darwin and modern science-第94节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


；　and　Southern　Oceans；　without　a　solitary　relic　of　the　great　islands　or　continents　supposed　to　have　sunk　beneath　their　waves。〃　　（〃Island　Life〃；　page　105。）

Writing　to　Wallace　（1876）；　Darwin　warmly　approves　the　former's　〃protest　against　sinking　imaginary　continents　in　a　quite　reckless　manner；　as　was　stated　by　Forbes；　followed；　alas；　by　Hooker；　and　caricatured　by　Wollaston　and　（Andrew）　Murray。〃　　（〃Life　and　Letters〃；　III。　page　230。）　　The　transport　question　thus　became　of　enormously　enhanced　importance。　　We　need　not　be　surprised　then　at　his　writing　to　Lyell　in　1856：〃I　cannot　avoid　thinking　that　Forbes's　'Atlantis'　was　an　ill…service　to　science；　as　checking　a　close　study　of　means　of　dissemination〃　（Ibid。　II。　page　78。）；　and　Darwin　spared　no　pains　to　extend　our　knowledge　of　them。　　He　implores　Hooker；　ten　years　later；　to　〃admit　how　little　is　known　on　the　subject；〃　and　summarises　with　some　satisfaction　what　he　had　himself　achieved：〃Remember　how　recently　you　and　others　thought　that　salt　water　would　soon　kill　seeds。。。Remember　that　no　one　knew　that　seeds　would　remain　for　many　hours　in　the　crops　of　birds　and　retain　their　vitality；　that　fish　eat　seeds；　and　that　when　the　fish　are　devoured　by　birds　the　seeds　can　germinate；　etc。　　Remember　that　every　year　many　birds　are　blown　to　Madeira　and　to　the　Bermudas。　　Remember　that　dust　is　blown　1000　miles　across　the　Atlantic。〃　　（〃More　Letters〃；　I。　page　483。）

It　has　always　been　the　fashion　to　minimise　Darwin's　conclusions；　and　these　have　not　escaped　objection。　　The　advocatus　diaboli　has　a　useful　function　in　science。　　But　in　attacking　Darwin　his　brief　is　generally　found　to　be　founded　on　a　slender　basis　of　facts。　　Thus　Winge　and　Knud　Andersen　have　examined　many　thousands　of　migratory　birds　and　found　〃that　their　crops　and　stomachs　were　always　empty。　　They　never　observed　any　seeds　adhering　to　the　feathers；　beaks　or　feet　of　the　birds。〃　　（R。F。　Scharff；　〃European　Animals〃；　page　64；　London；　1907。）　　The　most　considerable　investigation　of　the　problem　of　Plant　Dispersal　since　Darwin　is　that　of　Guppy。　　He　gives　a　striking　illustration　of　how　easily　an　observer　may　be　led　into　error　by　relying　on　negative　evidence。

〃When　Ekstam　published；　in　1895；　the　results　of　his　observations　on　the　plants　of　Nova　Zembla；　he　observed　that　he　possessed　no　data　to　show　whether　swimming　and　wading　birds　fed　on　berries；　and　he　attached　all　importance　to　dispersal　by　winds。　　On　subsequently　visiting　Spitzbergen　he　must　have　been　at　first　inclined；　therefore；　to　the　opinion　of　Nathorst；　who；　having　found　only　a　solitary　species　of　bird　（a　snow…sparrow）　in　that　region；　naturally　concluded　that　birds　had　been　of　no　importance　as　agents　in　the　plant…stocking。　　However；　Ekstam's　opportunities　were　greater；　and　he　tells　us　that　in　the　craws　of　six　specimens　of　Lagopus　hyperboreus　shot　in　Spitzbergen　in　August　he　found　represented　almost　25　per　cent。　of　the　usual　phanerogamic　flora　of　that　region　in　the　form　of　fruits；　seeds；　bulbils；　flower…buds；　leaf…buds；　etc。。。〃

〃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　admitte