the origin of species(物种起源)-第44章

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lied　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　blending　in　nature；　than　to　think　that　trees　have　been　specially　endowed　with　various　and　somewhat　analogous　degrees　of　difficulty　in　being　grafted　together　in　order　to　prevent　them　becoming　inarched　in　our　forests。　
The　sterility　of　first　crosses　between　pure　species；　which　have　their　reproductive　systems　perfect；　seems　to　depend　on　several　circumstances；　in　some　cases　largely　on　the　early　death　of　the　embryo。　The　sterility　of　hybrids；　which　have　their　reproductive　systems　imperfect；　and　which　have　had　this　system　and　their　whole　organisation　disturbed　by　being　compounded　of　two　distinct　species；　seems　closely　allied　to　that　sterility　which　so　frequently　affects　pure　species；　when　their　natural　conditions　of　life　have　been　disturbed。　This　view　is　supported　by　a　parallelism　of　another　kind；　namely；　that　the　crossing　of　forms　only　slightly　different　is　favourable　to　the　vigour　and　fertility　of　their　offspring；　and　that　slight　changes　in　the　conditions　of　life　are　apparently　favourable　to　the　vigour　and　fertility　of　all　organic　beings。　It　is　not　surprising　that　the　degree　of　difficulty　in　uniting　two　species；　and　the　degree　of　sterility　of　their　hybrid…offspring　should　generally　correspond；　though　due　to　distinct　causes；　for　both　depend　on　the　amount　of　difference　of　some　kind　between　the　species　which　are　crossed。　Nor　is　it　surprising　that　the　facility　of　effecting　a　first　cross；　the　fertility　of　the　hybrids　produced；　and　the　capacity　of　being　grafted　together　though　this　latter　capacity　evidently　depends　on　widely　different　circumstances　should　all　run；　to　a　certain　extent；　parallel　with　the　systematic　affinity　of　the　forms　which　are　subjected　to　experiment；　for　systematic　affinity　attempts　to　express　all　kinds　of　resemblance　between　all　species。　
First　crosses　between　forms　known　to　be　varieties；　or　sufficiently　alike　to　be　considered　as　varieties；　and　their　mongrel　offspring；　are　very　generally；　but　not　quite　universally；　fertile。　Nor　is　this　nearly　general　and　perfect　fertility　surprising；　when　we　remember　how　liable　we　are　to　argue　in　a　circle　with　respect　to　varieties　in　a　state　of　nature；　and　when　we　remember　that　the　greater　number　of　varieties　have　been　produced　under　domestication　by　the　selection　of　mere　external　differences；　and　not　of　differences　in　the　reproductive　system。　In　all　other　respects；　excluding　fertility；　there　is　a　close　general　resemblance　between　hybrids　and　mongrels。　Finally；　then；　the　facts　briefly　given　in　this　chapter　do　not　seem　to　me　opposed　to；　but　even　rather　to　support　the　view；　that　there　is　no　fundamental　distinction　between　species　and　varieties。　　
Chapter　9　…　On　the　Imperfection　of　the　Geological　Record

IN　the　sixth　chapter　I　enumerated　the　chief　objections　which　might　be　justly　urged　against　the　views　maintained　in　this　volume。　Most　of　them　have　now　been　discussed。　One；　namely　the　distinctness　of　specific　forms；　and　their　not　being　blended　together　by　innumerable　transitional　links；　is　a　very　obvious　difficulty。　I　assigned　reasons　why　such　links　do　not　commonly　occur　at　the　present　day；　under　the　circumstances　apparently　most　favourable　for　their　presence；　namely　on　an　extensive　and　continuous　area　with　graduated　physical　conditions。　I　endeavoured　to　show；　that　the　life　of　each　species　depends　in　a　more　important　manner　on　the　presence　of　other　already　defined　organic　forms；　than　on　climate；　and；　therefore；　that　the　really　governing　conditions　of　life　do　not　graduate　away　quite　insensibly　like　heat　or　moisture。　I　endeavoured；　also；　to　show　that　intermediate　varieties；　from　existing　in　lesser　numbers　than　the　forms　which　they　connect；　will　generally　be　beaten　out　and　exterminated　during　the　course　of　further　modification　and　improvement。　The　main　cause；　however；　of　innumerable　intermediate　links　not　now　occurring　everywhere　throughout　nature　depends　on　the　very　process　of　natural　selection；　through　which　new　varieties　continually　take　the　places　of　and　exterminate　their　parent…forms。　But　just　in　proportion　as　this　process　of　extermination　has　acted　on　an　enormous　scale；　so　must