the origin of species(物种起源)-第59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！


e　far…ranging　species　which　would　oftenest　produce　new　varieties；　and　the　varieties　would　at　first　generally　be　local　or　confined　to　one　place；　but　if　possessed　of　any　decided　advantage；　or　when　further　modified　and　improved；　they　would　slowly　spread　and　supplant　their　parent…forms。　When　such　varieties　returned　to　their　ancient　homes；　as　they　would　differ　from　their　former　state；　in　a　nearly　uniform；　though　perhaps　extremely　slight　degree；　they　would；　according　to　the　principles　followed　by　many　palaeontologists；　be　ranked　as　new　and　distinct　species。　
If　then；　there　be　some　degree　of　truth　in　these　remarks；　we　have　no　right　to　expect　to　find　in　our　geological　formations；　an　infinite　number　of　those　fine　transitional　forms；　which　on　my　theory　assuredly　have　connected　all　the　past　and　present　species　of　the　same　group　into　one　long　and　branching　chain　of　life。　We　ought　only　to　look　for　a　few　links；　some　more　closely；　some　more　distantly　related　to　each　other；　and　these　links；　let　them　be　ever　so　close；　if　found　in　different　stages　of　the　same　formation；　would；　by　most　palaeontologists；　be　ranked　as　distinct　species。　But　I　do　not　pretend　that　I　should　ever　have　suspected　how　poor　a　record　of　the　mutations　of　life；　the　best　preserved　geological　section　presented；　had　not　the　difficulty　of　our　not　discovering　innumerable　transitional　links　between　the　species　which　appeared　at　the　commencement　and　close　of　each　formation；　pressed　so　hardly　on　my　theory。　
On　the　sudden　appearance　of　whole　groups　of　Allied　Species。　The　abrupt　manner　in　which　whole　groups　of　species　suddenly　appear　in　certain　formations；　has　been　urged　by　several　palaeontologists；　for　instance；　by　Agassiz；　Pictet；　and　by　none　more　forcibly　than　by　Professor　Sedgwick；　as　a　fatal　objection　to　the　belief　in　the　transmutation　of　species。　If　numerous　species；　belonging　to　the　same　genera　or　families；　have　really　started　into　life　all　at　once；　the　fact　would　be　fatal　to　the　theory　of　descent　with　slow　modification　through　natural　selection。　For　the　development　of　a　group　of　forms；　all　of　which　have　descended　from　some　one　progenitor；　must　have　been　an　extremely　slow　process；　and　the　progenitors　must　have　lived　long　ages　before　their　modified　descendants。　But　we　continually　over…rate　the　perfection　of　the　geological　record；　and　falsely　infer；　because　certain　genera　or　families　have　not　been　found　beneath　a　certain　stage；　that　they　did　not　exist　before　that　stage。　We　continually　forget　how　large　the　world　is；　compared　with　the　area　over　which　our　geological　formations　have　been　carefully　examined；　we　forget　that　groups　of　species　may　elsewhere　have　long　existed　and　have　slowly　multiplied　before　they　invaded　the　ancient　archipelagoes　of　Europe　and　of　the　United　States。　We　do　not　make　due　allowance　for　the　enormous　intervals　of　time；　which　have　probably　elapsed　between　our　consecutive　formations；　longer　perhaps　in　some　cases　than　the　time　required　for　the　accumulation　of　each　formation。　These　intervals　will　have　given　time　for　the　multiplication　of　species　from　some　one　or　some　few　parent…forms；　and　in　the　succeeding　formation　such　species　will　appear　as　if　suddenly　created。　
I　may　here　recall　a　remark　formerly　made；　namely　that　it　might　require　a　long　succession　of　ages　to　adapt　an　organism　to　some　new　and　peculiar　line　of　life；　for　instance　to　fly　through　the　air；　but　that　when　this　had　been　effected；　and　a　few　species　had　thus　acquired　a　great　advantage　over　other　organisms；　a　comparatively　short　time　would　be　necessary　to　produce　many　divergent　forms；　which　would　be　able　to　spread　rapidly　and　widely　throughout　the　world。　
I　will　now　give　a　few　examples　to　illustrate　these　remarks；　and　to　show　how　liable　we　are　to　error　in　supposing　that　whole　groups　of　species　have　suddenly　been　produced。　I　may　recall　the　well…known　fact　that　in　geological　treatises；　published　not　many　years　ago；　the　great　class　of　mammals　was　always　spoken　of　as　having　abruptly　come　in　at　the　commencement　of　the　tertiary　series。　And　now　one　of　the　richest　known　accumulations　of　fossil　mammals　belongs　to　the　middle　of　the　secondary　series；　and　one　true　mammal　has　been　discovered　in　the　new　red　sandstone　at　nearly　the　commencement　of　this　great　series。　Cuvier　used　to　urge　that　no　monkey　occurred　in　any　tertiary　stratum；　but　now　extinct　species　have　been　discovered　in　India；　South　America；　and　in　Europe　even　as　far　back　as　the　eocene　stage。　The　most　striking　case；　however；　is　that　of　the　Whale　family；　as　these　animals　have　huge　bones；　are　marine；　and　range　over　the　world；　the　fact　of　not　a　single　bone　of　a　whale　having　been　discovered　in　any　secondary　formation；　seemed　fully　to　justify　the　belief　that　this　great　and　distinct　order　had　been　suddenly　produced　in　the　interval　between　the　latest　secondary　and　earliest　tertiary　formation。　But　now　we　may　read　in　the　Supplement　to　Lyell's　'Manual；'　published　in　1858；　clear　evidence　of　the　existence　of　whales　in　the　upper　greensand；　some　time　before　the　close　of　the　secondary　period。　
I　may　give　another　instance；　which　from　having　passed　under　my　own　eyes　has　much　struck　me。　In　a　memoir　on　Fossil　Sessile　Cirripedes；　I　have　stated　that；　from　the　number　of　existing　and　extinct　tertiary　species；　from　the　extraordinary　abundance　of　the　individuals　of　many　species　all　over　the　world；　from　the　Arctic　regions　to　the　equator；　inhabiting　various　zones　of　depths　from　the　upper　tidal　limits　to　50　fathoms；　from　the　perfect　manner　in　which　specimens　are　preserved　in　the　oldest　tertiary　beds；　from　the　ease　with　which　even　a　fragment　of　a　valve　can　be　recognised；　from　all　these　circumstances；　I　inferred　that　had　sessile　cirripedes　existed　during　the　secondary　periods；　they　would　certainly　have　been　preserved　and　discovered；　and　as　not　one　species　had　been　discovered　in　beds　of　this　age；　I　concluded　that　this　great　group　had　been　suddenly　developed　at　the　commencement　of　the　tertiary　series。　This　was　a　sore　trouble　to　me；　adding　as　I　thought　one　more　instance　of　the　abrupt　appearance　of　a　great　group　of　species。　But　my　work　had　hardly　been　published；　when　a　skilful　palaeontologist；　M。　Bosquet；　sent　me　a　drawing　of　a　perfect　specimen　of　an　unmistakeable　sessile　cirripede；　which　he　had　himself　extracted　from　the　chalk　of　Belgium。　And；　as　if　to　make　the　case　as　striking　as　possible；　this　sessile　cirripede　was　a　Chthamalus；　a　very　common；　large；　and　ubiquitous　genus；　of　which　not　one　specimen　has　as　yet　been　found　even　in　any　tertiary　stratum。　Hence　we　now　positively　know　that　sessile　cirripedes　existed　during　the　secondary　period；　and　these　cirripedes　might　have　been　the　progenitors　of　our　many　tertiary　and　existing　species。　
The　case　most　frequently　insisted　on　by　palaeontologists　of　the　apparently　sudden　appearance　of　a　whole　group　of　species；　is　that　of　the　teleostean　fishes；　low　down　in　the　Chalk　period。　This　group　includes　the　large　majority　of　existing　species。　Lately；　Professor　Pictet　has　carried　their　existence　one　sub…stage　further　back；　and　some　palaeontologists　believe　that　certain　much　older　fishes；　of　which　the　affinities　are　as　yet　imperfectly　known；　are　really　teleostean。　Assuming；　however；　that　the　whole　of　them　did　appear；　as　Agassiz　believes；　at　the　commencement　of　the　chalk　formation；　the　fact　would　certainly　be　highly　remarkable；　but　I　cannot　see　that　it　would　be　an　insuperable　difficulty　on　my　theory；　unless　it　could　likewise　be　shown　that　the　species　of　this　group　appeared　suddenly　and　simultaneously　throughout　the　world　at　this　same　period。　It　is　almost　superfluous　to　remark　that　hardly　any　fossil…fish　are　known　from　south　of　the　equator；　and　by　running　through　Pictet's　palaeontology　it　will　be　seen　that　very　few　species　are　known　from　several　formations　in　Europe。　Some　few　families　of　fish　now　have　a　confined　range；　the　teleostean　fish　might　formerly　have　had　a　similarly　confined　range；　and　after　having　been　largely　developed　in　some　one　sea；　might　have　spread　widely。　Nor　have　we　any　right　to　suppose　that　the　seas　of　the　world　have　always　been　so　freely　open　from　south　to　north　as　they　are　at　present。　Even　at　this　day；　if　the　Malay　Archipelago　were　converted　into　land；　the　tropical　parts　of　the　Indian　Ocean　would　form　a　large　and　perfectly　enclosed　basin；　in　which　any　great　group　of　marine　animals　might　be　multiplied；　and　here　they　would　remain　confined；　until　some　of　the　species　became　adapted　to　a　cooler　climate；　and　were　enabled　to　double　the　southern　capes　of　Africa　or　Australia；　and　thus　reach　other　and　distant　seas。　
From　these　and　similar　considerations；　but　chiefly　from　our　ignorance　of　the　geology　of　other　countries　beyond　the　confines　of　Europe　and　the　United　States；　and　from　the　revolution　in　our　palaeontological　ideas　on　many　points；　which　the　discoveries　of　even　the　last　dozen　years　have　effected；　it　seems　to　me　to　be　about　as　rash　in　us　to　dogmatize　on　the　succession　of　organic　beings　throughout　the　world；　as　it　would　be　for　a　naturalist　to　land　for　five　minutes　on　some　one　barren　point　in　Australia；　and　then　to　discuss　the　number　and　range　of　its　productions。　
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　numb