the origin of species(物种起源)-第43章

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；　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　essentially　related　to　the　principle　of　life。　
Fertility　of　Varieties　when　crossed；　and　of　their　Mongrel　off…spring。　It　may　be　urged；　as　a　most　forcible　argument；　that　there　must　be　some　essential　distinction　between　species　and　varieties；　and　that　there　must　be　some　error　in　all　the　foregoing　remarks；　inasmuch　as　varieties；　however　much　they　may　differ　from　each　other　in　external　appearance；　cross　with　perfect　facility；　and　yield　perfectly　fertile　offspring。　I　fully　admit　that　this　is　almost　invariably　the　case。　But　if　we　look　to　varieties　produced　under　nature；　we　are　immediately　involved　in　hopeless　difficulties；　for　if　two　hitherto　reputed　varieties　be　found　in　any　degree　sterile　together；　they　are　at　once　ranked　by　most　naturalists　as　species。　For　instance；　the　blue　and　red　pimpernel；　the　primrose　and　cowslip；　which　are　considered　by　many　of　our　best　botanists　as　varieties；　are　said　by　G？rtner　not　to　be　quite　fertile　when　crossed；　and　he　consequently　ranks　them　as　undoubted　species。　If　we　thus　argue　in　a　circle；　the　fertility　of　all　varieties　produced　under　nature　will　assuredly　have　to　be　granted。　
If　we　turn　to　varieties；　produced；　or　supposed　to　have　been　produced；　under　domestication；　we　are　still　involved　in　doubt。　For　when　it　is　stated；　for　instance；　that　the　German　Spitz　dog　unites　more　easily　than　other　dogs　with　foxes；　or　that　certain　South　American　indigenous　domestic　dogs　do　not　readily　cross　with　European　dogs；　the　explanation　which　will　occur　to　everyone；　and　probably　the　true　one；　is　that　these　dogs　have　descended　from　several　aboriginally　distinct　species。　Nevertheless　the　perfect　fertility　of　so　many　domestic　varieties；　differing　widely　from　each　other　in　appearance；　for　instance　of　the　pigeon　or　of　the　cabbage；　is　a　remarkable　fact；　more　especially　when　we　reflect　how　many　species　there　are；　which；　though　resembling　each　other　most　closely；　are　utterly　sterile　when　intercrossed。　Several　considerations；　however；　render　the　fertility　of　domestic　varieties　less　remarkable　than　at　first　appears。　It　can；　in　the　first　place；　be　clearly　shown　that　mere　external　dissimilarity　between　two　species　does　not　determine　their　greater　or　lesser　degree　of　sterility　when　crossed；　and　we　may　apply　the　same　rule　to　domestic　varieties。　In　the　second　place；　some　eminent　naturalists　believe　that　a　long　course　of　domestication　tends　to　eliminate　sterility　in　the　successive　generations　of　hybrids；　which　were　at　first　only　slightly　sterile；　and　if　this　be　so；　we　surely　ought　not　to　expect　to　find　sterility　both　appearing　and　disappearing　under　nearly　the　same　conditions　of　life。　Lastly；　and　this　seems　to　me　by　far　the　most　important　consideration；　new　races　of　animals　and　plants　are　produced　under　domestication　by　man's　methodical　and　unconscious　power　of　selection；　for　his　own　use　and　pleasure：　he　neither　wishes　to　select；　nor　could　select；　slight　differences　in　the　reproductive　system；　or　other　constitutional　difference　correlated　with　the　reproductive　system。　He　supplies　his　several　varieties　with　the　same　food；　treats　them　in　nearly　the　same　manner；　and　does　not　wish　to　alter　their　general　habits　of　life。　Nature　acts　uniformly　and　slowly　during　vast　periods　of　time　on　the　whole　organization；　in　any　way　which　may　be　for　each　creature's　own　good；　and　thus　she　may；　either　directly；　or　more　probably　indirectly；　through　correlation；　modify　the　reproductive　system　in　the　several　descendants　from　any　one　species。　Seeing　this　difference　in　the　process　of　selection；　as　carried　on　by　man　and　nature；　we　need　not　be　surprised　at　some　difference　in　the　result。　
I　have　as　yet　spoken　as　if　the　varieties　of　the　same　species　were　invariably　fertile　when　intercrossed。　But　it　seems　to　me　impossible　to　resist　the　evidence　of　the　existence　of　a　certain　amount　of　sterility　in　the　few　following　cases；　which　I　will　briefly　abstract。　The　evidence　is　at　least　as　good　as　that　from　which　we　believe　in　the　sterility　of　a　multitude　of　species。　The　evidence　is；　also；　derived　from　hostile　witnesses；　who　in　all　other　cases　consider　fertility　and　sterility　as　safe　criterions　of　specific　distinction。　G？rtner　kept　during　several　years　a　dwarf　kind　of　maize　with　yellow　seeds；　and　a　tall　variety　with　red　seeds；　growing　near　each　other　in　his　garden；　and　although　these　plants　have　separated　sexes；　they　never　naturally　crossed。　He　then　fertilized　thirteen　flowers　of　the　one　with　the　pollen　of　the　other；　but　only　a　single　head　produced　any　seed；　and　this　one　head　produced　only　five　grains。　Manipulation　in　this　case　could　not　have　been　injurious；　as　the　plants　have　separated　sexes。　No　one；　I　believe；　has　suspected　that　these　varieties　of　maize　are　distinct　species；　and　it　is　important　to　notice　that　the　hybrid　plants　thus　raised　were　themselves　perfectly　fertile；　so　that　even　G？rtner　did　not　venture　to　consider　the　two　varieties　as　specifically　distinct。　
Girou　de　Buzareingues　crossed　three　varieties　of　gourd；　which　like　the　maize　has　separated　sexes；　and　he　asserts　that　their　mutual　fertilization　is　by　so　much　the　less　easy　as　their　differences　are　greater。　How　far　these　experiments　may　be　trusted；　I　know　not；　but　the　forms　experimentised　on；　are　ranked　by　Sagaret；　who　mainly　founds　his　classification　by　the　test　of　infertility；　as　varieties。　
The　following　case　is　far　more　remarkable；　and　seems　at　first　quite　incredible；　but　it　is　the　result　of　an　astonishing　number　of　experiments　made　during　many　years　on　nine　species　of　Verbascum；　by　so　good　an　observer　and　so　hostile　a　witness；　as　G？rtner：　namely；　that　yellow　and　white　varieties　of　the　same　species　of　Verbascum　when　intercrossed　produce　less　seed；　than　do　either　coloured　varieties　when　fertilized　with　pollen　from　their　own　coloured　flowers。　Moreover；　he　asserts　that　when　yellow　and　white　varieties　of　one　species　are　crossed　with　yellow　and　white　varieties　of　a　distinct　species；　more　seed　is　produced　by　the　crosses　between　the　same　coloured　flowers；　than　between　those　which　are　differently　coloured。　Yet　these　varieties　of　Verbascum　present　no　other　difference　besides　the　mere　colour　of　the　flower；　and　one　variety　can　sometimes　be　raised　from　the　seed　of　the　other。　
From　observations　which　I　have　made　on　certain　varieties　of　hollyhock；　I　am　inclined　to　suspect　that　they　present　analogous　facts。　
K？lreuter；　whose　accuracy　has　been　confirmed　by　every　subsequent　observer；　has　proved　the　remarkable　fact；　that　one　variety　of　the　common　tobacco　is　more　fertile；　when　crossed　with　a　widely　distinct　species；　than　are　the　other　varieties。　He　experimentised　on　five　forms；　which　are　commonly　reputed　to　be　varieties；　and　which　he　tested　by　the　severest　trial；　namely；　by　reciprocal　crosses；　and　he　found　their　mongrel　offspring　perfectly　fertile。　But　one　of　these　five　varieties；　when　used　either　as　father　or　mother；　and　crossed　with　the　Nicotiana　glutinosa；　always　yielded　hybrids　not　so　sterile　as　those　which　were　produced　from　the　four　other　varieties　when　crossed　with　N。　glutinosa。　Hence　the　reproductive　system　of　this　one　variety　must　have　been　in　some　manner　and　in　some　degree　modified。　
From　these　facts；　from　the　great　difficulty　of　ascertaining　the　infertility　of　varieties　in　a　state　of　nature；　for　a　supposed　variety　if　infertile　in　any　degree　would　generally　be　ranked　as　species；　from　man　selecting　only　external　characters　in　the　production　of　the　most　distinct　domestic　varieties；　and　from　not　wishing　or　being　able　to　produce　recondite　and　functional　differences　in　the　reproductive　system；　from　these　several　considerations　and　facts；　I　do　not　think　that　the　very　general　fertility　of　varieties　can　be　proved　to　be　of　universal　occurrence；　or　to　form　a　fundamental　distinction　between　varieties　and　species。　The　general　fertility　of　varieties　does　not　seem　to　me　sufficient　to　overthrow　the　view　which　I　have　taken　with　respect　to　the　very　general；　but　not　invariable；　sterility　of　first　crosses　and　of　hybrids；　namely；　that　it　is　not　a　special　endowment；　but　is　incidental　on　slowly　acquired　modifications；　more　especially　in　the　reproductive　systems　of　the　forms　which　are　crossed。　
Hybrids　and　Mongrels　compared；　independently　of　their　fertility。　Independently　of　the　question　of　fertility；　the　offspring　of　species　when　crossed　and　of　varieties　when　crossed　may　be　compared　in　several　other　respects。　G？rtner；　whose　strong　wish　was　to　draw　a　marked　line　of　distinction　between　species　and　varieties；　could　find　very　few　and；　as　it　seems　to　me；　quite　unimportant　differences　between　the　so…called　hybrid　offspring　of　species；　and　the　so…called　mongrel　offspring　of　varieties。　And；　on　the　other　hand；　they　agree　most　closely　in　very　many　important　respects。　
I　shall　here　discuss　this　subject　with　extreme　brevity。　The　most　important　distinction　is；　that　in　the　first　generation　mongrels　are　more　variable　than　hybrids；　but　G？rtner　admits　that　hybrids　from　species　which　have　long　been　cultivated　are　often　variable　in　the　first　generation；　and　I　have　myself　seen　striking　instances　of　this　fact。　G？rtner　further　admits　that　hybrids　between　very　closely　allied　species　are　more　variable　than　those　from　very　distinct　species；　and　this　shows　that　the　differ