the origin of species(物种起源)-第47章

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！


ed；　for　the　formation　of　new　varieties　and　species；　but　during　such　periods　there　will　generally　be　a　blank　in　the　geological　record。　On　the　other　hand；　during　subsidence；　the　inhabited　area　and　number　of　inhabitants　will　decrease　（excepting　the　productions　on　the　shores　of　a　continent　when　first　broken　up　into　an　archipelago）；　and　consequently　during　subsidence；　though　there　will　be　much　extinction；　fewer　new　varieties　or　species　will　be　formed；　and　it　is　during　these　very　periods　of　subsidence；　that　our　great　deposits　rich　in　fossils　have　been　accumulated。　Nature　may　almost　be　said　to　have　guarded　against　the　frequent　discovery　of　her　transitional　or　linking　forms。　
From　the　foregoing　considerations　it　cannot　be　doubted　that　the　geological　record；　viewed　as　a　whole；　is　extremely　imperfect；　but　if　we　confine　our　attention　to　any　one　formation；　it　becomes　more　difficult　to　understand；　why　we　do　not　therein　find　closely　graduated　varieties　between　the　allied　species　which　lived　at　its　commencement　and　at　its　close。　Some　cases　are　on　record　of　the　same　species　presenting　distinct　varieties　in　the　upper　and　lower　parts　of　the　same　formation；　but；　as　they　are　rare；　they　may　be　here　passed　over。　Although　each　formation　has　indisputably　required　a　vast　number　of　years　for　its　deposition；　I　can　see　several　reasons　why　each　should　not　include　a　graduated　series　of　links　between　the　species　which　then　lived；　but　I　can　by　no　means　pretend　to　assign　due　proportional　weight　to　the　following　considerations。　
Although　each　formation　may　mark　a　very　long　lapse　of　years；　each　perhaps　is　short　compared　with　the　period　requisite　to　change　one　species　into　another。　I　am　aware　that　two　palaeontologists；　whose　opinions　are　worthy　of　much　deference；　namely　Bronn　and　Woodward；　have　concluded　that　the　average　duration　of　each　formation　is　twice　or　thrice　as　long　as　the　average　duration　of　specific　forms。　But　insuperable　difficulties；　as　it　seems　to　me；　prevent　us　coming　to　any　just　conclusion　on　this　head。　When　we　see　a　species　first　appearing　in　the　middle　of　any　formation；　it　would　be　rash　in　the　extreme　to　infer　that　it　had　not　elsewhere　previously　existed。　So　again　when　we　find　a　species　disappearing　before　the　uppermost　layers　have　been　deposited；　it　would　be　equally　rash　to　suppose　that　it　then　became　wholly　extinct。　We　forget　how　small　the　area　of　Europe　is　compared　with　the　rest　of　the　world；　nor　have　the　several　stages　of　the　same　formation　throughout　Europe　been　correlated　with　perfect　accuracy。　
With　marine　animals　of　all　kinds；　we　may　safely　infer　a　large　amount　of　migration　during　climatal　and　other　changes；　and　when　we　see　a　species　first　appearing　in　any　formation；　the　probability　is　that　it　only　then　first　immigrated　into　that　area。　It　is　well　known；　for　instance；　that　several　species　appeared　somewhat　earlier　in　the　palaeozoic　beds　of　North　America　than　in　those　of　Europe；　time　having　apparently　been　required　for　their　migration　from　the　American　to　the　European　seas。　In　examining　the　latest　deposits　of　various　quarters　of　the　world；　it　has　everywhere　been　noted；　that　some　few　still　existing　species　are　common　in　the　deposit；　but　have　become　extinct　in　the　immediately　surrounding　sea；　or；　conversely；　that　some　are　now　abundant　in　the　neighbouring　sea；　but　are　rare　or　absent　in　this　particular　deposit。　It　is　an　excellent　lesson　to　reflect　on　the　ascertained　amount　of　migration　of　the　inhabitants　of　Europe　during　the　Glacial　period；　which　forms　only　a　part　of　one　whole　geological　period；　and　likewise　to　reflect　on　the　great　changes　of　level；　on　the　inordinately　great　change　of　climate；　on　the　prodigious　lapse　of　time；　all　included　within　this　same　glacial　period。　Yet　it　may　be　doubted　whether　in　any　quarter　of　the　world；　sedimentary　deposits；　including　fossil　remains；　have　gone　on　accumulating　within　the　same　area　during　the　whole　of　this　period。　It　is　not；　for　instance；　probable　that　sediment　was　deposited　during　the　whole　of　the　glacial　period　near　the　mouth　of　the　Mississippi；　within　that　limit　of　depth　at　which　marine　animals　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　varieti