the origin of species(物种起源)-第52部分

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


als　can　flourish；　for　we　know　what　vast　geographical　changes　occurred　in　other　parts　of　America　during　this　space　of　time。　When　such　beds　as　were　deposited　in　shallow　water　near　the　mouth　of　the　Mississippi　during　some　part　of　the　glacial　period　shall　have　been　upraised；　organic　remains　will　probably　first　appear　and　disappear　at　different　levels；　owing　to　the　migration　of　species　and　to　geographical　changes。　And　in　the　distant　future；　a　geologist　examining　these　beds；　might　be　tempted　to　conclude　that　the　average　duration　of　life　of　the　embedded　fossils　had　been　less　than　that　of　the　glacial　period；　instead　of　having　been　really　far　greater；　that　is　extending　from　before　the　glacial　epoch　to　the　present　day。　
In　order　to　get　a　perfect　gradation　between　two　forms　in　the　upper　and　lower　parts　of　the　same　formation；　the　deposit　must　have　gone　on　accumulating　for　a　very　long　period；　in　order　to　have　given　sufficient　time　for　the　slow　process　of　variation；　hence　the　deposit　will　generally　have　to　be　a　very　thick　one；　and　the　species　undergoing　modification　will　have　had　to　live　on　the　same　area　throughout　this　whole　time。　But　we　have　seen　that　a　thick　fossiliferous　formation　can　only　be　accumulated　during　a　period　of　subsidence；　and　to　keep　the　depth　approximately　the　same；　which　is　necessary　in　order　to　enable　the　same　species　to　live　on　the　same　space；　the　supply　of　sediment　must　nearly　have　counterbalanced　the　amount　of　subsidence。　But　this　same　movement　of　subsidence　will　often　tend　to　sink　the　area　whence　the　sediment　is　derived；　and　thus　diminish　the　supply　whilst　the　downward　movement　continues。　In　fact；　this　nearly　exact　balancing　between　the　supply　of　sediment　and　the　amount　of　subsidence　is　probably　a　rare　contingency；　for　it　has　been　observed　by　more　than　one　palaeontologist；　that　very　thick　deposits　are　usually　barren　of　organic　remains；　except　near　their　upper　or　lower　limits。　
It　would　seem　that　each　separate　formation；　like　the　whole　pile　of　formations　in　any　country；　has　generally　been　intermittent　in　its　accumulation。　When　we　see；　as　is　so　often　the　case；　a　formation　composed　of　beds　of　different　mineralogical　composition；　we　may　reasonably　suspect　that　the　process　of　deposition　has　been　much　interrupted；　as　a　change　in　the　currents　of　the　sea　and　a　supply　of　sediment　of　a　different　nature　will　generally　have　been　due　to　geographical　changes　requiring　much　time。　Nor　will　the　closest　inspection　of　a　formation　give　any　idea　of　the　time　which　its　deposition　has　consumed。　Many　instances　could　be　given　of　beds　only　a　few　feet　in　thickness；　representing　formations；　elsewhere　thousands　of　feet　in　thickness；　and　which　must　have　required　an　enormous　period　for　their　accumulation；　yet　no　one　ignorant　of　this　fact　would　have　suspected　the　vast　lapse　of　time　represented　by　the　thinner　formation。　Many　cases　could　be　given　of　the　lower　beds　of　a　formation　having　been　upraised；　denuded；　submerged；　and　then　re…covered　by　the　upper　beds　of　the　same　formation；　facts；　showing　what　wide；　yet　easily　overlooked；　intervals　have　occurred　in　its　accumulation。　In　other　cases　we　have　the　plainest　evidence　in　great　fossilised　trees；　still　standing　upright　as　they　grew；　of　many　long　intervals　of　time　and　changes　of　level　during　the　process　of　deposition；　which　would　never　even　have　been　suspected；　had　not　the　trees　chanced　to　have　been　preserved：　thus；　Messrs　Lyell　and　Dawson　found　carboniferous　beds　1400　feet　thick　in　Nova　Scotia；　with　ancient　root…bearing　strata；　one　above　the　other；　at　no　less　than　sixty…eight　different　levels。　Hence；　when　the　same　species　occur　at　the　bottom；　middle；　and　top　of　a　formation；　the　probability　is　that　they　have　not　lived　on　the　same　spot　during　the　whole　period　of　deposition；　but　have　disappeared　and　reappeared；　perhaps　many　times；　during　the　same　geological　period。　So　that　if　such　species　were　to　undergo　a　considerable　amount　of　modification　during　any　one　geological　period；　a　section　would　not　probably　include　all　the　fine　intermediate　gradations　which　must　on　my　theory　have　existed　between　them；　but　abrupt；　though　perhaps　very　slight；　changes　of　form。　
It　is　all…important　to　remember　that　naturalists　have　no　golden　rule　by　which　to　distinguish　species　and　varieties；　they　grant　some　little　variability　to　each　species；　but　when　they　meet　with　a　somewhat　greater　amount　of　difference　between　any　two　forms；　they　rank　both　as　species；　unless　they　are　enabled　to　connect　them　together　by　close　intermediate　gradations。　And　this　from　the　reasons　just　assigned　we　can　seldom　hope　to　effect　in　any　one　geological　section。　Supposing　B　and　C　to　be　two　species；　and　a　third；　A；　to　be　found　in　an　underlying　bed；　even　if　A　were　strictly　intermediate　between　B　and　C；　it　would　simply　be　ranked　as　a　third　and　distinct　species；　unless　at　the　same　time　it　could　be　most　closely　connected　with　either　one　or　both　forms　by　intermediate　varieties。　Nor　should　it　be　forgotten；　as　before　explained；　that　A　might　be　the　actual　progenitor　of　B　and　C；　and　yet　might　not　at　all　necessarily　be　strictly　intermediate　between　them　in　all　points　of　structure。　So　that　we　might　obtain　the　parent…species　and　its　several　modified　descendants　from　the　lower　and　upper　beds　of　a　formation；　and　unless　we　obtained　numerous　transitional　gradations；　we　should　not　recognise　their　relationship；　and　should　consequently　be　compelled　to　rank　them　all　as　distinct　species。　
It　is　notorious　on　what　excessively　slight　differences　many　palaeontologists　have　founded　their　species；　and　they　do　this　the　more　readily　if　the　specimens　come　from　different　sub…stages　of　the　same　formation。　Some　experienced　conchologists　are　now　sinking　many　of　the　very　fine　species　of　D'Orbigny　and　others　into　the　rank　of　varieties；　and　on　this　view　we　do　find　the　kind　of　evidence　of　change　which　on　my　theory　we　ought　to　find。　Moreover；　if　we　look　to　rather　wider　intervals；　namely；　to　distinct　but　consecutive　stages　of　the　same　great　formation；　we　find　that　the　embedded　fossils；　though　almost　universally　ranked　as　specifically　different；　yet　are　far　more　closely　allied　to　each　other　than　are　the　species　found　in　more　widely　separated　formations；　but　to　this　subject　I　shall　have　to　return　in　the　following　chapter。　
One　other　consideration　is　worth　notice：　with　animals　and　plants　that　can　propagate　rapidly　and　are　not　highly　locomotive；　there　is　reason　to　suspect；　as　we　have　formerly　seen；　that　their　varieties　are　generally　at　first　local；　and　that　such　local　varieties　do　not　spread　widely　and　supplant　their　parent…forms　until　they　have　been　modified　and　perfected　in　some　considerable　degree。　According　to　this　view；　the　chance　of　discovering　in　a　formation　in　any　one　country　all　the　early　stages　of　transition　between　any　two　forms；　is　small；　for　the　successive　changes　are　supposed　to　have　been　local　or　confined　to　some　one　spot。　Most　marine　animals　have　a　wide　range；　and　we　have　seen　that　with　plants　it　is　those　which　have　the　widest　range；　that　oftenest　present　varieties；　so　that　with　shells　and　other　marine　animals；　it　is　probably　those　which　have　had　the　widest　range；　far　exceeding　the　limits　of　the　known　geological　formations　of　Europe；　which　have　oftenest　given　rise；　first　to　local　varieties　and　ultimately　to　new　species；　and　this　again　would　greatly　lessen　the　chance　of　our　being　able　to　trace　the　stages　of　transition　in　any　one　geological　formation。　
It　should　not　be　forgotten；　that　at　the　present　day；　with　perfect　specimens　for　examination；　two　forms　can　seldom　be　connected　by　intermediate　varieties　and　thus　proved　to　be　the　same　species；　until　many　specimens　have　been　collected　from　many　places；　and　in　the　case　of　fossil　species　this　could　rarely　be　effected　by　palaeontologists。　We　shall；　perhaps；　best　perceive　the　improbability　of　our　being　enabled　to　connect　species　by　numerous；　fine；　intermediate；　fossil　links；　by　asking　ourselves　whether；　for　instance；　geologists　at　some　future　period　will　be　able　to　prove；　that　our　different　breeds　of　cattle；　sheep；　horses；　and　dogs　have　descended　from　a　single　stock　or　from　several　aboriginal　stocks；　or；　again；　whether　certain　sea…shells　inhabiting　the　shores　of　North　America；　which　are　ranked　by　some　conchologists　as　distinct　species　from　their　European　representatives；　and　by　other　conchologists　as　only　varieties；　are　really　varieties　or　are；　as　it　is　called；　specifically　distinct。　This　could　be　effected　only　by　the　future　geologist　discovering　in　a　fossil　state　numerous　intermediate　gradations；　and　such　success　seems　to　me　improbable　in　the　highest　degree。　
Geological　research；　though　it　has　added　numerous　species　to　existing　and　extinct　genera；　and　has　made　the　intervals　between　some　few　groups　less　wide　than　they　otherwise　would　have　been；　yet　has　done　scarcely　anything　in　breaking　down　the　distinction　between　species；　by　connecting　them　together　by　numerous；　fine；　intermediate　varieties；　and　this　not　having　been　effected；　is　probably　the　gravest　and　most　obvious　of　all　the　many　objections　which　may　be　urged　against　my　views。　Hence　it　will　be　worth　while　to　sum　up　the　foregoing　remarks；　under　an　imaginary　illustration。　The　Malay　Archipelago　is　of　about　the　size　of　Europe　from　the　North　Cape　to　the　Mediterranean；　and　from　Britain　to　Russia；　and　therefore　equals　all　the　geological　formations　which　have　been　examined　with　any　accuracy；　excepting　those　of　the　United　States　of　America。　I　fully　agree　with　Mr　Godwin…Austen；　that　the　present　condition　of　the　Malay　Archipelago；　with　its　numerous　large　islands　separated　by　wide　and　shallow　seas；　probably　represents　the　former　state　of　Europe；　when　most　of　our　formations　were　accumulating。　The　Malay　Archipelago　is　one　of　the　richest　regions　of　the　whole　world　in　organic　beings；　yet　if　all　the　species　were　to　be　collected　which　have　ever　lived　there；　how　imperfectly　would　they　represent　the　natural　history　of　the　world！　
But　we　have　every　reason　to　believe　that　the　terrestrial　productions　of　the　archipelago　would　be　preserved　in　an　excessively　imperfect　manner　in　the　formations　which　we　suppose　to　be　there　accumulating。　I　suspect　that　not　many　of　the　strictly　littoral　animals；　or　of　those　which　lived　on　naked　submarine　rocks；　would　be　embedded；　and　those　embedded　in　gravel　or　sand；　would　not　endure　to　a　distant