darwin and modern science-第96部分

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　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　importance　of　what　I　may　call　short…distance　dispersal　as　a　necessary　condition　of　plant　life；　because　I　think　it　suggests　the　solution　of　a　difficulty　which　leads　Guppy　to　a　conclusion　with　which　I　am　unable　to　agree。　　But　the　work　which　he　has　done　taken　as　a　whole　appears　to　me　so　admirable　that　I　do　so　with　the　utmost　respect。　　He　points　out；　as　Bentham　had　already　done；　that　long…distance　dispersal　is　fortuitous。　　And　being　so　it　cannot　have　been　provided　for　by　previous　adaptation。　　He　says　（Guppy；　op。　cit。　II。　page　99。）：　　〃It　is　not　conceivable　that　an　organism　can　be　adapted　to　conditions　outside　its　environment。〃　　To　this　we　must　agree；　but；　it　may　be　asked；　do　the　general　means　of　plant　dispersal　violate　so　obvious　a　principle？　　He　proceeds：　　〃The　great　variety　of　the　modes　of　dispersal　of　seeds　is　in　itself　an　indication　that　the　dispersing　agencies　avail　themselves　in　a　hap…hazard　fashion　of　characters　and　capacities　that　have　been　developed　in　other　connections。〃　　（Loc。　cit。　page　102。）　　〃Their　utility　in　these　respects　is　an　accident　in　the　plant's　life。〃　　（Loc。　cit。　page　100。）　　He　attributes　this　utility　to　a　〃determining　agency；〃　an　influence　which　constantly　reappears　in　various　shapes　in　the　literature　of　Evolution　and　is　ultra…scientific　in　the　sense　that　it　bars　the　way　to　the　search　for　material　causes。　　He　goes　so　far　as　to　doubt　whether　fleshy　fruits　are　an　adaptation　for　the　dispersal　of　their　contained　seeds。　　（Loc。　cit。　page　102。）　　Writing　as　I　am　from　a　hillside　which　is　covered　by　hawthorn　bushes　sown　by　birds；　I　confess　I　can　feel　little　doubt　on　the　subject　myself。　　The　essential　fact　which　Guppy　brings　out　is　that　long…distance　unlike　short…distance　dispersal　is　not　universal　and　purposeful；　but　selective　and　in　that　sense　accidental。　　But　it　is　not　difficult　to　see　how　under　favouring　conditions　one　must　merge　into　the　other。

Guppy　has　raised　one　novel　point　which　can　only　be　briefly　referred　to　but　which　is　of　extreme　interest。　　There　are　grounds　for　thinking　that　flowers　and　insects　have　mutually　reacted　upon　one　another　in　their　evolution。　　Guppy　suggests　that　something　of　the　same　kind　may　be　true　of　birds。　　I　must　content　myself　with　the　quotation　of　a　single　sentence。　　〃With　the　secular　drying　of　the　globe　and　the　consequent　differentiation　of　climate　is　to　be　connected　the　suspension　to　a　great　extent　of　the　agency　of　birds　as　plant　dispersers　in　later　ages；　not　only　in　the　Pacific　Islands　but　all　over　the　tropics。　　The　changes　of　climate；　birds　and　plants　have　gone　on　together；　the　range　of　the　bird　being　controlled　by　the　climate；　and　the　distribution　of　the　plant　being　largely　dependent　on　the　bird。〃　　（Loc。cit。　II。　page　221。）

Darwin　was　clearly　prepared　to　go　further　than　Hooker　in　accounting　for　the　southern　flora　by　dispersion　from　the　north。　　Thus　he　says：　　〃We　must；　I　suppose；　admit　that　every　yard　of　land　has　been　successively　covered　with　a　beech…forest　between　the　Caucasus　and　Japan。〃　　（〃More　Letters〃；　II。　page　9。）　　Hooker　accounted　for　the　dissevered　condition　of　the　southern　flora　by　geographical　change；　but　this　Darwin　could　not　admit。　　He　suggested　to　Hooker　that　the　Australian　and　Cape　floras　might　have　had　a　point　of　connection　through　Abyssinia　（Ibid。　I。　page　447。）；　an　idea　which　was　promptly　snuffed　out。　　Similarly　he　remarked　to　Bentham　（1869）：　　〃I　suppose　you　think　that　the　Restiaceae；　Proteaceae；　etc。；　etc。　once　extended　over　the　whole　world；　leaving　fragments　in　the　south。〃　　（Ibid。　I。　page　380。）　　Eventually　he　conjectured　〃that　there　must　have　been　a　Tertiary　Antarctic　continent；　from　which　various　forms　radiated　to　the　southern　extremities　of　our　present　continents。〃　　（〃Life　and　Letters〃；　III。　page　231。）　　But　characteristically　he　could　not　admit　any　land　connections　and　trusted　to　〃floating　ice　for　transporting　seed。〃　　（〃More　Letters〃；　I。　page　116。）　　I　am　far　from　saying　that　this　theory　is　not　deserving　of　serious　attention；　though　there　seems　to　be　no　positive　evidence　to　support　it；　and　it　immediately　raises　the　difficulty　how　did　such　a　continent　come　to