darwin and modern science-第86部分

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　had　been　set　down　for　reading　at　the　Linnean　Society　on　the　same　day　as　Darwin's；　but　had　to　give　way。　　In　this　he　advocated　the　fixity　of　species。　　He　withdrew　it　after　hearing　Darwin's。　　We　can　hardly　realise　now　the　momentous　effect　on　the　scientific　thought　of　the　day　of　the　announcement　of　the　new　theory。　　Years　afterwards　（1882）　Bentham；　notwithstanding　his　habitual　restraint；　could　not　write　of　it　without　emotion。　　〃I　was　forced；　however　reluctantly；　to　give　up　my　long…cherished　convictions；　the　results　of　much　labour　and　study。〃　　The　revelation　came　without　preparation。　　Darwin；　he　wrote；　〃never　made　any　communications　to　me　in　relation　to　his　views　and　labours。〃　　But；　he　adds；　I。。。fully　adopted　his　theories　and　conclusions；　notwithstanding　the　severe　pain　and　disappointment　they　at　first　occasioned　me。〃　　（〃Life　and　Letters〃；　II。　page　294。）　　Scientific　history　can　have　few　incidents　more　worthy。　　I　do　not　know　what　is　most　striking　in　the　story；　the　pathos　or　the　moral　dignity　of　Bentham's　attitude。

Darwin　necessarily　restricted　himself　in　the　〃Origin〃　to　establishing　the　general　principles　which　would　account　for　the　facts　of　distribution；　as　a　part　of　his　larger　argument；　without　attempting　to　illustrate　them　in　particular　cases。　　This　he　appears　to　have　contemplated　doing　in　a　separate　work。　　But　writing　to　Hooker　in　1868　he　said：〃I　shall　to　the　day　of　my　death　keep　up　my　full　interest　in　Geographical　Distribution；　but　I　doubt　whether　I　shall　ever　have　strength　to　come　in　any　fuller　detail　than　in　the　〃Origin〃　to　this　grand　subject。〃　　（〃More　Letters〃；　II。　page　7。）　　This　must　be　always　a　matter　for　regret。　　But　we　may　gather　some　indication　of　his　later　speculations　from　the　letters；　the　careful　publication　of　which　by　F。　Darwin　has　rendered　a　service　to　science；　the　value　of　which　it　is　difficult　to　exaggerate。　　They　admit　us　to　the　workshop；　where　we　see　a　great　theory；　as　it　were；　in　the　making。　　The　later　ideas　that　they　contain　were　not　it　is　true　public　property　at　the　time。　　But　they　were　communicated　to　the　leading　biologists　of　the　day　and　indirectly　have　had　a　large　influence。

If　Darwin　laid　the　foundation；　the　present　fabric　of　Botanical　Geography　must　be　credited　to　Hooker。　　It　was　a　happy　partnership。　　The　far…seeing；　generalising　power　of　the　one　was　supplied　with　data　and　checked　in　conclusions　by　the　vast　detailed　knowledge　of　the　other。　　It　may　be　permitted　to　quote　Darwin's　generous　acknowledgment　when　writing　the　〃Origin〃：〃I　never　did　pick　any　one's　pocket；　but　whilst　writing　my　present　chapter　I　keep　on　feeling　（even　when　differing　most　from　you）　just　as　if　I　were　stealing　from　you；　so　much　do　I　owe　to　your　writings　and　conversation；　so　much　more　than　mere　acknowledgements　show。〃　　（〃Life　and　Letters〃；　II。　page　148　（footnote）。）　　Fourteen　years　before　he　had　written　to　Hooker：　　〃I　know　I　shall　live　to　see　you　the　first　authority　in　Europe　on。。。Geographical　Distribution。〃　　（Ibid。　I。　page　336。）　　We　owe　it　to　Hooker　that　no　one　now　undertakes　the　flora　of　a　country　without　indicating　the　range　of　the　species　it　contains。　　Bentham　tells　us：　　〃After　De　Candolle；　independently　of　the　great　works　of　Darwin。。。the　first　important　addition　to　the　science　of　geographical　botany　was　that　made　by　Hooker　in　his　〃Introductory　Essay　to　the　Flora　of　Tasmania〃；　which；　though　contemporaneous　only　with　the　〃Origin　of　Species〃；　was　drawn　up　with　a　general　knowledge　of　his　friend's　observations　and　views。〃　　（Pres。　Addr。　（1869）；　〃Proc。　Linn。　Soc。〃　1868…69；　page　lxxiv。）　　It　cannot　be　doubted　that　this　and　the　great　memoir　on　the　〃Distribution　of　Arctic　Plants〃　were　only　less　epoch…making　than　the　〃Origin〃　itself；　and　must　have　supplied　a　powerful　support　to　the　general　theory　of　organic　evolution。

Darwin　always　asserted　his　〃entire　ignorance　of　Botany。〃　　（〃More　Letters〃；　I。　page　400。）　　But　this　was　only　part　of　his　constant　half…humorous　self…　depreciation。　　He　had　been　a　pupil　of　Henslow；　and　it　is　evident　that　he　had　a　good　working　knowledge　of　systematic　botany。　　He　could　find　his　way　about　in　the　literature　and　always　cites　the　names　of　plants　with　scrupulous　accuracy。　　It　was　because　he　felt　the　want　of　such　a　work　for　his　own　researches　that　he　urged　the　preparation　of　the　〃Index　Kewensis〃；　and　undertook　to　defray　the　expense。　　It　has　been　thought　singular　that　he　should　have　been　elected　a　〃correspondant〃　of　the　Academie　des　Sciences　in　the　section　of　Botany；　but　it　is　not　surprising　that　his　work　in　Geographical　Botany　made　the　botanists　anxious　to　claim　him。　　His　heart　went　with　them。　　〃It　has　always　pleased　me；〃　he　tells　us；　〃to　exalt　plants　in　the　scale　of　organised　beings。〃　　（〃Life　and　Letters〃；　I。　page　98。）　　And　he　declares　that　he　finds　〃any　proposition　more　easily　tested　in　botanical　works　（Ibid。　II。　page　99。）　than　in　zoological。〃

In　the　〃Introductory　Essay〃　Hooker　dwelt　on　the　〃continuous　current　of　vegetation　from　Scandinavia　to　Tasmania〃　（〃Introductory　Essay　to　the　Flora　of　Tasmania〃；　London；　1859。　　Reprinted　from　the　〃Botany　of　the　Antarctic　Expedition〃；　Part　III。；　〃Flora　of　Tasmania〃；　Vol　I。　page　ciii。）；　but　finds　little　evidence　of　one　in　the　reverse　direction。　　〃In　the　New　World；　Arctic；　Scandinavian；　and　North　American　genera　and　species　are　continuously　extended　from　the　north　to　the　south　temperate　and　even　Antarctic　zones；　but　scarcely　one　Antarctic　species；　or　even　genus　advances　north　beyond　the　Gulf　of　Mexico〃　（page　civ。）。　　Hooker　considered　that　this　negatived　〃the　idea　that　the　Southern　and　Northern　Floras　have　had　common　origin　within　comparatively　modern　geological　epochs。〃　　（Loc。　cit。）　　This　is　no　doubt　a　correct　conclusion。　　But　it　is　difficult　to　explain　on　Darwin's　view　alone；　of　alternating　cold　in　the　two　hemispheres；　the　preponderant　migration　from　the　north　to　the　south。　　He　suggests；　therefore；　that　it　〃is　due　to　the　greater　extent　of　land　in　the　north　and　to　the　northern　forms。。。having。。。been　advanced　through　natural　selection　and　competition　to　a　higher　stage　of　perfection　or　dominating　power。〃　　（〃Origin　of　Species〃　（6th　edition）；　page　340；　cf。　also　〃Life　and　Letters〃；　II。　page　142。）　　The　present　state　of　the　Flora　of　New　Zealand　affords　a　striking　illustration　of　the　correctness　of　this　view。　　It　is　poor　in　species；　numbering　only　some　1400；　of　which　three…fourths　are　endemic。　　They　seem　however　quite　unable　to　resist　the　invasion　of　new　comers　and　already　600　species　of　foreign　origin　have　succeeded　in　establishing　themselves。

If　we　accept　the　general　configuration　of　the　earth's　surface　as　permanent　a　continuous　and　progressive　dispersal　of　species　from　the　centre　to　the　circumference；　i。e。　southwards；　seems　inevitable。　　If　an　observer　were　placed　above　a　point　in　St　George's　Channel　from　which　one　half　of　the　globe　was　visible　he　would　see　the　greatest　possible　quantity　of　land　spread　out　in　a　sort　of　stellate　figure。　　The　maritime　supremacy　of　the　English　race　has　perhaps　flowed　from　the　central　position　of　its　home。　　That　such　a　disposition　would　facilitate　a　centrifugal　migration　of　land　organisms　is　at　any　rate　obvious；　and　fluctuating　conditions　of　climate　operating　from　the　pole　would　supply　an　effective　means　of　propulsion。　　As　these　became　more　rigorous　animals　at　any　rate　would　move　southwards　to　escape　them。　　It　would　be　equally　the　case　with　plants　if　no　insuperable　obstacle　interposed。　　This　implies　a　mobility　in　plants；　notwithstanding　what　we　know　of　means　of　transport　which　is　at　first　sight　paradoxical。　　Bentham　has　stated　this　in　a　striking　way：　　〃Fixed　and　immovable　as　is　the　individual　plant；　there　is　no　class　in　which　the　race　is　endowed　with　greater　facilities　for　the　widest　dispersion。。。Plants　cast　away　their　offspring　in　a　dormant　state；　ready　to　be　carried　to　any　distance　by　those　external　agencies　which　we　may　deem　fortuitous；　but　without　which　many　a　race　might　perish　from　the　exhaustion　of　the　limited　spot　of　soil　in　which　it　is　rooted。〃　　（Pres。　Addr。（1869）；　〃Proc。　Linn。　Soc。〃　1868…69；　pages　lxvi；　lxvii。）

I　have　quoted　this　passage　from　Bentham　because　it　emphasises　a　point　which　Darwin　for　his　purpose　did　not　find　it　necessary　to　dwell　upon；　though　he　no　doubt　assumed　it。　　Dispersal　to　a　distance　is；　so　to　speak；　an　accidental　incident　in　the　life　of　a　species。　　Lepidium　Draba；　a　native　of　South…eastern　Europe；　owes　its　prevalence　in　the　Isle　of　Thanet　to　the　disastrous　Walcheren　expedition；　the　straw…stuffing　of　the　mattresses　of　the　fever…stricken　soldiers　who　were　landed　there　was　used　by　a　farmer　for　manure。　　Sir　Joseph　Hooker　（〃Royal　Institution　Lecture〃；　April　12；　1878。）　tells　us　that　landing　on　Lord　Auckland's　Island；　which　was　uninhabited；　〃the　first　evidence　I　met　with　of　its　having　been　previously　visited　by　man　was　the　English　chickweed；　and　this　I　traced　to　a　mound　that　marked　the　grave　of　a　British　sailor；　and　that　was　covered　with　the　plant；　doubtless　the　offspring　of　seed　that　had　adhered　to　the　spade　or　mattock　with　which　the　grave　had　been　dug。〃

Some　migration　from　the　spot　where　the　individuals　of　a　species　have　germinated　is　an　essential　provision　against　extinction。　　Their　descendants　otherwise　would　be　liable　to　suppression　by　more　vigorous　competitors。　　But　they　would　eventually　be　extinguished　inevitably；　as　pointed　out　by　Bentham；　by　the　exhaustion　of　at　any　rate　some　one　necessary　constituent　of　the　soil。　　Gilbert　showed　by　actual　analysis　that　the　production　of　a　〃fairy　ring〃　is　simply　due　to　the　using　up　by　the　fungi　of　the　available　nitrogen　in　the　enclosed　area　which　continually　enlarges　as　they　seek　a　fresh　supply　on　the　outside　margin。　　Anyone　who　cultivates　a　garden　can　easily　verify　the　fact　that　every　plant　has　some　adaptation　for　varying　degrees　of　seed…dispersal。　　It　cannot　be　doubted　that　slow　but　persistent　terrestrial　migration　has　played　an　enormous　part　in　bringing　about　existing　plant…distribution；　or　that　climatic　changes　would　intensify　the　effect　because　they　would　force　the　abandonment　of　a　former　area　and　the　occupation　of　a　new　one。　　We　are　compelled　to　admit　that　as　an　incident　of　the　Glacial　period　a　whole　flora　may　have　moved　down　and　up　a　mountain　side；　while　only　some　of　its　constituent　species　would　be　able　to　take　advantage　of　means　of　long…distance　transport。

I　have　dwelt　on　the　impor