the origin of species(物种起源)-第60节

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On　the　sudden　appearance　of　groups　of　Allied　Species　in　the　lowest　known　fossiliferous　strata。　There　is　another　and　allied　difficulty；　which　is　much　graver。　I　allude　to　the　manner　in　which　numbers　of　species　of　the　same　group；　suddenly　appear　in　the　lowest　known　fossiliferous　rocks。　Most　of　the　arguments　which　have　convinced　me　that　all　the　existing　species　of　the　same　group　have　descended　from　one　progenitor；　apply　with　nearly　equal　force　to　the　earliest　known　species。　For　instance；　I　cannot　doubt　that　all　the　Silurian　trilobites　have　descended　from　some　one　crustacean；　which　must　have　lived　long　before　the　Silurian　age；　and　which　probably　differed　greatly　from　any　known　animal。　Some　of　the　most　ancient　Silurian　animals；　as　the　Nautilus；　Lingula；　&c。；　do　not　differ　much　from　living　species；　and　it　cannot　on　my　theory　be　supposed；　that　these　old　species　were　the　progenitors　of　all　the　species　of　the　orders　to　which　they　belong；　for　they　do　not　present　characters　in　any　degree　intermediate　between　them。　If；　moreover；　they　had　been　the　progenitors　of　these　orders；　they　would　almost　certainly　have　been　long　ago　supplanted　and　exterminated　by　their　numerous　and　improved　descendants。　
Consequently；　if　my　theory　be　true；　it　is　indisputable　that　before　the　lowest　Silurian　stratum　was　deposited；　long　periods　elapsed；　as　long　as；　or　probably　far　longer　than；　the　whole　interval　from　the　Silurian　age　to　the　present　day；　and　that　during　these　vast；　yet　quite　unknown；　periods　of　time；　the　world　swarmed　with　living　creatures。　
To　the　question　why　we　do　not　find　records　of　these　vast　primordial　periods；　I　can　give　no　satisfactory　answer。　Several　of　the　most　eminent　geologists；　with　Sir　R。　Murchison　at　their　head；　are　convinced　that　we　see　in　the　organic　remains　of　the　lowest　Silurian　stratum　the　dawn　of　life　on　this　planet。　Other　highly　competent　judges；　as　Lyell　and　the　late　E。　Forbes；　dispute　this　conclusion。　We　should　not　forget　that　only　a　small　portion　of　the　world　is　known　with　accuracy。　M。　Barrande　has　lately　added　another　and　lower　stage　to　the　Silurian　system；　abounding　with　new　and　peculiar　species。　Traces　of　life　have　been　detected　in　the　Longmynd　beds　beneath　Barrande's　so…called　primordial　zone。　The　presence　of　phosphatic　nodules　and　bituminous　matter　in　some　of　the　lowest　azoic　rocks；　probably　indicates　the　former　existence　of　life　at　these　periods。　But　the　difficulty　of　understanding　the　absence　of　vast　piles　of　fossiliferous　strata；　which　on　my　theory　no　doubt　were　somewhere　accumulated　before　the　Silurian　epoch；　is　very　great。　If　these　most　ancient　beds　had　been　wholly　worn　away　by　denudation；　or　obliterated　by　metamorphic　action；　we　ought　to　find　only　small　remnants　of　the　formations　next　succeeding　them　in　age；　and　these　ought　to　be　very　generally　in　a　metamorphosed　condition。　But　the　descriptions　which　we　now　possess　of　the　Silurian　deposits　over　immense　territories　in　Russia　and　in　North　America；　do　not　support　the　view；　that　the　older　a　formation　is；　the　more　it　has　suffered　the　extremity　of　denudation　and　metamorphism。　
The　case　at　present　must　remain　inexplicable；　and　may　be　truly　urged　as　a　valid　argument　against　the　views　here　entertained。　To　show　that　it　may　hereafter　receive　some　explanation；　I　will　give　the　following　hypothesis。　From　the　nature　of　the　organic　remains；　which　do　not　appear　to　have　inhabited　profound　depths；　in　the　several　formations　of　Europe　and　of　the　United　States；　and　from　the　amount　of　sediment；　miles　in　thickness；　of　which　the　formations　are　composed；　we　may　infer　that　from　first　to　last　large　islands　or　tracts　of　land；　whence　the　sediment　was　derived；　occurred　in　the　neighbourhood　of　the　existing　continents　of　Europe　and　North　America。　But　we　do　not　know　what　was　the　state　of　things　in　the　intervals　between　the　successive　formations；　whether　Europe　and　the　United　States　during　these　intervals　existed　as　dry　land；　or　as　a　submarine　surface　near　land；　on　which　sediment　was　not　deposited；　or　again　as　the　bed　of　an　open　and　unfathomable　sea。　
Looking　to　the　existing　oceans；　which　are　thrice　as　extensive　as　the　land；　we　see　them　studded　with　many　islands；　but　not　one　oceanic　island　is　as　yet　known　to　afford　even　a　remnant　of　any　palaeozoic　or　secondary　formation。　Hence　we　may　perhaps　infer；　that　during　the　palaeozoic　and　secondary　periods；　neither　continents　nor　continental　islands　existed　where　our　oceans　now　extend；　for　had　they　existed　there；　palaeozoic　and　secondary　formations　would　in　all　probability　have　been　accumulated　from　sediment　derived　from　their　wear　and　tear；　and　would　have　been　at　least　partially　upheaved　by　the　oscillations　of　level；　which　we　may　fairly　conclude　must　have　intervened　during　these　enormously　long　periods。　If　then　we　may　infer　anything　from　these　facts；　we　may　infer　that　where　our　oceans　now　extend；　oceans　have　extended　from　the　remotest　period　of　which　we　have　any　record；　and　on　the　other　hand；　that　where　continents　now　exist；　large　tracts　of　land　have　existed；　subjected　no　doubt　to　great　oscillations　of　level；　since　the　earliest　silurian　period。　The　coloured　map　appended　to　my　volume　on　Coral　Reefs；　led　me　to　conclude　that　the　great　oceans　are　still　mainly　areas　of　subsidence；　the　great　archipelagoes　still　areas　of　oscillations　of　level；　and　the　continents　areas　of　elevation。　But　have　we　any　right　to　assume　that　things　have　thus　remained　from　eternity？　Our　continents　seem　to　have　been　formed　by　a　preponderance；　during　many　oscillations　of　level；　of　the　force　of　elevation；　but　may　not　the　areas　of　preponderant　movement　have　changed　in　the　lapse　of　ages？　At　a　period　immeasurably　antecedent　to　the　silurian　epoch；　continents　may　have　existed　where　oceans　are　now　spread　out；　and　clear　and　open　oceans　may　have　existed　where　our　continents　now　stand。　Nor　should　we　be　justified　in　assuming　that　if；　for　instance；　the　bed　of　the　Pacific　Ocean　were　now　converted　into　a　continent；　we　should　there　find　formations　older　than　the　silurian　strata；　supposing　such　to　have　been　formerly　deposited；　for　it　might　well　happen　that　strata　which　had　subsided　some　miles　nearer　to　the　centre　of　the　earth；　and　which　had　been　pressed　on　by　an　enormous　weight　of　superincumbent　water；　might　have　undergone　far　more　metamorphic　action　than　strata　which　have　always　remained　nearer　to　the　surface。　The　immense　areas　in　some　parts　of　the　world；　for　instance　in　South　America；　of　bare　metamorphic　rocks；　which　must　have　been　heated　under　great　pressure；　have　always　seemed　to　me　to　require　some　special　explanation；　and　we　may　perhaps　believe　that　we　see　in　these　large　areas；　the　many　formations　long　anterior　to　the　silurian　epoch　in　a　completely　metamorphosed　condition。　
The　several　difficulties　here　discussed；　namely　our　not　finding　in　the　successive　formations　infinitely　numerous　transitional　links　between　the　many　species　which　now　exist　or　have　existed；　the　sudden　manner　in　which　whole　groups　of　species　appear　in　our　European　formations；　the　almost　entire　absence；　as　at　present　known；　of　fossiliferous　formations　beneath　the　Silurian　strata；　are　all　undoubtedly　of　the　gravest　nature。　We　see　this　in　the　plainest　manner　by　the　fact　that　all　the　most　eminent　palaeontologists；　namely　Cuvier；　Owen；　Agassiz；　Barrande；　Falconer；　E。　Forbes；　&c。；　and　all　our　greatest　geologists；　as　Lyell；　Murchison；　Sedgwick；　&c。；　have　unanimously；　often　vehemently；　maintained　the　immutability　of　species。　But　I　have　reason　to　believe　that　one　great　authority；　Sir　Charles　Lyell；　from　further　reflexion　entertains　grave　doubts　on　this　subject。　I　feel　how　rash　it　is　to　differ　from　these　great　authorities；　to　whom；　with　others；　we　owe　all　our　knowledge。　Those　who　think　the　natural　geological　record　in　any　degree　perfect；　and　who　do　not　attach　much　weight　to　the　facts　and　arguments　of　other　kinds　even　in　this　volume；　will　undoubtedly　at　once　reject　my　theory。　For　my　part；　following　out　Lyell's　metaphor；　I　look　at　the　natural　geological　record；　as　a　history　of　the　world　imperfectly　kept；　and　written　in　a　changing　dialect；　of　this　history　we　possess　the　last　volume　alone；　relating　only　to　two　or　three　countries。　Of　this　volume；　only　here　and　there　a　short　chapter　has　been　preserved；　and　of　each　page；　only　here　and　there　a　few　lines。　Each　word　of　the　slowly…changing　language；　in　which　the　history　is　supposed　to　be　written；　being　more　or　less　different　in　the　interrupted　succession　of　chapters；　may　represent　the　apparently　abruptly　changed　forms　of　life；　entombed　in　our　consecutive；　but　widely　separated　formations。　On　this　view；　the　difficulties　above　discussed　are　greatly　diminished；　or　even　disappear。