the origin of species(物种起源)-第54节

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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　the　number　of　intermediate　varieties；　which　have　formerly　existed　on　the　earth；　be　truly　enormous。　Why　then　is　not　every　geological　formation　and　every　stratum　full　of　such　intermediate　links？　Geology　assuredly　does　not　reveal　any　such　finely　graduated　organic　chain；　and　this；　perhaps；　is　the　most　obvious　and　gravest　objection　which　can　be　urged　against　my　theory。　The　explanation　lies；　as　I　believe；　in　the　extreme　imperfection　of　the　geological　record。　
In　the　first　place　it　should　always　be　borne　in　mind　what　sort　of　intermediate　forms　must；　on　my　theory；　have　formerly　existed。　I　have　found　it　difficult；　when　looking　at　any　two　species；　to　avoid　picturing　to　myself；　forms　directly　intermediate　between　them。　But　this　is　a　wholly　false　view；　we　should　always　look　for　forms　intermediate　between　each　species　and　a　common　but　unknown　progenitor；　and　the　progenitor　will　generally　have　differed　in　some　respects　from　all　its　modified　descendants。　To　give　a　simple　illustration：　the　fantail　and　pouter　pigeons　have　both　descended　from　the　rock…pigeon；　if　we　possessed　all　the　intermediate　varieties　which　have　ever　existed；　we　should　have　an　extremely　close　series　between　both　and　the　rock…pigeon；　but　we　should　have　no　varieties　directly　intermediate　between　the　fantail　and　pouter；　none；　for　instance；　combining　a　tail　somewhat　expanded　with　a　crop　somewhat　enlarged；　the　characteristic　features　of　these　two　breeds。　These　two　breeds；　moreover；　have　become　so　much　modified；　that　if　we　had　no　historical　or　indirect　evidence　regarding　their　origin；　it　would　not　have　been　possible　to　have　determined　from　a　mere　comparison　of　their　structure　with　that　of　the　rock…pigeon；　whether　they　had　descended　from　this　species　or　from　some　other　allied　species；　such　as　C。　oenas。　
So　with　natural　species；　if　we　look　to　forms　very　distinct；　for　instance　to　the　horse　and　tapir；　we　have　no　reason　to　suppose　that　links　ever　existed　directly　intermediate　between　them；　but　between　each　and　an　unknown　common　parent。　The　common　parent　will　have　had　i