darwin and modern science-第103节

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f　explanation；　that　the　palms　themselves　might　have　changed　their　nature。　　One　of　the　most　valuable　aids　in　geological　research；　often　the　only　means　for　reconstructing　the　face　of　the　earth　in　by…gone　periods；　is　afforded　by　fossils；　but　only　the　morphologist　can　pronounce　as　to　their　trustworthiness　as　witnesses；　because　of　the　danger　of　mistaking　analogous　for　homologous　forms。　　This　difficulty　applies　equally　to　living　groups；　and　it　is　so　important　that　a　few　instances　may　not　be　amiss。

There　is　undeniable　similarity　between　the　faunas　of　Madagascar　and　South　America。　　This　was　supported　by　the　Centetidae　and　Dendrobatidae；　two　entire　〃families；〃　as　also　by　other　facts。　　The　value　of　the　Insectivores；　Solenodon　in　Cuba；　Centetes　in　Madagascar；　has　been　much　lessened　by　their　recognition　as　an　extremely　ancient　group　and　as　a　case　of　convergence；　but　if　they　are　no　longer　put　into　the　same　family；　this　amendment　is　really　to　a　great　extent　due　to　their　widely　discontinuous　distribution。　　The　only　systematic　difference　of　the　Dendrobatidae　from　the　Ranidae　is　the　absence　of　teeth；　morphologically　a　very　unimportant　character；　and　it　is　now　agreed；　on　the　strength　of　their　distribution；　that　these　little　arboreal；　conspicuously　coloured　frogs；　Dendrobates　in　South　America；　Mantella　in　Madagascar；　do　not　form　a　natural　group；　although　a　third　genus；　Cardioglossa　in　West　Africa；　seems　also　to　belong　to　them。　　If　these　creatures　lived　all　on　the　same　continent；　we　should　unhesitatingly　look　upon　them　as　forming　a　well…defined；　natural　little　group。　　On　the　other　hand　the　Aglossa；　with　their　three　very　divergent　genera；　namely　Pipa　in　South　America；　Xenopus　and　Hymenochirus　in　Africa；　are　so　well　characterised　as　one　ancient　group　that　we　use　their　distribution　unhesitatingly　as　a　hint　of　a　former　connection　between　the　two　continents。　　We　are　indeed　arguing　in　vicious　circles。　　The　Ratitae　as　such　are　absolutely　worthless　since　they　are　a　most　heterogeneous　assembly；　and　there　are　untold　groups；　of　the　artificiality　of　which　many　a　zoo…　geographer　had　not　the　slightest　suspicion　when　he　took　his　statistical　material；　the　genera　and　families；　from　some　systematic　catalogues　or　similar　lists。　　A　lamentable　instance　is　that　of　certain　flightless　Rails；　recently　extinct　or　sub…fossil；　on　the　isalnds　of　Mauritius；　Rodriguez　and　Chatham。　　Being　flightless　they　have　been　used　in　support　of　a　former　huge　Antarctic　continent；　instead　of　ruling　them　out　of　court　as　Rails　which；　each　in　its　island；　have　lost　the　power　of　flight；　a　process　which　must　have　taken　place　so　recently　that　it　is　difficult；　upon　morphological　grounds；　to　justify　their　separation　into　Aphanapteryx　in　Mauritius；　Erythromachus　in　Rodriguez　and　Diaphorapteryx　on　Chatham　Island。　　Morphologically　they　may　well　form　but　one　genus；　since　they　have　sprung　from　the　same　stock　and　have　developed　upon　the　same　lines；　they　are　therefore　monogenetic：　　but　since　we　know　that　they　have　become　what　they　are　independently　of　each　other　（now　unlike　any　other　Rails）；　they　are　polygenetic　and　therefore　could　not　form　one　genus　in　the　old　Darwinian　sense。　　Further；　they　are　not　a　case　of　convergence；　since　their　ancestry　is　not　divergent　but　leads　into　the　same　stratum。

THE　RECONSTRUCTION　OF　THE　GEOGRAPHY　OF　SUCCESSIVE　EPOCHS。

A　promising　method　is　the　study　by　the　specialist　of　a　large；　widely　distributed　group　of　animals　from　an　evolutionary　point　of　view。　　Good　examples　of　this　method　are　afforded　by　A。E。　Ortmann's　（〃The　geographical　distribution　of　Freshwater　Decapods　and　its　bearing　upon　ancient　geography〃；　〃Proc。　Amer。　Phil。　Soc。〃　Vol。　41；　1902。）　exhaustive　paper　and　by　A。W。　Grabau's　〃Phylogeny　of　Fusus　and　its　Allies〃　（〃Smithsonian　Misc。　Coll。〃　44；　1904。）　　After　many　important　groups　of　animals　have　been　treated　in　this　wayas　yet　sparingly　attemptedthe　results　as　to　hypothetical　land…connections　etc。　are　sure　to　be　corrective　and　supplementary；　and　their　problems　will　be　solved；　since　they　are　not　imaginary。

The　same　problems　are　attacked；　in　the　reverse　way；　by　starting　with　the　whole　fauna　of　a　country　and　thence；　so　to　speak；　letting　the　research　radiate。　　Some　groups　will　be　considered　as　autochthonous；　others　as　immigrants；　and　the　directions　followed　by　them　will　be　inquired　into；　the　search　may　lead　far　and　in　various　directions；　and　by　comparison　of　results；　by　making　compound　maps；　certain　routes　will　assume　definite　shape；　and　if　they　lead　across　straits　and　seas　they　are　warrants　to　search　for　land…connections　in　the　past。　　（A　fair　sample　of　this　method　is　C。H。　Eigenmann's　〃The　Freshwater　Fishes　of　South　and　Middle　America〃；　〃Popular　Science　Monthly〃；　Vol。　68；　1906。）　　There　are　now　not　a　few　maps　purporting　to　show　the　outlines　of　land　and　water　at　various　epochs。　　Many　of　these　attempts　do　not　tally　with　each　other；　owing　to　the　lamentable　deficiencies　of　geological　and　fossil　data；　but　the　bolder　the　hypothetical　outlines　are　drawn；　the　better；　and　this　is　preferable　to　the　insertion　of　bays　and　similar　detail　which　give　such　maps　a　fallacious　look　of　certainty　where　none　exists。　　Moreover　it　must　be　borne　in　mind　that；　when　we　draw　a　broad　continental　belt　across　an　ocean；　this　belt　need　never　have　existed　in　its　entirety　at　any　one　time。　　The　features　of　dispersal；　intended　to　be　explained　by　it；　would　be　accomplished　just　as　well　by　an　unknown　number　of　islands　which　have　joined　into　larger　complexes　while　elsewhere　they　subsided　again：　　like　pontoon…bridges　which　may　be　opened　anywhere；　or　like　a　series　of　superimposed　dissolving　views　of　land　and　sea…scapes。　　Hence　the　reconstructed　maps　of　Europe；　the　only　continent　tolerably　known；　show　a　considerable　number　of　islands　in　puzzling　changes；　while　elsewhere；　e。g。　in　Asia；　we　have　to　be　satisfied　with　sweeping　generalisations。

At　present　about　half…a…dozen　big　connections　are　engaging　our　attention；　leaving　as　comparatively　settled　the　extent　and　the　duration　of　such　minor　〃bridges〃　as　that　between　Africa　and　Madagascar；　Tasmania　and　Australia；　the　Antilles　and　Central　America；　Europe　and　North　Africa。　　（Not　a　few　of　those　who　are　fascinated　by；　and　satisfied　with；　the　statistical　aspect　of　distribution　still　have　a　strong　dislike　to　the　use　of　〃bridges〃　if　these　lead　over　deep　seas；　and　they　get　over　present　discontinuous　occurrences　by　a　former　〃universal　or　sub…universal　distribution〃　of　their　groups。　　This　is　indeed　an　easy　method　of　cutting　the　knot；　but　in　reality　they　shunt　the　question　only　a　stage　or　two　back；　never　troubling　to　explain　how　their　groups　managed　to　attain　to　that　sub…universal　range；　or　do　they　still　suppose　that　the　whole　world　was　originally　one　paradise　where　everything　lived　side　by　side；　until　sin　and　strife　and　glacial　epochs　left　nothing　but　scattered　survivors？

The　permanence　of　the　great　ocean…basins　had　become　a　dogma　since　it　was　found　that　a　universal　elevation　of　the　land　to　the　extent　of　100　fathoms　would　produce　but　little　changes；　and　when　it　was　shown　that　even　the　1000　fathom…line　followed　the　great　masses　of　land　rather　closely；　and　still　leaving　the　great　basins　（although　transgression　of　the　sea　to　the　same　extent　would　change　the　map　of　the　world　beyond　recognition）；　by　general　consent　one　mile　was　allowed　as　the　utmost　speculative　limit　of　subsidence。　Naturally　two　or　three　miles；　the　average　depth　of　the　oceans；　seems　enormous；　and　yet　such　a　difference　in　level　is　as　nothing　in　comparison　with　the　size　of　the　Earth。　　On　a　clay　model　globe　ten　feet　in　diameter　an　ocean　bed　three　miles　deep　would　scarcely　be　detected；　and　the　highest　mountains　would　be　smaller　than　the　unavoidable　grains　in　the　glazed　surface　of　our　model。　　There　are　but　few　countries　which　have　not　be　submerged　at　some　time　or　other。）

CONNECTION　OF　SOUTH　EASTERN　ASIA　WITH　AUSTRALIA。　　Neumayr's　Sino…Australian　continent　during　mid…Mesozoic　times　was　probably　a　much　changing　Archipelago；　with　final　separations　subsequent　to　the　Cretaceous　period。　　Henceforth　Australasia　was　left　to　its　own　fate；　but　for　a　possible　connection　with　the　antarctic　continent。

AFRICA；　MADAGASCAR；　INDIA。　　The　〃Lemuria〃　of　Sclater　and　Haeckel　cannot　have　been　more　than　a　broad　bridge　in　Jurassic　times；　whether　it　was　ever　available　for　the　Lemurs　themselves　must　depend　upon　the　time　of　its　duration；　the　more　recent　the　better；　but　it　is　difficult　to　show　that　it　lasted　into　the　Miocene。

AFRICA　AND　SOUTH　AMERICA。　　Since　the　opposite　coasts　show　an　entire　absence　of　marine　fossils　and　deposits　during　the　Mesozoic　period；　whilst　further　north　and　south　such　are　known　to　exist　and　are　mostly　identical　on　either　side；　Neumayr　suggested　the　existence　of　a　great　Afro…Son　American　mass　of　land　during　the　Jurassic　epoch。　　Such　land　is　almost　a　necessity　and　is　supported　by　many　facts；　it　would　easily　explain　the　distribution　of　numerous　groups　of　terrestrial　creatures。　　Moreover　to　the　north　of　this　hypothetical　land；　somewhere　across　from　the　Antilles　and　Guiana　to　North　Africa　and　South　Western　Europe；　existed　an　almost　identical　fauna　of　Corals　and　Molluscs；　indicating　either　a　coast…line　or　a　series　of　islands　interrupted　by　shallow　seas；　just　as　one　would　expect　if；　and　when；　a　Brazil…Ethiopian　mass　of　land　were