darwin and modern science- 第85部分

〃The　result　of　Ekstam's　observations　in　Spitzbergen　was　to　lead　him　to　attach　a　very　considerable　importance　in　plant　dispersal　to　the　agency　of　birds；　and　when　in　explanation　of　the　Scandinavian　elements　in　the　Spitzbergen　flora　he　had　to　choose　between　a　former　land　connection　and　the　agency　of　birds；　he　preferred　the　bird。〃　　（Guppy；　op。　cit。　II。　pages　511；　512。）

Darwin　objected　to　〃continental　extensions〃　on　geological　grounds；　but　he　also　objected　to　Lyell　that　they　do　not　〃account　for　all　the　phenomena　of　distribution　on　islands〃　（〃Life　and　Letters〃；　II。　page　77。）；　such　for　example　as　the　absence　of　Acacias　and　Banksias　in　New　Zealand。　　He　agreed　with　De　Candolle　that　〃it　is　poor　work　putting　together　the　merely　POSSIBLE　means　of　distribution。〃　　But　he　also　agreed　with　him　that　they　were　the　only　practicable　door　of　escape　from　multiple　origins。　　If　they　would　not　work　then　〃every　one　who　believes　in　single　centres　will　have　to　admit　continental　extensions〃　（Ibid。　II。　page　82。）；　and　that　he　regarded　as　a　mere　counsel　of　despair：〃to　make　continents；　as　easily　as　a　cook　does　pancakes。〃　　（Ibid。　II。　page　74。）

The　question　of　multiple　origins　however　presented　itself　in　another　shape　where　the　solution　was　much　more　difficult。　　The　problem；　as　stated　by　Darwin；　is　this：〃The　identity　of　many　plants　and　animals；　on　mountain…　summits；　separated　from　each　other　by　hundreds　of　miles　of　lowlands。。。without　the　apparent　possibility　of　their　having　migrated　from　one　point　to　the　other。〃　　He　continues；　〃even　as　long　ago　as　1747；　such　facts　led　Gmelin　to　conclude　that　the　same　species　must　have　been　independently　created　at　several　distinct　points；　and　we　might　have　remained　in　this　same　belief；　had　not　Agassiz　and　others　called　vivid　attention　to　the　Glacial　period；　which　affords。。。a　simple　explanation　of　the　facts。〃　　（〃Origin　of　Species〃　（6th　edition）　page　330。）

The　〃simple　explanation〃　was　substantially　given　by　E。　Forbes　in　1846。　　It　is　scarcely　too　much　to　say　that　it　belongs　to　the　same　class　of　fertile　and　far…reaching　ideas　as　〃natural　selection〃　itself。　　It　is　an　extraordinary　instance；　if　one　were　wanted　at　all；　of　Darwin's　magnanimity　and　intense　modesty　that　though　he　had　arrived　at　the　theory　himself；　he　acquiesced　in　Forbes　receiving　the　well…merited　credit。　　〃I　have　never；〃　he　says；　〃of　course　alluded　in　print　to　my　having　independently　worked　out　this　view。〃　　But　he　would　have　been　more　than　human　if　he　had　not　added：　〃I　was　forestalled　in。。。one　important　point；　which　my　vanity　has　always　made　me　regret。〃　　（〃Life　and　Letters〃；　I。　page　88。）

Darwin；　however；　by　applying　the　theory　to　trans…tropical　migration；　went　far　beyond　Forbes。　　The　first　enunciation　to　this　is　apparently　contained　in　a　letter　to　Asa　Gray　in　1858。　　The　whole　is　too　long　to　quote；　but　the　pith　is　contained　in　one　paragraph。　　〃There　is　a　considerable　body　of　geological　evidence　that　during　the　Glacial　epoch　the　whole　world　was　colder；　I　inferred　that；。。。from　erratic　boulder　phenomena　carefully　observed　by　me　on　both　the　east　and　west　coast　of　South　America。　　Now　I　am　so　bold　as　to　believe　that　at　the　height　of　the　Glacial　epoch；　AND　WHEN　ALL　TROPICAL　PRODUCTIONS　MUST　HAVE　BEEN　CONSIDERABLY　DISTRESSED；　several　temperate　forms　slowly　travelled　into　the　heart　of　the　Tropics；　and　even　reached　the　southern　hemisphere；　and　some　few　southern　forms　penetrated　in　a　reverse　direction　northward。〃　　（〃Life　and　Letters〃；　II。　page　136。）　　Here　again　it　is　clear　that　though　he　credits　Agassiz　with　having　called　vivid　attention　to　the　Glacial　period；　he　had　himself　much　earlier　grasped　the　idea　of　periods　of　refrigeration。

Putting　aside　the　fact；　which　has　only　been　made　known　to　us　since　Darwin's　death；　that　he　had　anticipated　Forbes；　it　is　clear　that　he　gave　the　theory　a　generality　of　which　the　latter　had　no　conception。　　This　is　pointed　out　by　Hooker　in　his　classical　paper　〃On　the　Distribution　of　Arctic　Plants〃　（1860）。　　〃The　theory　of　a　southern　migration　of　northern　types　being　due　to　the　cold　epochs　preceding　and　during　the　glacial；　originated；　I　believe；　with　the　late　Edward　Forbes；　the　extended　one；　of　the　trans…tropical　migration；　is　Mr　Darwin's。〃　　（〃Linn。　Trans。〃　XXIII。　page　253。　　The　attempt　appears　to　have　been　made　to　claim　for　Heer　priority　in　what　I　may　term　for　short　the　arctic…alpine　theory　（Scharff；　〃European　Animals〃；　page　128）。　　I　find　no　suggestion　of　his　having　hit　upon　it　in　his　correspondence　with　Darwin　or　Hooker。　　Nor　am　I　aware　of　any　reference　to　his　having　done　so　in　his　later　publications。　　I　am　indebted　to　his　biographer；　Professor　Schroter；　of　Zurich；　for　an　examination　of　his　earlier　papers　with　an　equally　negative　result。）　　Assuming　that　local　races　have　derived　from　a　common　ancestor；　Hooker's　great　paper　placed　the　fact　of　the　migration　on　an　impregnable　basis。　　And；　as　he　pointed　out；　Darwin　has　shown　that　〃such　an　explanation　meets　the　difficulty　of　accounting　for　the　restriction　of　so　many　American　and　Asiatic　arctic　types　to　their　own　peculiar　longitudinal　zones；　and　for　what　is　a　far　greater　difficulty；　the　representation　of　the　same　arctic　genera　by　most　closely　allied　species　in　different　longitudes。〃

The　facts　of　botanical　geography　were　vital　to　Darwin's　argument。　　He　had　to　show　that　they　admitted　of　explanation　without　assuming　multiple　origins　for　species；　which　would　be　fatal　to　the　theory　of　Descent。　　He　had　therefore　to　strengthen　and　extend　De　Candolle's　work　as　to　means　of　transport。　　He　refused　to　supplement　them　by　hypothetical　geographical　changes　for　which　there　was　no　independent　evidence：　　this　was　simply　to　attempt　to　explain　ignotum　per　ignotius。　　He　found　a　real　and；　as　it　has　turned　out；　a　far…reaching　solution　in　climatic　change　due　to　cosmical　causes　which　compelled　the　migration　of　species　as　a　condition　of　their　existence。　　The　logical　force　of　the　argument　consists　in　dispensing　with　any　violent　assumption；　and　in　showing　that　the　principle　of　descent　is　adequate　to　explain　the　ascertained　facts。

It　does　not；　I　think；　detract　from　the　merit　of　Darwin's　conclusions　that　the　tendency　of　modern　research　has　been　to　show　that　the　effects　of　the　Glacial　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　Linn