the origin of species(物种起源)-第26章

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ut　in　which　both　parents　have　lost　some　character　which　their　progenitor　possessed；　the　tendency；　whether　strong　or　weak；　to　reproduce　the　lost　character　might　be；　as　was　formerly　remarked；　for　all　that　we　can　see　to　the　contrary；　transmitted　for　almost　any　number　of　generations。　When　a　character　which　has　been　lost　in　a　breed；　reappears　after　a　great　number　of　generations；　the　most　probable　hypothesis　is；　not　that　the　offspring　suddenly　takes　after　an　ancestor　some　hundred　generations　distant；　but　that　in　each　successive　generation　there　has　been　a　tendency　to　reproduce　the　character　in　question；　which　at　last；　under　unknown　favourable　conditions；　gains　an　ascendancy。　For　instance；　it　is　probable　that　in　each　generation　of　the　barb…pigeon；　which　produces　most　rarely　a　blue　and　black…barred　bird；　there　has　been　a　tendency　in　each　generation　in　the　plumage　to　assume　this　colour。　This　view　is　hypothetical；　but　could　be　supported　by　some　facts；　and　I　can　see　no　more　abstract　improbability　in　a　tendency　to　produce　any　character　being　inherited　for　an　endless　number　of　generations；　than　in　quite　useless　or　rudimentary　organs　being；　as　we　all　know　them　to　be；　thus　inherited。　Indeed；　we　may　sometimes　observe　a　mere　tendency　to　produce　a　rudiment　inherited：　for　instance；　in　the　common　snapdragon　（Antirrhinum）　a　rudiment　of　a　fifth　stamen　so　often　appears；　that　this　plant　must　have　an　inherited　tendency　to　produce　it。　
As　all　the　species　of　the　same　genus　are　supposed；　on　my　theory；　to　have　descended　from　a　common　parent；　it　might　be　expected　that　they　would　occasionally　vary　in　an　analogous　manner；　so　that　a　variety　of　one　species　would　resemble　in　some　of　its　characters　another　species；　this　other　species　being　on　my　view　only　a　well…marked　and　permanent　variety。　But　characters　thus　gained　would　probably　be　of　an　unimportant　nature；　for　the　presence　of　all　important　characters　will　be　governed　by　natural　selection；　in　accordance　with　the　diverse　habits　of　the　species；　and　will　not　be　left　to　the　mutual　action　of　the　conditions　of　life　and　of　a　similar　inherited　constitution。　It　might　further　be　expected　that　the　species　of　the　same　genus　would　occasionally　exhibit　reversions　to　lost　ancestral　characters。　As；　however；　we　never　know　the　exact　character　of　the　common　ancestor　of　a　group；　we　could　not　distinguish　these　two　cases：　if；　for　instance；　we　did　not　know　that　the　rock…pigeon　was　not　feather…footed　or　turn…crowned；　we　could　not　have　told；　whether　these　characters　in　our　domestic　breeds　were　reversions　or　only　analogous　variations；　but　we　might　have　inferred　that　the　blueness　was　a　case　of　reversion；　from　the　number　of　the　markings；　which　are　correlated　with　the　blue　tint；　and　which　it　does　not　appear　probable　would　all　appear　together　from　simple　variation。　More　especially　we　might　have　inferred　this；　from　the　blue　colour　and　marks　so　often　appearing　when　distinct　breeds　of　diverse　colours　are　crossed。　Hence；　though　under　nature　it　must　generally　be　left　doubtful；　what　cases　are　reversions　to　an　anciently　existing　character；　and　what　are　new　but　analogous　variations；　yet　we　ought；　on　my　theory；　sometimes　to　find　the　varying　offspring　of　a　species　assuming　characters　（either　from　reversion　or　from　analogous　variation）　which　already　occur　in　some　members　of　the　same　group。　And　this　undoubtedly　is　the　case　in　nature。　
A　considerable　part　of　the　difficulty　in　recognising　a　variable　species　in　our　systematic　works；　is　due　to　its　varieties　mocking；　as　it　were；　come　of　the　other　species　of　the　same　genus。　A　considerable　catalogue；　also；　could　be　given　of　forms　intermediate　between　two　other　forms；　which　themselves　must　be　doubtfully　ranked　as　either　varieties　or　species；　that　the　one　in　varying　has　assumed　some　of　the　characters　of　the　other；　so　as　to　produce　the　intermediate　form。　But　the　best　evidence　is　afforded　by　parts　or　organs　of　an　important　and　uniform　nature　occasionally　varying　so　as　to　acquire；　in　some　degree；　the　character　of　the　same　part　or　organ　in　an　allied　species。　I　have　collected　a　long　list　of　such　cases；　but　here；　as　before；　I　lie　under　a　great　disadvantage　in　not　being　able　to　give　them。　I　can　only　repeat　that　such　cases　certainly　do　occur；　and　seem　to　me　very　remarkable。　
I　will；　however；　give　one　curious　and　complex　case；　not　indeed　as　affecting　any　important　character；　but　from　occurring　in　several　species　of　the　same　genus；　partly　under　domestication　and　partly　under　nature。　It　is　a　case　apparently　of　reversion。　The　ass　not　rarely　has　very　distinct　transverse　bars　on　its　legs；　like　those　of　a　zebra：　it　has　been　asserted　that　these　are　plainest　in　the　foal；　and　from　inquiries　which　I　have　made；　I　believe　this　to　be　true。　It　has　also　been　asserted　that　the　stripe　on　each　shoulder　is　sometimes　double。　The　shoulder…stripe　is　certainly　very　variable　in　length　and　outline。　A　white　ass；　but　not　an　albino；　has　been　described　without　either　spinal　or　shoulder…stripe；　and　these　stripes　are　sometimes　very　obscure；　or　actually　quite　lost；　in　dark…coloured　asses。　The　koulan　of　Pallas　is　said　to　have　been　seen　with　a　double　shoulder…stripe；　but　traces　of　it；　as　stated　by　Mr　Blyth　and　others；　occasionally　appear：　and　I　have　been　informed　by　Colonel　Poole　that　foals　of　this　species　are　generally　striped　on　the　legs；　and　faintly　on　the　shoulder。　The　quagga；　though　so　plainly　barred　like　a　zebra　over　the　body；　is　without　bars　on　the　legs；　but　Dr　Gray　has　figured　one　specimen　with　very　distinct　zebra…like　bars　on　the　hocks。　
With　respect　to　the　horse；　I　have　collected　cases　in　England　of　the　spinal　stripe　in　horses　of　the　most　distinct　breeds；　and　of　all　colours；　transverse　bars　on　the　legs　are　not　rare　in　duns；　mouse…duns；　and　in　one　instance　in　a　chestnut：　a　faint　shoulder…stripe　may　sometimes　be　seen　in　duns；　and　I　have　seen　a　trace　in　a　bay　horse。　My　son　made　a　careful　examination　and　sketch　for　me　of　a　dun　Belgian　cart…horse　with　a　double　stripe　on　each　shoulder　and　with　leg…stripes；　and　a　man；　whom　I　can　implicitly　trust；　has　examined　for　me　a　small　dun　Welch　pony　with　three　short　parallel　stripes　on　each　shoulder。　
In　the　north…west　part　of　India　the　Kattywar　breed　of　horses　is　so　generally　striped；　that；　as　I　hear　from　Colonel　Poole；　who　examined　the　breed　for　the　Indian　Government；　a　horse　without　stripes　is　not　considered　as　purely…bred。　The　spine　is　always　striped；　the　legs　are　generally　barred；　and　the　shoulder…stripe；　which　is　sometimes　double　and　sometimes　treble；　is　common；　the　side　of　the　face；　moreover；　is　sometimes　striped。　The　stripes　are　plainest　in　the　foal；　and　sometimes　quite　disappear　in　old　horses。　Colonel　Poole　has　seen　both　gray　and　bay　Kattywar　horses　striped　when　first　foaled。　I　have；　also；　reason　to　suspect；　from　information　given　me　by　Mr。　W。　W。　Edwards；　that　with　the　English　race…horse　the　spinal　stripe　is　much　commoner　in　the　foal　than　in　the　full…grown　animal。　Without　here　entering　on　further　details；　I　may　state　that　I　have　collected　cases　of　leg　and　shoulder　stripes　in　horses　of　very　different　breeds；　in　various　countries　from　Britain　to　Eastern　China；　and　from　Norway　in　the　north　to　the　Malay　Archipelago　in　the　south。　In　all　parts　of　the　world　these　stripes　occur　far　oftenest　in　duns　and　mouse…duns；　by　the　term　dun　a　large　range　of　colour　is　included；　from　one　between　brown　and　black　to　a　close　approach　to　cream…colour。　
I　am　aware　that　Colonel　Hamilton　Smith；　who　has　written　on　this　subject；　believes　that　the　several　breeds　of　the　horse　have　descended　from　several　aboriginal　species　one　of　which；　the　dun；　was　striped；　and　that　the　above…described　appearances　are　all　due　to　ancient　crosses　with　the　dun　stock。　But　I　am　not　at　all　satisfied　with　this　theory；　and　should　be　loth　to　apply　it　to　breeds　so　distinct　as　the　heavy　Belgian　cart…horse；　Welch　ponies；　cobs；　the　lanky　Kattywar　race；　&c。；　inhabiting　the　most　distant　parts　of　the　world。　
Now　let　us　turn　to　the　effects　of　crossing　the　several　species　of　the　horse…genus。　Rollin　asserts；　that　the　common　mule　from　the　ass　and　horse　is　particularly　apt　to　have　bars　on　its　legs。　I　once　saw　a　mule　with　its　legs　so　much　striped　that　any　one　at　first　would　have　thought　that　it　must　have　been　the　product　of　a　zebra；　and　Mr。　W。　C。　Martin；　in　his　excellent　treatise　on　the　horse；　has　given　a　figure　of　a　similar　mule。　In　four　coloured　drawings；　which　I　have　seen；　of　hybrids　between　the　ass　and　zebra；　the　legs　were　much　more　plainly　barred　than　the　rest　of　the　body；　and　in　one　of　them　there　was　a　double　shoulder…stripe。　In　Lord　Moreton's　famous　hybrid　from　a　chestnut　mare　and　male　quagga；　the　hybrid；　and　even　the　pure　offspring　subsequently　produced　from　the　mare　by　a　black　Arabian　sire；　were　much　more　plainly　barred　across　the　legs　than　is　even　the　pure　quagga。　Lastly；　and　this　is　another　most　remarkable　case；　a　hybrid　has　been　figured　by　Dr　Gray　（and　he　informs　me　that　he　knows　of　a　second　case）　from　the　ass　and　the　hemionus；　and　this　hybrid；　though　the　ass　seldom　has　stripes　on　its　legs　and　the　hemionus　has　none　and　has　not　even　a　shoulder…stripe；　nevertheless　had　all　four　legs　barred；　and　had　three　short　shoulder…stripes；　like　those　on　the　dun　Welch　pony；　and　even　had　some　zebra…like　stripes　on　the　sides　of　its　face。　With　respect　to　this　last　fact；　I　was　so　convinced　that　not　even　a　stripe　of　colour　appears　from　what　would　commonly　be　called　an　accident；　that　I　was　led　solely　from　the　occurrence　of　the　face…stripes　on　this　hybrid　from　the　ass　and　hemionus；　to　ask　Colonel　Poole　whether　such　face…stripes　ever　occur　in　the　eminently　striped　Kattywar　breed　of　horses；　and　was；　as　we　have　seen；　answered　in　the　affirmative。　
What　now　are　we　to　say　to　these　several　facts？　We　see　several　very　distinct　species　of　the　horse…genus　becoming；　by　simple　variation；　striped　on　the　legs　like　a　zebra；　or　striped　on　the　shoulders　like　an　ass。　In　the　horse　we　see　this　tendency　strong　whenever　a　dun　tint　appears　a　tint　which　approaches　to　that　of　the　general　colouring　of　the　other　species　of　the　genus。　The　appearance　of　the　stripes　is　not　accompanied　by　any　change　of　form　or　by　any　other　new　character。　We　see　this　tendency　to　become　striped　most　strongly　displayed　in　hybrids　from　between　several　of　the　most　distinct　species。　Now　observe　the　case　of　the　several　breeds　of　pigeons：　they　are　descended　from　a　pigeon　（including　two　or　three　sub…species　or　geographical　races）　of　a　bluish　colour；　with　certain　bars　and　other　marks；　and　when　any　breed　assumes　by　simple　variation　a　bluish　tint；　these　bars　and　other　marks　invariably　reappear；　but　without　any　other　change　of　form　or　character。　When　the　oldest　and　truest　breeds　of　various　colours　are　crossed；　we　see　a　strong　tendency　for　the　blue　tint　and　bars　and　marks　to　reappear　in　the　mongrels。　I　have　stated　that　the　most　probable　hypothesis　to　account　for　the　reappearance　of　very　ancient　characters；　is　that　there　is　a　tendency　in　the　young　of　each　successive　generation　to　produce　the　long…lost　character；　and　that　this　tendency；　from　unknown　causes；　sometimes　prevails。　And　we　have　just　seen　that　in　several　species　of　the　horse…genus　the　stripes　are　either　plainer　or　appear　more　commonly　in　the　young　than　in　the　old。　Call　the　breeds　of　pigeons；　some　of　which　have　bred　true　for　centuries；　species；　and　how　exactly　parallel　is　the　case　with　that　of　t