darwin and modern science-第83章

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　to　the　glacial　epoch。〃　　And　next　he　considers　how；　and　by　what　changes；　the　rehabilitation　and　distribution　of　these　lands　themselves　were　effected　subsequent　to　that　period。　　Further；　he　deserves　credit　for　having　cleared　up　a　misunderstanding　of　the　idea　of　specific　centres　of　creation。　　Whilst　for　instance　Schmarda　assumed　without　hesitation　that　the　same　species；　if　occurring　at　places　separated　by　great　distances；　or　apparently　insurmountable　barriers；　had　been　there　created　independently　（multiple　centres）；　Lyell　and　Darwin　held　that　each　species　had　only　one　single　centre；　and　with　this　view　most　of　us　agree；　but　their　starting　point　was　to　them　represented　by　one　individual；　or　rather　one　single　pair。　　According　to　Murray；　on　the　other　hand；　this　centre　of　a　species　is　formed　by　all　the　individuals　of　a　species；　all　of　which　equally　undergo　those　changes　which　new　conditions　may　impose　upon　them。　　In　this　respect　a　new　species　has　a　multiple　origin；　but　this　in　a　sense　very　different　from　that　which　was　upheld　by　L。　Agassiz。　　As　Murray　himself　puts　it：　　〃To　my　multiple　origin；　communication　and　direct　derivation　is　essential。　　The　species　is　compounded　of　many　influences　brought　together　through　many　individuals；　and　distilled　by　Nature　into　one　species；　and；　being　once　established　it　may　roam　and　spread　wherever　it　finds　the　conditions　of　life　not　materially　different　from　those　of　its　original　centre。〃　　（Murray；　〃The　Geographical　Distribution　of　Mammals〃；　page　14。　　London；　1866。）　　This　declaration　fairly　agrees　with　more　modern　views；　and　it　must　be　borne　in　mind　that　the　application　of　the　single…centre　principle　to　the　genera；　families　and　larger　groups　in　the　search　for　descent　inevitably　leads　to　one　creative　centre　for　the　whole　animal　kingdom；　a　condition　as　unwarrantable　as　the　myth　of　Adam　and　Eve　being　the　first　representatives　of　Mankind。

It　looks　as　if　it　had　required　almost　ten　years　for　〃The　Origin　of　Species〃　to　show　its　full　effect；　since　the　year　1868　marks　the　publication　of　Haeckel's　〃Naturliche　Schoepfungsgeschichte〃　in　addition　to　other　great　works。　　The　terms　〃Oecology〃　（the　relation　of　organisms　to　their　environment）　and　〃Chorology〃　（their　distribution　in　space）　had　been　given　us　in　his　〃Generelle　Morphologie〃　in　1866。　　The　fourteenth　chapter　of　the　〃History　of　Creation〃　is　devoted　to　the　distribution　of　organisms；　their　chorology；　with　the　emphatic　assertion　that　〃not　until　Darwin　can　chorology　be　spoken　of　as　a　separate　science；　since　he　supplied　the　acting　causes　for　the　elucidation　of　the　hitherto　accumulated　mass　of　facts。〃　　A　map　（a　〃hypothetical　sketch〃）　shows　the　monophyletic　origin　and　the　routes　of　distribution　of　Man。

Natural　Selection　may　be　all…mighty；　all…sufficient；　but　it　requires　time；　so　much　that　the　countless　aeons　required　for　the　evolution　of　the　present　fauna　were　soon　felt　to　be　one　of　the　most　serious　drawbacks　of　the　theory。　　Therefore　every　help　to　ease　and　shorten　this　process　should　have　been　welcomed。　　In　1868　M。　Wagner　（The　first　to　formulate　clearly　the　fundamental　idea　of　a　theory　of　migration　and　its　importance　in　the　origin　of　new　species　was　L。　von　Buch；　who　in　his　〃Physikalische　Beschreibung　der　Canarischen　Inseln〃；　written　in　1825；　wrote　as　follows：　　〃Upon　the　continents　the　individuals　of　the　genera　by　spreading　far；　form；　through　differences　of　the　locality；　food　and　soil；　varieties　which　finally　become　constant　as　new　species；　since　owing　to　the　distances　they　could　never　be　crossed　with　other　varieties　and　thus　be　brought　back　to　the　main　type。　　Next　they　may　again；　perhaps　upon　different　roads；　return　to　the　old　home　where　they　find　the　old　type　likewise　changed；　both　having　become　so　different　that　they　can　interbreed　no　longer。　　Not　so　upon　islands；　where　the　individuals　shut　up　in　narrow　valleys　or　within　narrow　districts；　can　always　meet　one　another　and　thereby　destroy　every　new　attempt　towards　the　fixing　of　a　new　variety。〃　　Clearly　von　Buch　explains　here　why　island　types　remain　fixed；　and　why　these　types　themselves　have　become　so　different　from　their　continental　congeners。Actually　von　Buch　is　aware　of　a　most　important　point；　the　difference　in　the　process　of　development　which　exists　between　a　new　species　b；　which　is　the　result　of　an　ancestral　species　a　having　itself　changed　into　b　and　thereby　vanished　itself；　and　a　new　species　c　which　arose　through　separation　out　of　the　same　ancestral　a；　which　itself　persists　as　such　unaltered。　　Von　Buch's　prophetic　view　seems　to　have　escaped　Lyell's　and　even　Wagner's　notice。）　came　to　the　rescue　with　his　〃Darwin'sche　Theorie　und　das　Migrations…Gesetz　der　Organismen〃。　　（Leipzig；　1868。）　　He　shows　that　migration；　i。e。　change　of　locality；　implies　new　environmental　conditions　（never　mind　whether　these　be　new　stimuli　to　variation；　or　only　acting　as　their　selectors　or　censors）；　and　moreover　secures　separation　from　the　original　stock　and　thus　eliminates　or　lessens　the　reactionary　dangers　of　panmixia。　　Darwin　accepted　Wagner's　theory　as　〃advantageous。〃　　Through　the　heated　polemics　of　the　more　ardent　selectionists　Wagner's　theory　came　to　grow　into　an　alternative　instead　of　a　help　to　the　theory　of　selectional　evolution。　　Separation　is　now　rightly　considered　a　most　important　factor　by　modern　students　of　geographical　distribution。

For　the　same　year；　1868；　we　have　to　mention　Huxley；　whose　Arctogaea　and　Notogaea　are　nothing　less　than　the　reconstructed　main　masses　of　land　of　the　Mesozoic　period。　　Beyond　doubt　the　configuration　of　land　at　that　remote　period　has　left　recognisable　traces　in　the　present　continents；　but　whether　they　can　account　for　the　distribution　of　such　a　much　later　group　as　the　Gallinaceous　birds　is　more　than　questionable。　　In　any　case　he　took　for　his　text　a　large　natural　group　of　birds；　cosmopolitan　as　a　whole；　but　with　a　striking　distribution。　　The　Peristeropodes；　or　pigeon…footed　division；　are　restricted　to　the　Australian　and　Neotropical　regions；　in　distinction　to　the　Alectoropodes　（with　the　hallux　inserted　at　a　level　above　the　front　toes）　which　inhabit　the　whole　of　the　Arctogaea；　only　a　few　members　having　spread　into　the　South　World。　　Further；　as　Asia　alone　has　its　Pheasants　and　allies；　so　is　Africa　characterised　by　its　Guinea…fowls　and　relations；　America　has　the　Turkey　as　an　endemic　genus；　and　the　Grouse　tribe　in　a　wider　sense　has　its　centre　in　the　holarctic　region：　　a　splendid　object　lesson　of　descent；　world…wide　spreading　and　subsequent　differentiation。　　Huxley；　by　the　way；　was　the　firstat　least　in　private　talkto　state　that　it　will　be　for　the　morphologist；　the　well…trained　anatomist；　to　give　the　casting　vote　in　questions　of　geographical　distribution；　since　he　alone　can　determine　whether　we　have　to　deal　with　homologous；　or　analogous；　convergent；　representative　forms。

It　seems　late　to　introduce　Wallace's　name　in　1876；　the　year　of　the　publication　of　his　standard　work。　　（〃The　Geographical　Distribution　of　Animals〃；　2　vols。　　London；　1876。）　　We　cannot　do　better　than　quote　the　author's　own　words；　expressing　the　hope　that　his　〃book　should　bear　a　similar　relation　to　the　eleventh　and　twelfth　chapters　of　the　〃Origin　of　Species〃　as　Darwin's　〃Animals　and　Plants　under　Domestication〃　does　to　the　first　chapter　of　that　work；〃　and　to　add　that　he　has　amply　succeeded。　　Pleading　for　a　few　primary　centres　he　accepts　Sclater's　six　regions　and　does　not　follow　Huxley's　courageous　changes　which　Sclater　himself　had　accepted　in　1874。　　Holding　the　view　of　the　permanence　of　the　oceans　he　accounts　for　the　colonisation　of　outlying　islands　by　further　elaborating　the　views　of　Lyell　and　Darwin；　especially　in　his　fascinating　〃Island　Life〃；　with　remarkable　chapters　on　the　Ice　Age；　Climate　and　Time　and　other　fundamental　factors。　　His　method　of　arriving　at　the　degree　of　relationship　of　the　faunas　of　the　various　regions　is　eminently　statistical。　　Long　lists　of　genera　determine　by　their　numbers　the　affinity　and　hence　the　source　of　colonisation。　　In　order　to　make　sure　of　his　material　he　performed　the　laborious　task　of　evolving　a　new　classification　of　the　host　of　Passerine　birds。　　This　statistical　method　has　been　followed　by　many　authors；　who；　relying　more　upon　quantity　than　quality；　have　obscured　the　fact　that　the　key　to　the　present　distribution　lies　in　the　past　changes　of　the　earth's　surface。　　However；　with　Wallace　begins　the　modern　study　of　the　geographical　distribution　of　animals　and　the　sudden　interest　taken　in　this　subject　by　an　ever　widening　circle　of　enthusiasts　far　beyond　the　professional　brotherhood。

A　considerable　literature　has　since　grown　up；　almost　bewildering　in　its　range；　diversity　of　aims　and　style　of　procedure。　　It　is　a　chaos；　with　many　paths　leading　into　the　maze；　but　as　yet　very　few　take　us　to　a　position　commanding　a　view　of　the　whole　intricate　terrain　with　its　impenetrable　tangle　and　pitfalls。

One　line　of　research；　not　initiated　but　greatly　influenced　by　Wallace's　works；　became　so　prominent　as　to　almost　constitute　a　period　which　may　be　characterised　as　that　of　the　search　by　specialists　for　either　the　justification　or　the　amending　of　his　regions。　　As　class　after　class　of　animals　was　brought　up　to　reveal　the　secret　of　the　true　regions；　some　authors　saw　in　their　different　results　nothing　but　the　faultiness　of　previously　established　regions；　others　looked　upon　eventual　agreements　as　their　final　corroboration；　especially　when　for　instance　such　diverse　groups　as　mammals　and　scorpions　could；　with　some　ingenuity；　be　made　to　harmonise。　　But　the　obvious　result　of　all　these　efforts　was　the　growing　knowledge　that　almost　every　class　seemed　to　follow　principles　of　its　own。　　The　regions　tallied　neither　in　extent　nor　in　numbers；　although　most　of　them　gravitated　more　and　more　towards　three　centres；　namely　Australia；　South　America　and　the　rest　of　the　world。　　Still　zoologists　persisted　in　the　search；　and　the　various　modes　and　capabilities　of　dispersal　of　the　respective　groups　were　thought　sufficient　explanation　of　the　divergent　results　in　trying　to　bring　the　mapping　of　the　world　under　one　scheme。

Contemporary　literature　is　full　of　devices　for　the　mechanical　dispersal　of　animals。　　Marine　currents；　warm　and　cold；　were　favoured　all　the　more　since　they　showed　the　probable　original　homes　of　the　creatures　in　question。　　If　these　could　not　stand　sea…water；　they　floated　upon　logs　or　icebergs；　or　they　were　blown　across　by　storms；　fishes　were　lifted　over　barriers　by　waterspouts；　and　there　is　on　record　even　an　hypothetical　land　tortoise；　full　of　eggs；　which　colonised　an　oceanic　island　after　a　perilous　sea　voyage　upon　a　tree　trunk。　　Accidents　will　happen；　and　beyond　doubt　many　freaks　of　discontinuous　distribution　have　to　be　accounted　for　by　some　such　means。　　But　whilst　sufficient　for　the　scanty　settlers　of　true　oceanic　islands；　they　cannot　be　held　seriously　to　account　for　the　rich　fauna　of　a　large　continent；　over　which　palaeontology　shows　us　that　the　immigrants　have　passed　like　waves。　　It　should　also　be　borne　in　mind　that　there　is　a　great　difference　between　flotsam　and　jetsam。　　A　current　is　an　extension　of　the　same　medium　and　the　animals　in　it　may　suffer　no　change　during　even　a　long　voyage；　since　they　may　be　brought　from　one　litoral　to　another　where　they　will　still　be　in　the　same　or　but　slightly　altered　environment。　　But　the　jetsam　is　in　the　position　of　a　passe