the origin of species(物种起源)-第53节

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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　difference　in　the　degree　of　variability　graduates　away。　When　mongrels　and　the　more　fertile　hybrids　are　propagated　for　several　generations　an　extreme　amount　of　variability　in　their　offspring　is　notorious；　but　some　few　cases　both　of　hybrids　and　mongrels　long　retaining　uniformity　of　character　could　be　given。　The　variability；　however；　in　the　successive　generations　of　mongrels　is；　perhaps；　greater　than　in　hybrids。　
This　greater　variability　of　mongrels　than　of　hybrids　does　not　seem　to　me　at　all　surprising。　For　the　parents　of　mongrels　are　varieties；　and　mostly　domestic　varieties　（very　few　experiments　having　been　tried　on　natural　varieties）；　and　this　implies　in　most　cases　that　there　has　been　recent　variability；　and　therefore　we　might　expect　that　such　variability　would　often　continue　and　be　super…added　to　that　arising　from　the　mere　act　of　crossing。　The　slight　degree　of　variability　in　hybrids　from　the　first　cross　or　in　the　first　generation；　in　contrast　with　their　extreme　variability　in　the　succeeding　generations；　is　a　curious　fact　and　deserves　attention。　For　it　bears　on　and　corroborates　the　view　which　I　have　taken　on　the　cause　of　ordinary　variability；　namely；　that　it　is　due　to　the　reproductive　system　being　eminently　sensitive　to　any　change　in　the　conditions　of　life；　being　thus　often　rendered　either　impotent　or　at　least　incapable　of　its　proper　function　of　producing　offspring　identical　with　the　parent…form。　Now　hybrids　in　the　first　generation　are　descended　from　species　（excluding　those　long　cultivated）　which　have　not　had　their　reproductive　systems　in　any　way　affected；　and　they　are　not　variable；　but　hybrids　themselves　have　their　reproductive　systems　seriously　affected；　and　their　descendants　are　highly　variable。　
But　to　return　to　our　comparison　of　mongrels　and　hybrids：　G？rtner　states　that　mongrels　are　more　liable　than　hybrids　to　revert　to　either　parent…form；　but　this；　if　it　be　true；　is　certainly　only　a　difference　in　degree。　G？rtner　further　insists　that　when　any　two　species；　although　most　closely　allied　to　each　other；　are　crossed　with　a　third　species；　the　hybrids　are　widely　different　from　each　other；　whereas　if　two　very　distinct　varieties　of　one　species　are　crossed　with　another　species；　the　hybrids　do　not　differ　much。　But　this　conclusion；　as　far　as　I　can　make　out；　is　founded　on　a　single　experiment；　and　seems　directly　opposed　to　the　results　of　several　experiments　made　by　K？lreuter。　
These　alone　are　the　unimportant　differences；　which　G？rtner　is　able　to　point　out；　between　hybrid　and　mongrel　plants。　On　the　other　hand；　the　resemblance　in　mongrels　and　in　hybrids　to　their　respective　parents；　more　especially　in　hybrids　produced　from　nearly　related　species；　follows　according　to　G？rtner　the　same　laws。　When　two　species　are　crossed；　one　has　sometimes　a　prepotent　power　of　impressing　its　likeness　on　the　hybrid；　and　so　I　believe　it　to　be　with　varieties　of　plants。　With　animals　one　variety　certainly　often　has　this　prepotent　power　over　another　variety。　Hybrid　plants　produced　from　a　reciprocal　cross；　generally　resemble　each　other　closely；　and　so　it　is　with　mongrels　from　a　reciprocal　cross。　Both　hybrids　and　mongrels　can　be　reduced　to　either　pure　parent…form；　by　repeated　crosses　in　successive　generations　with　either　parent。　
These　several　remarks　are　apparently　applicable　to　animals；　but　the　subject　is　here　excessively　complicated；　partly　owing　to　the　existence　of　secondary　sexual　characters；　but　more　especially　owing　to　prepotency　in　transmitting　likeness　running　more　strongly　in　one　sex　than　in　the　other；　both　when　one　species　is　crossed　with　another；　and　when　one　variety　is　crossed　with　another　variety。　For　instance；　I　think　those　authors　are　right；　who　maintain　that　the　ass　has　a　prepotent　power　over　the　horse；　so　that　both　the　mule　and　the　hinny　more　resemble　the　ass　than　the　horse；　but　that　the　prepotency　runs　more　strongly　in　the　male…ass　than　in　the　female；　so　that　the　mule；　which　is　the　offspring　of　the　male…ass　and　mare；　is　more　like　an　ass；　than　is　the　hinny；　which　is　the　offspring　of　the　female…ass　and　stallion。　
Much　stress　has　been　laid　by　som