darwin and modern science-第115章

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ey　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　passenger　who　has　been　carried　off　by　the　wrong　train。　　Almost　every　year　some　American　land　birds　arrive　at　our　western　coasts　and　none　of　them　have　gained　a　permanent　footing　although　such　visits　must　have　taken　place　since　prehistoric　times。　　It　was　therefore　argued　that　only　those　groups　of　animals　should　be　used　for　locating　and　defining　regions　which　were　absolutely　bound　to　the　soil。　　This　method　likewise　gave　results　not　reconcilable　with　each　other；　even　when　the　distribution　of　fossils　was　taken　into　account；　but　it　pointed　to　the　absolute　necessity　of　searching　for　former　land…connections　regardless　of　their　extent　and　the　present　depths　to　which　they　may　have　sunk。

That　the　key　to　the　present　distribution　lies　in　the　past　had　been　felt　long　ago；　but　at　last　it　was　appreciated　that　the　various　classes　of　animals　and　plants　have　appeared　in　successive　geological　epochs　and　also　at　many　places　remote　from　each　other。　　The　key　to　the　distribution　of　any　group　lies　in　the　configuration　of　land　and　water　of　that　epoch　in　which　it　made　its　first　appearance。　　Although　this　sounds　like　a　platitude；　it　has　frequently　been　ignored。　　If；　for　argument's　sake；　Amphibia　were　evolved　somewhere　upon　the　great　southern　land…mass　of　Carboniferous　times　（supposed　by　some　to　have　stretched　from　South　America　across　Africa　to　Australia）；　the　distribution　of　this　developing　class　must　have　proceeded　upon　lines　altogether　different　from　that　of　the　mammals　which　dated　perhaps　from　lower　Triassic　times；　when　the　old　south　continental　belt　was　already　broken　up。　　The　broad　lines　of　this　distribution　could　never　coincide　with　that　of　the　other；　older　class；　no　matter　whether　the　original　mammalian　centre　was　in　the　Afro…Indian；　Australian；　or　Brazilian　portion。　　If　all　the　various　groups　of　animals　had　come　into　existence　at　the　same　time　and　at　the　same　place；　then　it　would　be　possible；　with　sufficient　geological　data；　to　construct　a　map　showing　the　generalised　results　applicable　to　the　whole　animal　kingdom。　　But　the　premises　are　wrong。　　Whatever　regions　we　may　seek　to　establish　applicable　to　all　classes；　we　are　necessarily　mixing　up　several　principles；　namely　geological；　historical；　i。e。　evolutionary；　with　present　day　statistical　facts。　　We　might　as　well　attempt　one　compound　picture　representing　a　chick's　growth　into　an　adult　bird　and　a　child's　growth　into　manhood。

In　short　there　are　no　general　regions；　not　even　for　each　class　separately；　unless　this　class　be　one　which　is　confined　to　a　comparatively　short　geological　period。　　Most　of　the　great　classes　have　far　too　long　a　history　and　have　evolved　many　successive　main　groups。　　Let　us　take　the　mammals。　　Marsupials　live　now　in　Australia　and　in　both　Americas；　because　they　already　existed　in　Mesozoic　times；　Ungulata　existed　at　one　time　or　other　all　over　the　world　except　in　Australia；　because　they　are　post…Cretaceous；　Insectivores；　although　as　old　as　any　Placentalia；　are　cosmopolitan　excepting　South　America　and　Australia；　Stags　and　Bears；　as　examples　of　comparatively　recent　Arctogaeans；　are　found　everywhere　with　the　exception　of　Ethiopia　and　Australia。　　Each　of　these　groups　teaches　a　valuable　historical　lesson；　but　when　these　are　combined　into　the　establishment　of　a　few　mammalian　〃realms；〃　they　mean　nothing　but　statistical　majorities。　　If　there　is　one　at　all；　Australia　is　such　a　realm　backed　against　the　rest　of　the　world；　but　as　certainly　it　is　not　a　mammalian　creative　centre！

Well　then；　if　the　idea　of　generally　applicable　regions　is　a　mare's　nest；　as　was　the　search　for　the　Holy　Grail；　what　is　the　object　of　the　study　of　geographical　distribution？　　It　is　nothing　less　than　the　history　of　the　evolution　of　life　in　space　and　time　in　the　widest　sense。　　The　attempt　to　account　for　the　present　distribution　of　any　group　of　organisms　involves　the　aid　of　every　branch　of　science。　　It　bids　fair　to　become　a　history　of　the　world。　　It　started　in　a　mild；　statistical　way；　restricting　itself　to　the　present　fauna　and　flora　and　to　the　present　configuration　of　land　and　water。　　Next　came　Oceanography　concerned　with　the　depths　of　the　seas；　their　currents　and　temperatures；　then　enquiries　into　climatic　changes；　culminating　in　irreconcilable　astronomical　hypotheses　as　to　glacial　epochs；　theories　about　changes　of　the　level　of　the　seas；　mainly　from　the　point　of　view　of　the　physicist　and　astronomer。　　Then　came　more　and　more　to　the　front　the　importance　of　the　geological　record；　hand　in　hand　with　the　palaeontological　data　and　the　search　for　the　natural　affinities；　the　genetic　system　of　the　organisms。　　Now　and　then　it　almost　seems　as　if　the　biologists　had　done　their　share　by　supplying　the　problems　and　that　the　physicists　and　geologists　would　settle　them；　but　in　reality　it　is　not　so。　　The　biologists　not　only　set　the　problems；　they　alone　can　check　the　offered　solutions。　　The　mere　fact　of　palms　having　flourished　in　Miocene　Spitzbergen　led　to　an　hypothetical　shifting　of　the　axis　of　the　world　rather　than　to　the　assumption；　by　way　of　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　m