darwin and modern science-第95节

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acial　period　were　less　simple；　more　localised　and　less　general　than　he　perhaps　supposed。　　He　admitted　that　〃equatorial　refrigeration。。。must　have　been　small。〃　　（〃More　Letters〃；　I。　page　177。）　　It　may　prove　possible　to　dispense　with　it　altogether。　　One　cannot　but　regret　that　as　he　wrote　to　Bates：〃the　sketch　in　the　'Origin'　gives　a　very　meagre　account　of　my　fuller　MS。　essay　on　this　subject。〃　　（Loc。　cit。）　　Wallace　fully　accepted　〃the　effect　of　the　Glacial　epoch　in　bringing　about　the　present　distribution　of　Alpine　and　Arctic　plants　in　the　NORTHERN　HEMISPHERE；〃　but　rejected　〃the　lowering　of　the　temperature　of　the　tropical　regions　during　the　Glacial　period〃　in　order　to　account　for　their　presence　in　the　SOUTHERN　hemisphere。　　（〃More　Letters〃；　II。　page　25　（footnote　1）。）　　The　divergence　however　does　not　lie　very　deep。　　Wallace　attaches　more　importance　to　ordinary　means　of　transport。　　〃If　plants　can　pass　in　considerable　numbers　and　variety　over　wide　seas　and　oceans；　it　must　be　yet　more　easy　for　them　to　traverse　continuous　areas　of　land；　wherever　mountain…chains　offer　suitable　stations。〃　　（〃Island　Life〃　（2nd　edition）；　London；　1895；　page　512。）　　And　he　argues　that　such　periodical　changes　of　climate；　of　which　the　Glacial　period　may　be　taken　as　a　type；　would　facilitate　if　not　stimulate　the　process。　　（Loc。　cit。　page　518。）

It　is　interesting　to　remark　that　Darwin　drew　from　the　facts　of　plant　distribution　one　of　his　most　ingenious　arguments　in　support　of　this　theory。　　（See　〃More　Letters〃；　I。　page　424。）　　He　tells　us；　〃I　was　led　to　anticipate　that　the　species　of　the　larger　genera　in　each　country　would　oftener　present　varieties；　than　the　species　of　the　smaller　genera。〃　　（〃Origin〃；　page　44。）　　He　argues　〃where；　if　we　may　use　the　expression；　the　manufactory　of　species　has　been　active；　we　ought　generally　to　find　the　manufactory　still　in　action。〃　　（Ibid。　page　45。）　　This　proved　to　be　the　case。　　But　the　labour　imposed　upon　him　in　the　study　was　immense。　　He　tabulated　local　floras　〃belting　the　whole　northern　hemisphere〃　（〃More　Letters〃；　I。　page　107。）；　besides　voluminous　works　such　as　De　Candolle's　〃Prodromus〃。　　The　results　scarcely　fill　a　couple　of　pages。　　This　is　a　good　illustration　of　the　enormous　pains　which　he　took　to　base　any　statement　on　a　secure　foundation　of　evidence；　and　for　this　the　world；　till　the　publication　of　his　letters；　could　not　do　him　justice。　　He　was　a　great　admirer　of　Herbert　Spencer；　whose　〃prodigality　of　original　thought〃　astonished　him。　　〃But；〃　he　says；　〃the　reflection　constantly　recurred　to　me　that　each　suggestion；　to　be　of　real　value　to　service；　would　require　years　of　work。〃　　（Ibid。　II。　page　235。）

At　last　the　ground　was　cleared　and　we　are　led　to　the　final　conclusion。　　〃If　the　difficulties　be　not　insuperable　in　admitting　that　in　the　long　course　of　time　all　the　individuals　of　the　same　species　belonging　to　the　same　genus；　have　proceeded　from　some　one　source；　then　all　the　grand　leading　facts　of　geographical　distribution　are　explicable　on　the　theory　of　migration；　together　with　subsequent　modification　and　the　multiplication　of　new　forms。〃　（〃Origin〃；　page　360。）　　In　this　single　sentence　Darwin　has　stated　a　theory　which；　as　his　son　F。　Darwin　has　said　with　justice；　has　〃revolutionized　botanical　geography。〃　　（〃The　Botanical　Work　of　Darwin〃；　〃Ann。　Bot。〃　1899；　　page　xi。）　　It　explains　how　physical　barriers　separate　and　form　botanical　regions；　how　allied　species　become　concentrated　in　the　same　areas；　how；　under　similar　physical　conditions；　plants　may　be　essentially　dissimilar；　showing　that　descent　and　not　the　surroundings　is　the　controlling　factor；　how　insular　floras　have　acquired　their　peculiarities；　in　short　how　the　most　various　and　apparently　uncorrelated　problems　fall　easily　and　inevitably　into　line。

The　argument　from　plant　distribution　was　in　fact　irresistible。　　A　proof；　if　one　were　wanted；　was　the　immediate　conversion　of　what　Hooker　called　〃the　stern　keen　intellect〃　（〃More　Letters〃；　I。　page　134。）　of　Bentham；　by　general　consent　the　leading　botanical　systematist　at　the　time。　　It　is　a　striking　historical　fact　that　a　paper　of　his　own　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；　Arct