests　on　no　direct　evidence。　I　believe；　for　instance；　that　our　dogs　have　descended　from　several　wild　stocks；　yet；　with　perhaps　the　exception　of　certain　indigenous　domestic　dogs　of　South　America；　all　are　quite　fertile　together；　and　analogy　makes　me　greatly　doubt；　whether　the　several　aboriginal　species　would　at　first　have　freely　bred　together　and　have　produced　quite　fertile　hybrids。　So　again　there　is　reason　to　believe　that　our　European　and　the　humped　Indian　cattle　are　quite　fertile　together；　but　from　facts　communicated　to　me　by　Mr　Blyth；　I　think　they　mu