the origin of species(物种起源)-第51部分

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　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　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　duri