darwin and modern science-第49章

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The　appearance　of　〃The　Origin　of　Species〃　in　1859　revolutionised　all　the　biological　sciences。　　From　the　very　nature　of　the　case；　Darwin　was　compelled　to　give　careful　consideration　to　the　palaeontological　evidence；　indeed；　it　was　the　palaeontology　and　modern　distribution　of　animals　in　South　America　which　first　led　him　to　reflect　upon　the　great　problem。　　In　his　own　words：　　〃I　had　been　deeply　impressed　by　discovering　in　the　Pampean　formation　great　fossil　animals　covered　with　armour　like　that　on　the　existing　armadillos；　secondly；　by　the　manner　in　which　closely　allied　animals　replace　one　another　in　proceeding　southward　over　the　Continent；　and　thirdly；　by　the　South　American　character　of　most　of　the　productions　of　the　Galapagos　archipelago；　and　more　especially　by　the　manner　in　which　they　differ　slightly　on　each　island　of　the　group。〃　　（〃Life　and　Letters　of　Charles　Darwin〃；　I。　page　82。）　　In　the　famous　tenth　and　eleventh　chapters　of　the　〃Origin〃；　the　palaeontological　evidence　is　examined　at　length　and　the　imperfection　of　the　geological　record　is　strongly　emphasised。　　The　conclusion　is　reached；　that；　in　view　of　this　extreme　imperfection；　palaeontology　could　not　reasonably　be　expected　to　yield　complete　and　convincing　proof　of　the　evolutionary　theory。　　〃I　look　at　the　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。〃　　（〃Origin　of　Species〃；　page　289。）　　Yet；　aside　from　these　inevitable　difficulties；　he　concludes；　that　〃the　other　great　leading　facts　in　palaeontology　agree　admirably　with　the　theory　of　descent　with　modification　through　variation　and　natural　selection。〃　　（Ibid。　page　313。）

Darwin's　theory　gave　an　entirely　new　significance　and　importance　to　palaeontology。　　Cuvier's　conception　of　the　science　had　been　a　limited；　though　a　lofty　one。　　〃How　glorious　it　would　be　if　we　could　arrange　the　organised　products　of　the　universe　in　their　chronological　order！。。。The　chronological　succession　of　organised　forms；　the　exact　determination　of　those　types　which　appeared　first；　the　simultaneous　origin　of　certain　species　and　their　gradual　decay；　would　perhaps　teach　us　as　much　about　the　mysteries　of　organisation　as　we　can　possibly　learn　through　experiments　with　living　organisms。〃　　（Zittel　op。　cit。　page　140。）　　This；　however；　was　rather　the　expression　of　a　hope　for　the　distant　future　than　an　account　of　what　was　attainable；　and　in　practice　the　science　remained　almost　purely　descriptive；　until　Darwin　gave　it　a　new　standpoint；　new　problems　and　an　altogether　fresh　interest　and　charm。　　The　revolution　thus　accomplished　is　comparable　only　to　that　produced　by　the　Copernican　astronomy。

From　the　first　it　was　obvious　that　one　of　the　most　searching　tests　of　the　evolutionary　theory　would　be　given　by　the　advance　of　palaeontological　discovery。　　However　imperfect　the　geological　record　might　be；　its　ascertained　facts　would　necessarily　be　consistent；　under　any　reasonable　interpretation；　with　the　demands　of　a　true　theory；　otherwise　the　theory　would　eventually　be　overwhelmed　by　the　mass　of　irreconcilable　data。　　A　very　great　stimulus　was　thus　given　to　geological　investigation　and　to　the　exploration　of　new　lands。　　In　the　last　forty　years；　the　examination　of　North　and　South　America；　of　Africa　and　Asia　has　brought　to　light　many　chapters　in　the　history　of　life；　which　are　astonishingly　full　and　complete。　　The　flood　of　new　material　continues　to　accumulate　at　such　a　rate　that　it　is　impossible　to　keep　abreast　of　it；　and　the　very　wealth　of　the　collections　is　a　source　of　difficulty　and　embarrassment。　　In　modern　palaeontology　phylogenetic　questions　and　problems　occupy　a　foremost　place　and；　as　a　result　of　the　labours　of　many　eminent　investigators　in　many　lands；　it　may　be　said　that　this　science　has　proved　to　be　one　of　the　most　solid　supports　of　Darwin's　theory。　　True；　there　are　very　many　unsolved　problems；　and　the　discouraged　worker　is　often　tempted　to　believe　that　the　fossils　raise　more　questions　than　they　answer。　　Yet；　on　the　other　hand；　the　whole　trend　of　the　evidence　is　so　strongly　in　favour　of　the　evolutionary　doctrine；　that　no　other　interpretation　seems　at　all　rational。

To　present　any　adequate　account　of　the　palaeontological　record　from　the　evolutionary　standpoint；　would　require　a　large　volume　and　a　singularly　unequal；　broken　and　disjointed　history　it　would　be。　　Here　the　record　is　scanty；　interrupted；　even　unintelligible；　while　there　it　is　crowded　with　embarrassing　wealth　of　material；　but　too　often　these　full　chapters　are　separated　by　such　stretches　of　unrecorded　time；　that　it　is　difficult　to　connect　them。　　It　will　be　more　profitable　to　present　a　few　illustrative　examples　than　to　attempt　an　outline　of　the　whole　history。

At　the　outset；　the　reader　should　be　cautioned　not　to　expect　too　much；　for　the　task　of　determining　phylogenies　fairly　bristles　with　difficulties　and　encounters　many　unanswered　questions。　　Even　when　the　evidence　seems　to　be　as　copious　and　as　complete　as　could　be　wished；　different　observers　will　put　different　interpretations　upon　it；　as　in　the　notorious　case　of　the　Steinheim　shells。　　（In　the　Miocene　beds　of　Steinheim；　Wurtemberg；　occur　countless　fresh…water　shells；　which　show　numerous　lines　of　modification；　but　these　have　been　very　differently　interpreted　by　different　writers。）　　The　ludicrous　discrepances　which　often　appear　between　the　phylogenetic　〃trees〃　of　various　writers　have　cast　an　undue　discredit　upon　the　science　and　have　led　many　zoologists　to　ignore　palaeontology　altogether　as　unworthy　of　serious　attention。　　One　principal　cause　of　these　discrepant　and　often　contradictory　results　is　our　ignorance　concerning　the　exact　modes　of　developmental　change。　　What　one　writer　postulates　as　almost　axiomatic；　another　will　reject　as　impossible　and　absurd。　　Few　will　be　found　to　agree　as　to　how　far　a　given　resemblance　is　offset　by　a　given　unlikeness；　and　so　long　as　the　question　is　one　of　weighing　evidence　and　balancing　probabilities；　complete　harmony　is　not　to　be　looked　for。　　These　formidable　difficulties　confront　us　even　in　attempting　to　work　out　from　abundant　material　a　brief　chapter　in　the　phylogenetic　history　of　some　small　and　clearly　limited　group；　and　they　become　disproportionately　greater；　when　we　extend　our　view　over　vast　periods　of　time　and　undertake　to　determine　the　mutual　relationships　of　classes　and　types。　　If　the　evidence　were　complete　and　available；　we　should　hardly　be　able　to　unravel　its　infinite　complexity；　or　to　find　a　clue　through　the　mazes　of　the　labyrinth。　　〃Our　ideas　of　the　course　of　descent　must　of　necessity　be　diagrammatic。〃　　（D。H。　Scott；　〃Studies　in　Fossil　Botany〃；　page　524。　　London；　1900。）

Some　of　the　most　complete　and　convincing　examples　of　descent　with　modification　are　to　be　found　among　the　mammals；　and　nowhere　more　abundantly　than　in　North　America；　where　the　series　of　continental　formations；　running　through　the　whole　Tertiary　period；　is　remarkably　full。　　Most　of　these　formations　contain　a　marvellous　wealth　of　mammalian　remains　and　in　an　unusual　state　of　preservation。　　The　oldest　Eocene　（Paleocene）　has　yielded　a　mammalian　fauna　which　is　still　of　prevailingly　Mesozoic　character；　and　contains　but　few　forms　which　can　be　regarded　as　ancestral　to　those　of　later　times。　　The　succeeding　fauna　of　the　lower　Eocene　proper　（Wasatch　stage）　is　radically　different　and；　while　a　few　forms　continue　over　from　the　Paleocene；　the　majority　are　evidently　recent　immigrants　from　some　region　not　yet　identified。　　From　the　Wasatch　onward；　the　development　of　many　phyla　may　be　traced　in　almost　unbroken　continuity；　though　from　time　to　time　the　record　is　somewhat　obscured　by　migrations　from　the　Old　World　and　South　America。　　As　a　rule；　however；　it　is　easy　to　distinguish　between　the　immigrant　and　the　indigenous　elements　of　the　fauna。

From　their　gregarious　habits　and　individual　abundance；　the　history　of　many　hoofed　animals　is　preserved　with　especial　clearness。　　So　well　known　as　to　have　become　a　commonplace；　is　the　phylogeny　of　the　horses；　which；　contrary　to　all　that　would　have　been　expected；　ran　the　greater　part　of　its　course　in　North　America。　　So　far　as　it　has　yet　been　traced；　the　line　begins　in　the　lower　Eocene　with　the　genus　Eohippus；　a　little　creature　not　much　larger　than　a　cat；　which　has　a　short　neck；　relatively　short　limbs；　and　in　particular；　short　feet；　with　four　functional　digits　and　a　splint…like　rudiment　in　the　fore…foot；　three　functional　digits　and　a　rudiment　in　the　hind…foot。　　The　forearm　bones　（ulna　and　radius）　are　complete　and　separate；　as　are　also　the　bones　of　the　lower　leg　（fibula　and　tibia）。　　The　skull　has　a　short　face；　with　the　orbit；　or　eye…socket；　incompletely　enclosed　with　bone；　and　the　brain…case　is　slender　and　of　small　capacity。　　The　teeth　are　short…　crowned；　the　incisors　without　〃mark；〃　or　enamel　pit；　on　the　cutting　edge；　the　premolars　are　all　smaller　and　simpler　than　the　molars。　　The　pattern　of　the　upper　molars　is　so　entirely　different　from　that　seen　in　the　modern　horses　that；　without　the　intermediate　connecting　steps；　no　one　would　have　ventured　to　derive　the　later　from　the　earlier　plan。　　This　pattern　is　quadritubercular；　with　four　principal；　conical　cusps　arranged　in　two　transverse　pairs；　forming　a　square；　and　two　minute　cuspules　between　each　transverse　pair；　a　tooth　which　is　much　more　pig…like　than　horse…like。　　In　the　lower　molars　the　cusps　have　already　united　to　form　two　crescents；　one　behind　the　other；　forming　a　pattern　which　is　extremely　common　in　the　early　representatives　of　many　different　families；　both　of　the　Perissodactyla　and　the　Artiodactyla。　　In　spite　of　the　manifold　differences　in　all　parts　of　the　skeleton　between　Eohippus　and　the　recent　horses；　the　former　has　stamped　upon　it　an　equine　character　which　is　unmistakable；　though　it　can　hardly　be　expressed　in　words。

Each　one　of　the　different　Eocene　and　Oligocene　horizons　has　its　characteristic　genus　of　horses；　showing　a　slow；　steady　progress　in　a　definite　direction；　all　parts　of　the　structure　participating　in　the　advance。　　It　is　not　necessary　to　follow　each　of　these　successive　steps　of　change；　but　it　should　be　emphasised　that　the　changes　are　gradual　and　uninterrupted。　　The　genus　Mesohippus；　of　the　middle　Oligocene；　may　be　selected　as　a　kind　of　half…way　stage　in　the　long　progression。　　Comparing　Mesohippus　with　Eohippus；　we　observe　that　the　former　is　much　larger；　some　species　attaining　the　size　of　a　sheep；　and　has　a　relatively　longer　neck；　longer　limbs　and　much　more　elongate　feet；　which　are　tridactyl；　and　the　middle　toe　is　so　enlarged　that　it　bears　most　of　the　weight；　while　the　lateral　digits　are　very　much　more　slender。　　The　fore…arm　bones　have　begun　to　co…ossify　and　the　ulna　is　greatly　reduced；　while　the　fibula；　though　still　complete；　is　hardly　more　than　a　thread　of　bone。　　The　skull　has　a　longer　face　and　a　nearly　enclosed　orbit；　and　the　brain…case　is　fuller　and　more　capacious；　the　internal　cast　of　which　shows　that　the　brain　was　richly　convoluted。　　The　teeth　are　still　very　short…crowned；　but　the　upper　incisors　plainly　show　the　beginning　of　the　〃mark〃；　the　premolars　have　assumed　the　molar　form；　and　the　upper　molars；　though　plainly　derived　from　those　o