the origin of species(物种起源)-第56节

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hich　the　sea　commonly　wears　away　a　line　of　cliff　of　any　given　height；　we　could　measure　the　time　requisite　to　have　denuded　the　Weald。　This；　of　course；　cannot　be　done；　but　we　may；　in　order　to　form　some　crude　notion　on　the　subject；　assume　that　the　sea　would　eat　into　cliffs　500　feet　in　height　at　the　rate　of　one　inch　in　a　century。　This　will　at　first　appear　much　too　small　an　allowance；　but　it　is　the　same　as　if　we　were　to　assume　a　cliff　one　yard　in　height　to　be　eaten　back　along　a　whole　line　of　coast　at　the　rate　of　one　yard　in　nearly　every　twenty…two　years。　I　doubt　whether　any　rock；　even　as　soft　as　chalk；　would　yield　at　this　rate　excepting　on　the　most　exposed　coasts；　though　no　doubt　the　degradation　of　a　lofty　cliff　would　be　more　rapid　from　the　breakage　of　the　fallen　fragments。　On　the　other　hand；　I　do　not　believe　that　any　line　of　coast；　ten　or　twenty　miles　in　length；　ever　suffers　degradation　at　the　same　time　along　its　whole　indented　length；　and　we　must　remember　that　almost　all　strata　contain　harder　layers　or　nodules；　which　from　long　resisting　attrition　form　a　breakwater　at　the　base。　Hence；　under　ordinary　circumstances；　I　conclude　that　for　a　cliff　500　feet　in　height；　a　denudation　of　one　inch　per　century　for　the　whole　length　would　be　an　ample　allowance。　At　this　rate；　on　the　above　data；　the　denudation　of　the　Weald　must　have　required　306；662；400　years；　or　say　three　hundred　million　years。　
The　action　of　fresh　water　on　the　gently　inclined　Wealden　district；　when　upraised；　could　hardly　have　been　great；　but　it　would　somewhat　reduce　the　above　estimate。　On　the　other　hand；　during　oscillations　of　level；　which　we　know　this　area　has　undergone；　the　surface　may　have　existed　for　millions　of　years　as　land；　and　thus　have　escaped　the　action　of　the　sea：　when　deeply　submerged　for　perhaps　equally　long　periods；　it　would；　likewise；　have　escaped　the　action　of　the　coast…waves。　So　that　in　all　probability　a　far　longer　period　than　300　million　years　has　elapsed　since　the　latter　part　of　the　Secondary　period。　
I　have　made　these　few　remarks　because　it　is　highly　important　for　us　to　gain　some　notion；　however　imperfect；　of　the　lapse　of　years。　During　each　of　these　years；　over　the　whole　world；　the　land　and　the　water　has　been　peopled　by　hosts　of　living　forms。　What　an　infinite　number　of　generations；　which　the　mind　cannot　grasp；　must　have　succeeded　each　other　in　the　long　roll　of　years！　Now　turn　to　our　richest　geological　museums；　and　what　a　paltry　display　we　behold！　
On　the　poorness　of　our　Palaeontological　collections。　That　our　Palaeontological　collections　are　very　imperfect；　is　admitted　by　every　one。　The　remark　of　that　admirable　Palaeontologist；　the　late　Edward　Forbes；　should　not　be　forgotten；　namely；　that　numbers　of　our　fossil　species　are　known　and　named　from　single　and　often　broken　specimens；　or　from　a　few　specimens　collected　on　some　one　spot。　Only　a　small　portion　of　the　surface　of　the　earth　has　been　geologically　explored；　and　no　part　with　sufficient　care；　as　the　important　discoveries　made　every　year　in　Europe　prove。　No　organism　wholly　soft　can　be　preserved。　Shells　and　bones　will　decay　and　disappear　when　left　on　the　bottom　of　the　sea；　where　sediment　is　not　accumulating。　I　believe　we　are　continually　taking　a　most　erroneous　view；　when　we　tacitly　admit　to　ourselves　that　sediment　is　being　deposited　over　nearly　the　whole　bed　of　the　sea；　at　a　rate　sufficiently　quick　to　embed　and　preserve　fossil　remains。　Throughout　an　enormously　large　proportion　of　the　ocean；　the　bright　blue　tint　of　the　water　bespeaks　its　purity。　The　many　cases　on　record　of　a　formation　conformably　covered；　after　an　enormous　interval　of　time；　by　another　and　later　formation；　without　the　underlying　bed　having　suffered　in　the　interval　any　wear　and　tear；　seem　explicable　only　on　the　view　of　the　bottom　of　the　sea　not　rarely　lying　for　ages　in　an　unaltered　condition。　The　remains　which　do　become　embedded；　if　in　sand　or　gravel；　will　when　the　beds　are　upraised　generally　be　dissolved　by　the　percolation　of　rain…water。　I　suspect　that　but　few　of　the　very　many　animals　which　live　on　the　beach　between　high　and　low　watermark　are　preserved。　For　instance；　the　several　species　of　the　Chthamalinae　（a　sub…family　of　sessile　cirripedes）　coat　the　rocks　all　over　the　world　in　infinite　numbers：　they　are　all　strictly　littoral；　with　the　exception　of　a　single　Mediterranean　species；　which　inhabits　deep　water　and　has　been　found　fossil　in　Sicily；　whereas　not　one　other　species　has　hitherto　been　found　in　any　tertiary　formation：　yet　it　is　now　known　that　the　genus　Chthamalus　existed　during　the　chalk　period。　The　molluscan　genus　Chiton　offers　a　partially　analogous　case。　
With　respect　to　the　terrestrial　productions　which　lived　during　the　Secondary　and　Palaeozoic　periods；　it　is　superfluous　to　state　that　our　evidence　from　fossil　remains　is　fragmentary　in　an　extreme　degree。　For　instance；　not　a　land　shell　is　known　belonging　to　either　of　these　vast　periods；　with　one　exception　discovered　by　Sir　C。　Lyell　in　the　carboniferous　strata　of　North　America。　I　n　regard　to　mammiferous　remains；　a　single　glance　at　the　historical　table　published　in　the　Supplement　to　Lyell's　Manual；　will　bring　home　the　truth；　how　accidental　and　rare　is　their　preservation；　far　better　than　pages　of　detail。　Nor　is　their　rarity　surprising；　when　we　remember　how　large　a　proportion　of　the　bones　of　tertiary　mammals　have　been　discovered　either　in　caves　or　in　lacustrine　deposits；　and　that　not　a　cave　or　true　lacustrine　bed　is　known　belonging　to　the　age　of　our　secondary　or　palaeozoic　formations。　
But　the　imperfection　in　the　geological　record　mainly　results　from　another　and　more　important　cause　than　any　of　the　foregoing；　namely；　from　the　several　formations　being　separated　from　each　other　by　wide　intervals　of　time。　When　we　see　the　formations　tabulated　in　written　works；　or　when　we　follow　them　in　nature；　it　is　difficult　to　avoid　believing　that　they　are　closely　consecutive。　But　we　know；　for　instance；　from　Sir　R。　Murchison's　great　work　on　Russia；　what　wide　gaps　there　are　in　that　country　between　the　superimposed　formations；　so　it　is　in　North　America；　and　in　many　other　parts　of　the　world。　The　most　skilful　geologist；　if　his　attention　had　been　exclusively　confined　to　these　large　territories；　would　never　have　suspected　that　during　the　periods　which　were　blank　and　barren　in　his　own　country；　great　piles　of　sediment；　charged　with　new　and　peculiar　forms　of　life；　had　elsewhere　been　accumulated。　And　if　in　each　separate　territory；　hardly　any　idea　can　be　formed　of　the　length　of　time　which　has　elapsed　between　the　consecutive　formations；　we　may　infer　that　this　could　nowhere　be　ascertained。　The　frequent　and　great　changes　in　the　mineralogical　composition　of　consecutive　formations；　generally　implying　great　changes　in　the　geography　of　the　surrounding　lands；　whence　the　sediment　has　been　derived；　accords　with　the　belief　of　vast　intervals　of　time　having　elapsed　between　each　formation。　
But　we　can；　I　think；　see　why　the　geological　formations　of　each　region　are　almost　invariably　intermittent；　that　is；　have　not　followed　each　other　in　close　sequence。　Scarcely　any　fact　struck　me　more　when　examining　many　hundred　miles　of　the　South　American　coasts；　which　have　been　upraised　several　hundred　feet　within　the　recent　period；　than　the　absence　of　any　recent　deposits　sufficiently　extensive　to　last　for　even　a　short　geological　period。　Along　the　whole　west　coast；　which　is　inhabited　by　a　peculiar　marine　fauna；　tertiary　beds　are　so　scantily　developed；　that　no　record　of　several　successive　and　peculiar　marine　faunas　will　probably　be　preserved　to　a　distant　age。　A　little　reflection　will　explain　why　along　the　rising　coast　of　the　western　side　of　South　America；　no　extensive　formations　with　recent　or　tertiary　remains　can　anywhere　be　found；　though　the　supply　of　sediment　must　for　ages　have　been　great；　from　the　enormous　degradation　of　the　coast…rocks　and　from　muddy　streams　entering　the　sea。　The　explanation；　no　doubt；　is；　that　the　littoral　and　sub…littoral　deposits　are　continually　worn　away；　as　soon　as　they　are　brought　up　by　the　slow　and　gradual　rising　of　the　land　within　the　grinding　action　of　the　coast…waves。　
We　may；　I　think；　safely　conclude　that　sediment　must　be　accumulated　in　extremely　thick；　solid；　or　extensive　masses；　in　order　to　withstand　the　incessant　action　of　the　waves；　when　first　upraised　and　during　subsequent　oscillations　of　level。　Such　thick　and　extensive　accumulations　of　sediment　may　be　formed　in　two　ways；　either；　in　profound　depths　of　the　sea；　in　which　case；　judging　from　the　researches　of　E。　Forbes；　we　may　conclude　that　the　bottom　will　be　inhabited　by　extremely　few　animals；　and　the　mass　when　upraised　will　give　a　most　imperfect　record　of　the　forms　of　life　which　then　existed；　or；　sediment　may　be　accumulated　to　any　thickness　and　extent　over　a　shallow　bottom；　if　it　continue　slowly　to　subside。　In　this　latter　case；　as　long　as　the　rate　of　subsidence　and　supply　of　sediment　nearly　balance　each　other；　the　sea　will　remain　shallow　and　favourable　for　life；　and　thus　a　fossiliferous　formation　thick　enough；　when　upraised；　to　resist　any　amount　of　degradation；　may　be　formed。　
I　am　convinced　that　all　our　ancient　formations；　which　are　rich　in　fossils；　have　thus　been　formed　during　subsidence。　Since　publishing　my　views　on　this　subject　in　1845；　I　have　watched　the　progress　of　Geolog