darwin and modern science-第97节

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e　evidence　to　support　it；　and　it　immediately　raises　the　difficulty　how　did　such　a　continent　come　to　be　stocked？

We　must；　however；　agree　with　Hooker　that　the　common　origin　of　the　northern　and　southern　floras　must　be　referred　to　a　remote　past。　　That　Darwin　had　this　in　his　mind　at　the　time　of　the　publication　of　the　〃Origin〃　is　clear　from　a　letter　to　Hooker。　　〃The　view　which　I　should　have　looked　at　as　perhaps　most　probable　（though　it　hardly　differs　from　yours）　is　that　the　whole　world　during　the　Secondary　ages　was　inhabited　by　marsupials；　araucarias　（Mem。Fossil　wood　of　this　nature　in　South　America）；　Banksia；　etc。；　and　that　these　were　supplanted　and　exterminated　in　the　greater　area　of　the　north；　but　were　left　alive　in　the　south。〃　　（Ibid。　I。　page　453。）　　Remembering　that　Araucaria；　unlike　Banksia；　belongs　to　the　earlier　Jurassic　not　to　the　angiospermous　flora；　this　view　is　a　germinal　idea　of　the　widest　generality。

The　extraordinary　congestion　in　species　of　the　peninsulas　of　the　Old　World　points　to　the　long…continued　action　of　a　migration　southwards。　　Each　is　in　fact　a　cul…de…sac　into　which　they　have　poured　and　from　which　there　is　no　escape。　　On　the　other　hand　the　high　degree　of　specialisation　in　the　southern　floras　and　the　little　power　the　species　possess　of　holding　their　own　in　competition　or　in　adaptation　to　new　conditions　point　to　long…　continued　isolation。　　〃An　island。。。will　prevent　free　immigration　and　competition；　hence　a　greater　number　of　ancient　forms　will　survive。〃　　（Ibid。　I。　page　481。）　　But　variability　is　itself　subject　to　variation。　　The　nemesis　of　a　high　degree　of　protected　specialisation　is　the　loss　of　adaptability。　　（See　Lyell；　〃The　Geological　Evidences　of　the　Antiquity　of　Man〃；　London；　1863；　page　446。）　　It　is　probable　that　many　elements　of　the　southern　flora　are　doomed：　　there　is；　for　example；　reason　to　think　that　the　singular　Stapelieae　of　S。　Africa　are　a　disappearing　group。　　The　tree　Lobelias　which　linger　in　the　mountains　of　Central　Africa；　in　Tropical　America　and　in　the　Sandwich　Islands　have　the　aspect　of　extreme　antiquity。　　I　may　add　a　further　striking　illustration　from　Professor　Seward：　　〃The　tall；　graceful　fronds　of　Matonia　pectinata；　forming　miniature　forests　on　the　slopes　of　Mount　Ophir　and　other　districts　in　the　Malay　Peninsula　in　association　with　Dipteris　conjugata　and　Dipteris　lobbiana；　represent　a　phase　of　Mesozoic　life　which　survives　

'Like　a　dim　picture　of　the　drowned　past。'〃　（〃Report　of　the　73rd　Meeting　of　the　British　Assoc。〃　　（Southport；　1903）；　London；　1904；　page　844。）

The　Matonineae　are　ferns　with　an　unusually　complex　vascular　system　and　were　abundant　〃in　the　northern　hemisphere　during　the　earlier　part　of　the　Mesozoic　era。〃

It　was　fortunate　for　science　that　Wallace　took　up　the　task　which　his　colleague　had　abandoned。　　Writing　to　him　on　the　publication　of　his　〃Geographical　Distribution　of　Animals〃　Darwin　said：　　〃I　feel　sure　that　you　have　laid　a　broad　and　safe　foundation　for　all　future　work　on　Distribution。　How　interesting　it　will　be　to　see　hereafter　plants　treated　in　strict　relation　to　your　views。〃　　（〃More　Letters〃；　II。　page　12。）　　This　hope　was　fulfilled　in　〃Island　Life〃。　　I　may　quote　a　passage　from　it　which　admirably　summarises　the　contrast　between　the　northern　and　the　southern　floras。

〃Instead　of　the　enormous　northern　area；　in　which　highly　organised　and　dominant　groups　of　plants　have　been　developed　gifted　with　great　colonising　and　aggressive　powers；　we　have　in　the　south　three　comparatively　small　and　detached　areas；　in　which　rich　floras　have　been　developed　with　SPECIAL　adaptations　to　soil；　climate；　and　organic　environment；　but　comparatively　impotent　and　inferior　beyond　their　own　domain。〃　　（Wallace；　〃Island　Life〃；　pages　527；　528。）

It　will　be　noticed　that　in　the　summary　I　have　attempted　to　give　of　the　history　of　the　subject；　efforts　have　been　concentrated　on　bringing　into　relation　the　temperate　floras　of　the　northern　and　southern　hemispheres；　but　no　account　has　been　taken　of　the　rich　tropical　vegetation　which　belts　the　world　and　little　to　account　for　the　original　starting…point　of　existing　vegetation　generally。　　It　must　be　remembered　on　the　one　hand　that　our　detailed　knowledge　of　the　floras　of　the　tropics　is　still　very　incomplete　and　far　inferior　to　that　of　temperate　regions；　on　the　other　hand　palaeontological　discoveries　have　put　the　problem　in　an　entirely　new　light。　　Well　might　Darwin；　writing　to　Heer　in　1875；　say：　　〃Many　as　have　been　the　wonderful　discoveries　in　Geology　during　the　last　half…century；　I　think　none　have　exceeded　in　interest　your　results　with　respect　to　the　plants　which　formerly　existed　in　the　arctic　regions。〃　　（〃More　Letters〃；　II。　page　240。）

As　early　as　1848　Debey　had　described　from　the　Upper　Cretaceous　rocks　of　Aix…la…Chapelle　Flowering　plants　of　as　high　a　degree　of　development　as　those　now　existing。　　The　fact　was　commented　upon　by　Hooker　（〃Introd。　Essay　to　the　Flora　of　Tasmania〃；　page　xx。）；　but　its　full　significance　seems　to　have　been　scarcely　appreciated。　　For　it　implied　not　merely　that　their　evolution　must　have　taken　place　but　the　foundations　of　existing　distribution　must　have　been　laid　in　a　preceding　age。　　We　now　know　from　the　discoveries　of　the　last　fifty　years　that　the　remains　of　the　Neocomian　flora　occur　over　an　area　extending　through　30　deg　of　latitude。　　The　conclusion　is　irresistible　that　within　this　was　its　centre　of　distribution　and　probably　of　origin。

Darwin　was　immensely　impressed　with　the　outburst　on　the　world　of　a　fully　fledged　angiospermous　vegetation。　　He　warmly　approved　the　brilliant　theory　of　Saporta　that　this　happened　〃as　soon　（as）　flower…frequenting　insects　were　developed　and　favoured　intercrossing。〃　　（〃More　Letters〃；　II。　page　21。）　　Writing　to　him　in　1877　he　says：　　〃Your　idea　that　dicotyledonous　plants　were　not　developed　in　force　until　sucking　insects　had　been　evolved　seems　to　me　a　splendid　one。　　I　am　surprised　that　the　idea　never　occurred　to　me；　but　this　is　always　the　case　when　one　first　hears　a　new　and　simple　explanation　of　some　mysterious　phenomenon。〃　　（〃Life　and　Letters〃；　III。　page　285。　　Substantially　the　same　idea　had　occurred　earlier　to　F。W。A。　Miquel。　　Remarking　that　〃sucking　insects　（Haustellata）。。。perform　in　nature　the　important　duty　of　maintaining　the　existence　of　the　vegetable　kingdom；　at　least　as　far　as　the　higher　orders　are　concerned；〃　he　points　our　that　〃the　appearance　in　great　numbers　of　haustellate　insects　occurs　at　and　after　the　Cretaceous　epoch；　when　the　plants　with　pollen　and　closed　carpels　（Angiosperms）　are　found；　and　acquire　little　by　little　the　preponderance　in　the　vegetable　kingdom。〃　　〃Archives　Neerlandaises〃；　III。　（1868）。　　English　translation　in　〃Journ。　of　Bot。〃　1869；　page　101。）

Even　with　this　help　the　abruptness　still　remains　an　almost　insoluble　problem；　though　a　forecast　of　floral　structure　is　now　recognised　in　some　Jurassic　and　Lower　Cretaceous　plants。　　But　the　gap　between　this　and　the　structural　complexity　and　diversity　of　angiosperms　is　enormous。　　Darwin　thought　that　the　evolution　might　have　been　accomplished　during　a　period　of　prolonged　isolation。　　Writing　to　Hooker　（1881）　he　says：　　〃Nothing　is　more　extraordinary　in　the　history　of　the　Vegetable　Kingdom；　as　it　seems　to　me；　than　the　APPARENTLY　very　sudden　or　abrupt　development　of　the　higher　plants。　I　have　sometimes　speculated　whether　there　did　not　exist　somewhere　during　long　ages　an　extremely　isolated　continent；　perhaps　near　the　South　Pole。〃　　（〃Life　and　Letters〃；　III。　page　248。）

The　present　trend　of　evidence　is；　however；　all　in　favour　of　a　northern　origin　for　flowering　plants；　and　we　can　only　appeal　to　the　imperfection　of　the　geological　record　as　a　last　resource　to　extricate　us　from　the　difficulty　of　tracing　the　process。　　But　Darwin's　instinct　that　at　some　time　or　other　the　southern　hemisphere　had　played　an　important　part　in　the　evolution　of　the　vegetable　kingdom　did　not　mislead　him。　　Nothing　probably　would　have　given　him　greater　satisfaction　than　the　masterly　summary　in　which　Seward　has　brought　together　the　evidence　for　the　origin　of　the　Glossopteris　flora　in　Gondwana　land。

〃A　vast　continental　area；　of　which　remnants　are　preserved　in　Australia；　South　Africa　and　South　America。。。A　tract　of　enormous　extent　occupying　an　area；　part　of　which　has　since　given　place　to　a　southern　ocean；　while　detached　masses　persist　as　portions　of　more　modern　continents；　which　have　enabled　us　to　read　in　their　fossil　plants　and　ice…scratched　boulders　the　records　of　a　lost　continent；　in　which　the　Mesozoic　vegetation　of　the　northern　continent　had　its　birth。〃　　（〃Encycl。　Brit。〃　（10th　edition　1902）；　Vol。　XXXI。　（〃Palaeobotany；　Mesozoic〃）；　page　422。）　　Darwin　would　probably　have　demurred　on　physical　grounds　to　the　extent　of　the　continent；　and　preferred　to　account　for　the　transoceanic　distribution　of　its　flora　by　the　same　means　which　must　have　accomplished　it　on　land。

It　must　in　fairness　be　added　that　Guppy's　later　views　give　some　support　to　the　conjectural　existence　of　the　〃lost　continent。〃　　〃The　distribution　of　the　genus　Dammara〃　（Agathis）　led　him　to　modify　his　earlier　conclusions。　　He　tells　us：〃In　my　volume　on　the　geology　of　Vanua　Levu　it　was　shown　that　the　Tertiary　period　was　an　age　of　submergence　in　the　Western　Pacific；　and　a　disbelief　in　any　previous　continental　condition　was　expressed。　　My　la