darwin and modern science-第98节

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　in　the　Western　Pacific；　and　a　disbelief　in　any　previous　continental　condition　was　expressed。　　My　later　view　is　more　in　accordance　with　that　of　Wichmann；　who；　on　geological　grounds；　contended　that　the　islands　of　the　Western　Pacific　were　in　a　continental　condition　during　the　Palaeozoic　and　Mesozoic　periods；　and　that　their　submergence　and　subsequent　emergence　took　place　in　Tertiary　times。〃　　（Guppy；　op。　cit。　II。　page　304。）

The　weight　of　the　geological　evidence　I　am　unable　to　scrutinise。　　But　though　I　must　admit　the　possibility　of　some　unconscious　bias　in　my　own　mind　on　the　subject；　I　am　impressed　with　the　fact　that　the　known　distribution　of　the　Glossopteris　flora　in　the　southern　hemisphere　is　precisely　paralleled　by　that　of　Proteaceae　and　Restiaceae　in　it　at　the　present　time。　　It　is　not　unreasonable　to　suppose　that　both　phenomena；　so　similar；　may　admit　of　the　same　explanation。　　I　confess　it　would　not　surprise　me　if　fresh　discoveries　in　the　distribution　of　the　Glossopteris　flora　were　to　point　to　the　possibility　of　its　also　having　migrated　southwards　from　a　centre　of　origin　in　the　northern　hemisphere。

Darwin；　however；　remained　sceptical　〃about　the　travelling　of　plants　from　the　north　EXCEPT　DURING　THE　TERTIARY　PERIOD。〃　　But　he　added；　〃such　speculations　seem　to　me　hardly　scientific；　seeing　how　little　we　know　of　the　old　floras。〃　　（〃Life　and　Letters〃；　III。　page　247。）　　That　in　later　geological　times　the　south　has　been　the　grave　of　the　weakened　offspring　of　the　aggressive　north　can　hardly　be　doubted。　　But　if　we　look　to　the　Glossopteris　flora　for　the　ancestry　of　Angiosperms　during　the　Secondary　period；　Darwin's　prevision　might　be　justified；　though　he　has　given　us　no　clue　as　to　how　he　arrived　at　it。

It　may　be　true　that　technically　Darwin　was　not　a　botanist。　　But　in　two　pages　of　the　〃Origin〃　he　has　given　us　a　masterly　explanation　of　〃the　relationship；　with　very　little　identity；　between　the　productions　of　North　America　and　Europe。〃　　（Pages　333；　334。）　　He　showed　that　this　could　be　accounted　for　by　their　migration　southwards　from　a　common　area；　and　he　told　Wallace　that　he　〃doubted　much　whether　the　now　called　Palaearctic　and　Neartic　regions　ought　to　be　separated。〃　　（〃Life　and　Letters〃；　III。　page　230。）　　Catkin…bearing　deciduous　trees　had　long　been　seen　to　justify　Darwin's　doubt：　　oaks；　chestnuts；　beeches；　hazels；　hornbeams；　birches；　alders；　willows　and　poplars　are　common　both　to　the　Old　and　New　World。　　Newton　found　that　the　separate　regions　could　not　be　sustained　for　birds；　and　he　is　now　usually　followed　in　uniting　them　as　the　Holartic。　　One　feels　inclined　to　say　in　reading　the　two　pages；　as　Lord　Kelvin　did　to　a　correspondent　who　asked　for　some　further　development　of　one　of　his　papers；　It　is　all　there。　　We　have　only　to　apply　the　principle　to　previous　geological　ages　to　understand　why　the　flora　of　the　Southern　United　States　preserves　a　Cretaceous　facies。　　Applying　it　still　further　we　can　understand　why；　when　the　northern　hemisphere　gradually　cooled　through　the　Tertiary　period；　the　plants　of　the　Eocene　〃suggest　a　comparison　of　the　climate　and　forests　with　those　of　the　Malay　Archipelago　and　Tropical　America。〃　　（Clement　Reid；　〃Encycl。　Brit。〃　（10th　edition）；　Vol。　XXXI。　　（〃Palaeobotany；　Tertiary〃）；　page　435。）　　Writing　to　Asa　Gray　in　1856　with　respect　to　the　United　States　flora；　Darwin　said　that　〃nothing　has　surprised　me　more　than　the　greater　generic　and　specific　affinity　with　East　Asia　than　with　West　America。〃　　（〃More　Letters〃；　I。　page　434。）　　The　recent　discoveries　of　a　Tulip　tree　and　a　Sassafras　in　China　afford　fresh　illustrations。　　A　few　years　later　Asa　Gray　found　the　explanation　in　both　areas　being　centres　of　preservation　of　the　Cretaceous　flora　from　a　common　origin。　　It　is　interesting　to　note　that　the　paper　in　which　this　was　enunciated　at　once　established　his　reputation。

In　Europe　the　latitudinal　range　of　the　great　mountain　chains　gave　the　Miocene　flora　no　chance　of　escape　during　the　Glacial　period；　and　the　Mediterranean　appears　to　have　equally　intercepted　the　flow　of　alpine　plants　to　the　Atlas。　　（John　Ball　in　Appendix　G；　page　438；　in　〃Journal　of　a　Tour　in　Morocco　and　the　Great　Atlas〃；　J。D。　Hooker　and　J。　Ball；　London；　1878。）　　In　Southern　Europe　the　myrtle；　the　laurel；　the　fig　and　the　dwarf…palm　are　the　sole　representatives　of　as　many　great　tropical　families。　　Another　great　tropical　family；　the　Gesneraceae　has　left　single　representatives　from　the　Pyrenees　to　the　Balkans；　and　in　the　former　a　diminutive　yam　still　lingers。　These　are　only　illustrations　of　the　evidence　which　constantly　accumulates　and　which　finds　no　rational　explanation　except　that　which　Darwin　has　given　to　it。

The　theory　of　southward　migration　is　the　key　to　the　interpretation　of　the　geographical　distribution　of　plants。　　It　derived　enormous　support　from　the　researches　of　Heer　and　has　now　become　an　accepted　commonplace。　　Saporta　in　1888　described　the　vegetable　kingdom　as　〃emigrant　pour　suivre　une　direction　determinee　et　marcher　du　nord　au　sud；　a　la　recherche　de　regions　et　de　stations　plus　favorables；　mieux　appropriees　aux　adaptations　acquises；　a　meme　que　la　temperature　terrestre　perd　ses　conditions　premieres。〃　　（〃Origine　Paleontologique　des　arbres〃；　Paris；　1888；　page　28。）　　If；　as　is　so　often　the　case；　the　theory　now　seems　to　be　a　priori　inevitable；　the　historian　of　science　will　not　omit　to　record　that　the　first　germ　sprang　from　the　brain　of　Darwin。

In　attempting　this　sketch　of　Darwin's　influence　on　Geographical　Distribution；　I　have　found　it　impossible　to　treat　it　from　an　external　point　of　view。　　His　interest　in　it　was　unflagging；　all　I　could　say　became　necessarily　a　record　of　that　interest　and　could　not　be　detached　from　it。　　He　was　in　more　or　less　intimate　touch　with　everyone　who　was　working　at　it。　In　reading　the　letters　we　move　amongst　great　names。　　With　an　extraordinary　charm　of　persuasive　correspondence　he　was　constantly　suggesting；　criticising　and　stimulating。　　It　is　hardly　an　exaggeration　to　say　that　from　the　quiet　of　his　study　at　Down　he　was　founding　and　directing　a　wide…world　school。

POSTSCRIPTUM。

Since　this　essay　was　put　in　type　Dr　Ernst's　striking　account　of　the　〃New　Flora　of　the　Volcanic　Island　of　Krakatau〃　（Cambridge；　1909。）　has　reached　me。　　All　botanists　must　feel　a　debt　of　gratitude　to　Prof。　Seward　for　his　admirable　translation　of　a　memoir　which　in　its　original　form　is　practically　unprocurable　and　to　the　liberality　of　the　Cambridge　University　Press　for　its　publication。　　In　the　preceding　pages　I　have　traced　the　laborious　research　by　which　the　methods　of　Plant　Dispersal　were　established　by　Darwin。　　In　the　island　of　Krakatau　nature　has　supplied　a　crucial　experiment　which；　if　it　had　occurred　earlier；　would　have　at　once　secured　conviction　of　their　efficiency。　　A　quarter　of　a　century　ago　every　trace　of　organic　life　in　the　island　was　〃destroyed　and　buried　under　a　thick　covering　of　glowing　stones。〃　　Now；　it　is　〃again　covered　with　a　mantle　of　green；　the　growth　being　in　places　so　luxuriant　that　it　is　necessary　to　cut　one's　way　laboriously　through　the　vegetation。〃　　（Op。　cit。　page　4。）　　Ernst　traces　minutely　how　this　has　been　brought　about　by　the　combined　action　of　wind；　birds　and　sea　currents；　as　means　of　transport。　　The　process　will　continue；　and　he　concludes：〃At　last　after　a　long　interval　the　vegetation　on　the　desolated　island　will　again　acquire　that　wealth　of　variety　and　luxuriance　which　we　see　in　the　fullest　development　which　Nature　has　reached　in　the　primaeval　forest　in　the　tropics。〃　　（Op。　cit。　page　72。）　　The　possibility　of　such　a　result　revealed　itself　to　the　insight　of　Darwin　with　little　encouragement　or　support　from　contemporary　opinion。

One　of　the　most　remarkable　facts　established　by　Ernst　is　that　this　has　not　been　accomplished　by　the　transport　of　seeds　alone。　　〃Tree　stems　and　branches　played　an　important　part　in　the　colonisation　of　Krakatau　by　plants　and　animals。　　Large　piles　of　floating　trees；　stems；　branches　and　bamboos　are　met　with　everywhere　on　the　beach　above　high…water　mark　and　often　carried　a　considerable　distance　inland。　　Some　of　the　animals　on　the　island；　such　as　the　fat　Iguana　（Varanus　salvator）　which　suns　itself　in　the　beds　of　streams；　may　have　travelled　on　floating　wood；　possibly　also　the　ancestors　of　the　numerous　ants；　but　certainly　plants。〃　　（Op。　cit。　page　56。）　　Darwin　actually　had　a　prevision　of　this。　　Writing　to　Hooker　he　says：〃Would　it　not　be　a　prodigy　if　an　unstocked　island　did　not　in　the　course　of　ages　receive　colonists　from　coasts　whence　the　currents　flow；　trees　are　drifted　and　birds　are　driven　by　gales？〃　　（〃More　Letters〃；　I。　page　483。）　　And　ten　years　earlier：〃I　must　believe　in　the。。。whole　plant　or　branch　being　washed　into　the　sea；　with　floods　and　slips　and　earthquakes；　this　must　continually　be　happening。〃　　（〃Life　and　Letters〃；　II。　pages　56；　57。）　　If　we　give　to　〃continually〃　a　cosmic　measure；　can　the　fact　be　doubted？　　All　this；　in　the　light　of　our　present　knowledge；　is　too　obvious　to　us　to　admit　of　discussion。　　But　it　seems　to　me　nothing　less　than　pathetic　to　see　how　in　the　teeth　of　the　obsession　as　to　continental　extension；　Darwin　fought　single…handed　for　what　we　now　know　to　be　the　truth。

Guppy's　heart　failed　him　when　he　had　to　deal　with　the　isolated　case　of　Agathis　which　alone　seemed　inexplicable　by　known　means　of　transport。　　But　when　we