darwin and modern science-第79章

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n；　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　be　stocked？

We　must；　however；　agree　with　Hooker　that　the　common　origin　of　the　northern　and　southern　floras　must　be　referred　to　a　remote　past。　　That　Darwin　had　this　in　his　mind　at　the　time　of　the　publication　of　the　〃Origin〃　is　clear　from　a　letter　to　Hooker。　　〃The　view　which　I　should　have　looked　at　as　perhaps　most　probable　（though　it　hardly　differs　from　yours）　is　that　the　whole　world　during　the　Secondary　ages　was　inhabited　by　marsupials；　araucarias　（Mem。Fossil　wood　of　this　nature　in　South　America）；　Banksia；　etc。；　and　that　these　were　supplanted　and　exterminated　in　the　greater　area　of　the　north；　but　were　left　alive　in　the　south。〃　　（Ibid。　I。　page　453。）　　Remembering　that　Araucaria；　unlike　Banksia；　belongs　to　the　earlier　Jurassic　not　to　the　angiospermous　flora；　this　view　is　a　germinal　idea　of　the　widest　generality。

The　extraordinary　congestion　in　species　of　the　peninsulas　of　the　Old　World　points　to　the　long…continued　action　of　a　migration　southwards。　　Each　is　in　fact　a　cul…de…sac　into　which　they　have　poured　and　from　which　there　is　no　escape。　　On　the　other　hand　the　high　degree　of　specialisation　in　the　southern　floras　and　the　little　power　the　species　possess　of　holding　their　own　in　competition　or　in　adaptation　to　new　conditions　point　to　long…　continued　isolation。　　〃An　island。。。will　prevent　free　immigration　and　competition；　hence　a　greater　number　of　ancient　forms　will　survive。〃　　（Ibid。　I。　page　481。）　　But　variability　is　itself　subject　to　variation。　　The　nemesis　of　a　high　degree　of　protected　specialisation　is　the　loss　of　adaptability。　　（See　Lyell；　〃The　Geological　Evidences　of　the　Antiquity　of　Man〃；　London；　1863；　page　446。）　　It　is　probable　that　many　elements　of　the　southern　flora　are　doomed：　　there　is；　for　example；　reason　to　think　that　the　singular　Stapelieae　of　S。　Africa　are　a　disappearing　group。　　The　tree　Lobelias　which　linger　in　the　mountains　of　Central　Africa；　in　Tropical　America　and　in　the　Sandwich　Islands　have　the　aspect　of　extreme　antiquity。　　I　may　add　a　further　striking　illustration　from　Professor　Seward：　　〃The　tall；　graceful　fronds　of　Matonia　pectinata；　forming　miniature　forests　on　the　slopes　of　Mount　Ophir　and　other　districts　in　the　Malay　Peninsula　in　association　with　Dipteris　conjugata　and　Dipteris　lobbiana；　represent　a　phase　of　Mesozoic　life　which　survives　

'Like　a　dim　picture　of　the　drowned　past。'〃　（〃Report　of　the　73rd　Meeting　of　the　British　Assoc。〃　　（Southport；　1903）；　London；　1904；　page　844。）

The　Matonineae　are　ferns　with　an　unusually　complex　vascular　system　and　were　abundant　〃in　the　northern　hemisphere　during　the　earlier　part　of　the　Mesozoic　era。〃

It　was　fortunate　for　science　that　Wallace　took　up　the　task　which　his　colleague　had　abandoned。　　Writing　to　him　on　the　publication　of　his　〃Geographical　Distribution　of　Animals〃　Darwin　said：　　〃I　feel　sure　that　you　have　laid　a　broad　and　safe　foundation　for　all　future　work　on　Distribution。　How　interesting　it　will　be　to　see　hereafter　plants　treated　in　strict　relation　to　your　views。〃　　（〃More　Letters〃；　II。　page　12。）　　This　hope　was　fulfilled　in　〃Island　Life〃。　　I　may　quote　a　passage　from　it　which　admirably　summarises　the　contrast　between　the　northern　and　the　southern　floras。

〃Instead　of　the　enormous　northern　area；　in　which　highly　organised　and　dominant　groups　of　plants　have　been　developed　gifted　with　great　colonising　and　aggressive　powers；　we　have　in　the　south　three　comparatively　small　and　detached　areas；　in　which　rich　floras　have　been　developed　with　SPECIAL　adaptations　to　soil；　climate；　and　organic　environment；　but　comparatively　impotent　and　inferior　beyond　their　own　domain。〃　　（Wallace；　〃Island　Life〃；　pages　527；　528。）

It　will　be　noticed　that　in　the　summary　I　have　attempted　to　give　of　the　history　of　the　subject；　efforts　have　been　concentrated　on　bringing　into　relation　the　temperate　floras　of　the　northern　and　southern　hemispheres；　but　no　account　has　been　taken　of　the　rich　tropical　vegetation　which　belts　the　world　and　little　to　account　for　the　original　starting…point　of　existing　vegetation　generally。　　It　must　be　remembered　on　the　one　hand　that　our　detailed　knowledge　of　the　floras　of　the　tropics　is　still　very　incomplete　and　far　inferior　to　that　of　temperate　regions；　on　the　other　hand　palaeontological　discoveries　have　put　the　problem　in　an　entirely　new　light。　　Well　might　Darwin；　writing　to　Heer　in　1875；　say：　　〃Many　as　have　been　the　wonderful　discoveries　in　Geology　during　the　last　half…century；　I　think　none　have　exceeded　in　interest　your　results　with　respect　to　the　plants　which　formerly　existed　in　the　arctic　regions。〃　　（〃More　Letters〃；　II。　page　240。）

As　early　as　1848　Debey　had　described　from　the　Upper　Cretaceous　rocks　of　Aix…la…Chapelle　Flowering　plants　of　as　high　a　degree　of　development　as　those　now　existing。　　The　fact　was　commented　upon　by　Hooker　（〃Introd。　Essay　to　the　Flora　of　Tasmania〃；　page　xx。）；　but　its　full　significance　seems　to　have　been　scarcely　appreciated。　　For　it　implied　not　merely　that　their　evolution　must　have　taken　place　but　the　foundations　of　existing　distribution　must　have　been　laid　in　a　preceding　age。　　We　now　know　from　the　discoveries　of　the　last　fifty