the origin of species(物种起源)-第60章

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ductive　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　some　authors　on　the　supposed　fact；　that　mongrel　animals　alone　are　born　closely　like　one　of　their　parents；　but　it　can　be　shown　that　this　does　sometimes　occur　with　hybrids；　yet　I　grant　much　less　frequently　with　hybrids　than　with　mongrels。　Looking　to　the　cases　which　I　have　collected　of　cross…bred　animals　closely　resembling　one　parent；　the　resemblances　seem　chiefly　confined　to　characters　almost　monstrous　in　their　nature；　and　which　have　suddenly　appeared　such　as　albinism；　melanism；　deficiency　of　tail　or　horns；　or　additional　fingers　and　toes；　and　do　not　relate　to　characters　which　have　been　slowly　acquired　by　selection。　Consequently；　sudden　reversions　to　the　perfect　character　of　either　parent　would　be　more　likely　to　occur　with　mongrels；　which　are　descended　from　varieties　often　suddenly　produced　and　semi…monstrous　in　character；　than　with　hybrids；　which　are　descended　from　species　slowly　and　naturally　produced。　On　the　whole　I　entirely　agree　with　Dr　Prosper　Lucas；　who；　after　arranging　an　enormous　body　of　facts　with　respect　to　animals；　comes　to　the　conclusion；　that　the　laws　of　resemblance　of　the　child　to　its　parents　are　the　same；　whether　the　two　parents　differ　much　or　little　from　each　other；　namely　in　the　union　of　individuals　of　the　same　variety；　or　of　different　varieties；　or　of　distinct　species。　
Laying　aside　the　question　of　fertility　and　sterility；　in　all　other　respects　there　seems　to　be　a　general　and　close　similarity　in　the　offspring　of　crossed　species；　and　of　crossed　varieties。　If　we　look　at　species　as　having　been　specially　created；　and　at　varieties　as　having　been　produced　by　secondary　laws；　this　similarity　would　be　an　astonishing　fact。　But　it　harmonizes　perfectly　with　the　view　that　there　is　no　essential　distinction　between　species　and　varieties。　
Summary　of　Chapter。　First　crosses　between　forms　sufficiently　distinct　to　be　ranked　as　species；　and　their　hybrids；　are　very　generally；　but　not　universally；　sterile。　The　sterility　is　of　all　degrees；　and　is　often　so　slight　that　the　two　most　careful　experimentalists　who　have　ever　lived；　have　come　to　diametrically　opposite　conclusions　in　ranking　forms　by　this　test。　The　sterility　is　innately　variable　in　individuals　of　the　same　species；　and　is　eminently　susceptible　of　favourable　and　unfavourable　conditions。　The　degree　of　sterility　does　not　strictly　follow　systematic　affinity；　but　is　governed　by　several　curious　and　complex　laws。　It　is　generally　different；　and　sometimes　widely　different；　in　reciprocal　crosses　between　the　same　two　species。　It　is　not　always　equal　in　degree　in　a　first　cross　and　in　the　hybrid　produced　from　this　cross。　
In　the　same　manner　as　in　grafting　trees；　the　capacity　of　one　species　or　variety　to　take　on　another；　is　incidental　on　generally　unknown　differences　in　their　vegetative　systems；　so　in　crossing；　the　greater　or　less　facility　of　one　species　to　unite　with　another；　is　incidental　on　unknown　differences　in　their　reproductive　systems。　There　is　no　more　reason　to　think　that　species　have　been　specially　endowed　with　various　degrees　of　sterility　to　prevent　them　crossing　and　ble