darwin and modern science-第101节

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that　it　will　ever　be　proved　that　within　the　recent　period　continents　which　are　now　quite　separate；　have　been　continuously；　or　almost　continuously；　united　with　each　other；　and　with　the　many　existing　oceanic　islands。〃　　（Ibid。　page　357。）　　Again；　〃believing。。。that　our　continents　have　long　remained　in　nearly　the　same　relative　position；　though　subjected　to　large；　but　partial　oscillations　of　level；〃　that　means　to　say　within　the　period　of　existing　species；　or　〃within　the　recent　period。〃　　（Ibid。　page。　370。）　　The　difficulty　was　to　a　great　extent　one　of　his　own　making。　　Whilst　almost　everybody　else　believed　in　the　immutability　of　the　species；　which　implies　an　enormous　age；　logically　since　the　dawn　of　creation；　to　him　the　actually　existing　species　as　the　latest　results　of　evolution；　were　necessarily　something　very　new；　so　young　that　only　the　very　latest　of　the　geological　epochs　could　have　affected　them。　　It　has　since　come　to　our　knowledge　that　a　great　number　of　terrestrial　〃recent〃　species；　even　those　of　the　higher　classes　of　Vertebrates；　date　much　farther　back　than　had　been　thought　possible。　　Many　of　them　reach　well　into　the　Miocene；　a　time　since　which　the　world　seems　to　have　assumed　the　main　outlines　of　the　present　continents。

In　the　year　1866　appeared　A。　Murray's　work　on　the　〃Geographical　Distribution　of　Mammals〃；　a　book　which　has　perhaps　received　less　recognition　than　it　deserves。　　His　treatment　of　the　general　introductory　questions　marks　a　considerable　advance　of　our　problem；　although；　and　partly　because；　he　did　not　entirely　agree　with　Darwin's　views　as　laid　down　in　the　first　edition　of　〃The　Origin　of　Species〃；　which　after　all　was　the　great　impulse　given　to　Murray's　work。　　Like　Forbes　he　did　not　shrink　from　assuming　enormous　changes　in　the　configuration　of　the　continents　and　oceans　because　the　theory　of　descent；　with　its　necessary　postulate　of　great　migrations；　required　them。　　He　stated；　for　instance；　〃that　a　Miocene　Atlantis　sufficiently　explains　the　common　distribution　of　animals　and　plants　in　Europe　and　America　up　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　tha