the origin of species(物种起源)-第57节

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！


subsidence。　Since　publishing　my　views　on　this　subject　in　1845；　I　have　watched　the　progress　of　Geology；　and　have　been　surprised　to　note　how　author　after　author；　in　treating　of　this　or　that　great　formation；　has　come　to　the　conclusion　that　it　was　accumulated　during　subsidence。　I　may　add；　that　the　only　ancient　tertiary　formation　on　the　west　coast　of　South　America；　which　has　been　bulky　enough　to　resist　such　degradation　as　it　has　as　yet　suffered；　but　which　will　hardly　last　to　a　distant　geological　age；　was　certainly　deposited　during　a　downward　oscillation　of　level；　and　thus　gained　considerable　thickness。　
All　geological　facts　tell　us　plainly　that　each　area　has　undergone　numerous　slow　oscillations　of　level；　and　apparently　these　oscillations　have　affected　wide　spaces。　Consequently　formations　rich　in　fossils　and　sufficiently　thick　and　extensive　to　resist　subsequent　degradation；　may　have　been　formed　over　wide　spaces　during　periods　of　subsidence；　but　only　where　the　supply　of　sediment　was　sufficient　to　keep　the　sea　shallow　and　to　embed　and　preserve　the　remains　before　they　had　time　to　decay。　On　the　other　hand；　as　long　as　the　bed　of　the　sea　remained　stationary；　thick　deposits　could　not　have　been　accumulated　in　the　shallow　parts；　which　are　the　most　favourable　to　life。　Still　less　could　this　have　happened　during　the　alternate　periods　of　elevation；　or；　to　speak　more　accurately；　the　beds　which　were　then　accumulated　will　have　been　destroyed　by　being　upraised　and　brought　within　the　limits　of　the　coast…action。　
Thus　the　geological　record　will　almost　necessarily　be　rendered　intermittent。　I　feel　much　confidence　in　the　truth　of　these　views；　for　they　are　in　strict　accordance　with　the　general　principles　inculcated　by　Sir　C。　Lyell；　and　E。　Forbes　independently　arrived　at　a　similar　conclusion。　
One　remark　is　here　worth　a　passing　notice。　During　periods　of　elevation　the　area　of　the　land　and　of　the　adjoining　shoal　parts　of　the　sea　will　be　increased；　and　new　stations　will　often　be　formed；　all　circumstances　most　favourable；　as　previously　explained；　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　t