darwin and modern science-第70节

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um　in　an　Orchid；　or　of　the　pappus　in　a　Composite。

Did　space　allow；　other　examples　might　be　added。　　We　may　venture　to　maintain　that　the　glimpses　which　the　fossil　record　allows　us　into　early　stages　in　the　evolution　of　organs　now　of　high　systematic　importance；　by　no　means　justify　the　belief　in　any　essential　distinction　between　morphological　and　adaptive　characters。

Another　point；　closely　connected　with　Darwin's　theory；　on　which　the　fossil　history　of　plants　has　been　supposed　to　have　some　bearing；　is　the　question　of　Mutation；　as　opposed　to　indefinite　variation。　　Arber　and　Parkin；　in　their　interesting　memoir　on　the　Origin　of　Angiosperms；　have　suggested　calling　in　Mutation　to　explain　the　apparently　sudden　transition　from　the　cycadean　to　the　angiospermous　type　of　foliage；　in　late　Mesozoic　times；　though　they　express　themselves　with　much　caution；　and　point　out　〃a　distinct　danger　that　Mutation　may　become　the　last　resort　of　the　phylogenetically　destitute〃！

The　distinguished　French　palaeobotanists；　Grand'Eury　（C。　Grand'Eury；　〃Sur　les　mutations　de　quelques　Plantes　fossiles　du　Terrain　houiller〃。　　〃Comptes　Rendus〃；　CXLII。　page　25；　1906。）　and　Zeiller　（R。　Zeiller　〃Les　Vegetaux　fossiles　et　leurs　Enchainements〃；　〃Revue　du　Mois〃；　III。　February；　1907。）；　are　of　opinion；　to　quote　the　words　of　the　latter　writer；　that　the　facts　of　fossil　Botany　are　in　agreement　with　the　sudden　appearance　of　new　forms；　differing　by　marked　characters　from　those　that　have　given　them　birth；　he　adds　that　these　results　give　more　amplitude　to　this　idea　of　Mutation；　extending　it　to　groups　of　a　higher　order；　and　even　revealing　the　existence　of　discontinuous　series　between　the　successive　terms　of　which　we　yet　recognise　bonds　of　filiation。　　（Loc。　cit。　page　23。）

If　Zeiller's　opinion　should　be　confirmed；　it　would　no　doubt　be　a　serious　blow　to　the　Darwinian　theory。　　As　Darwin　said：　　〃Under　a　scientific　point　of　view；　and　as　leading　to　further　investigation；　but　little　advantage　is　gained　by　believing　that　new　forms　are　suddenly　developed　in　an　inexplicable　manner　from　old　and　widely　different　forms；　over　the　old　belief　in　the　creation　of　species　from　the　dust　of　the　earth。〃　　（〃Origin　of　Species〃；　page　424。）

It　most　however　be　pointed　out；　that　such　mutations　as　Zeiller；　and　to　some　extent　Arber　and　Parkin；　appear　to　have　in　view；　bridging　the　gulf　between　different　Orders　and　Classes；　bear　no　relation　to　any　mutations　which　have　been　actually　observed；　such　as　the　comparatively　small　changes；　of　sub…　specific　value；　described　by　De　Vries　in　the　type…case　of　Oenothera　Lamarckiana。　　The　results　of　palaeobotanical　research　have　undoubtedly　tended　to　fill　up　gaps　in　the　Natural　System　of　plantsthat　many　such　gaps　still　persist　is　not　surprising；　their　presence　may　well　serve　as　an　incentive　to　further　research　but　does　not；　as　it　seems　to　the　writer；　justify　the　assumption　of　changes　in　the　past；　wholly　without　analogy　among　living　organisms。

As　regards　the　succession　of　species；　there　are　no　greater　authorities　than　Grand'Eury　and　Zeiller；　and　great　weight　must　be　attached　to　their　opinion　that　the　evidence　from　continuous　deposits　favours　a　somewhat　sudden　change　from　one　specific　form　to　another。　　At　the　same　time　it　will　be　well　to　bear　in　mind　that　the　subject　of　the　〃absence　of　numerous　intermediate　varieties　in　any　single　formation〃　was　fully　discussed　by　Darwin。　　（〃Origin　of　Species〃；　pages　275…282；　and　page　312。）；　the　explanation　which　he　gave　may　go　a　long　way　to　account　for　the　facts　which　recent　writers　have　regarded　as　favouring　the　theory　of　saltatory　mutation。

The　rapid　sketch　given　in　the　present　essay　can　do　no　more　than　call　attention　to　a　few　salient　points；　in　which　the　palaeontological　records　of　plants　has　an　evident　bearing　on　the　Darwinian　theory。　　At　the　present　day　the　whole　subject　of　palaeobotany　is　a　study　in　evolution；　and　derives　its　chief　inspiration　from　the　ideas　of　Darwin　and　Wallace。　　In　return　it　contributes　something　to　the　verification　of　their　teaching；　the　recent　progress　of　the　subject；　in　spite　of　the　immense　difficulties　which　still　remain；　has　added　fresh　force　to　Darwin's　statement　that　〃the　great　leading　facts　in　palaeontology　agree　admirably　with　the　theory　of　descent　with　modification　through　variation　and　natural　selection。〃　　（Ibid。　page　313。）


XIII。　　THE　INFLUENCE　OF　ENVIRONMENT　ON　THE　FORMS　OF　PLANTS。

By　GEORG　KLEBS；　PH。D。　Professor　of　Botany　in　the　University　of　Heidelberg。

The　dependence　of　plants　on　their　environment　became　the　object　of　scientific　research　when　the　phenomena　of　life　were　first　investigated　and　physiology　took　its　place　as　a　special　branch　of　science。　　This　occurred　in　the　course　of　the　eighteenth　century　as　the　result　of　the　pioneer　work　of　Hales；　Duhamel；　Ingenhousz；　Senebier　and　others。　　In　the　nineteenth　century；　particularly　in　the　second　half；　physiology　experienced　an　unprecedented　development　in　that　it　began　to　concern　itself　with　the　experimental　study　of　nutrition　and　growth；　and　with　the　phenomena　associated　with　stimulus　and　movement；　on　the　other　hand；　physiology　neglected　phenomena　connected　with　the　production　of　form；　a　department　of　knowledge　which　was　the　province　of　morphology；　a　purely　descriptive　science。　　It　was　in　the　middle　of　the　last　century　that　the　growth　of　comparative　morphology　and　the　study　of　phases　of　development　reached　their　highest　point。

The　forms　of　plants　appeared　to　be　the　expression　of　their　inscrutable　inner　nature；　the　stages　passed　through　in　the　development　of　the　individual　were　regarded　as　the　outcome　of　purely　internal　and　hidden　laws。　The　feasibility　of　experimental　inquiry　seemed　therefore　remote。　　Meanwhile；　the　recognition　of　the　great　importance　of　such　a　causal　morphology　emerged　from　the　researches　of　the　physiologists　of　that　time；　more　especially　from　those　of　Hofmeister　（Hofmeister；　〃Allgemeine　Morphologie〃；　Leipzig；　1868；　page　579。）；　and　afterwards　from　the　work　of　Sachs。　　（Sachs；　〃Stoff　und　Form　der　Pflanzenorgane〃；　Vol。　I。　1880；　Vol。　II。　1882。　　〃Gesammelte　Abhandlungen　uber　Pflanzen…Physiologie〃；　II。　Leipzig；　1893。）　　Hofmeister；　in　speaking　of　this　line　of　inquiry；　described　it　as　〃the　most　pressing　and　immediate　aim　of　the　investigator　to　discover　to　what　extent　external　forces　acting　on　the　organism　are　of　importance　in　determining　its　form。〃　　This　advance　was　the　outcome　of　the　influence　of　that　potent　force　in　biology　which　was　created　by　Darwin's　〃Origin　of　Species〃　（1859）。

The　significance　of　the　splendid　conception　of　the　transformation　of　species　was　first　recognised　and　discussed　by　Lamarck　（1809）；　as　an　explanation　of　transformation　he　at　once　seized　upon　the　ideaan　intelligible　viewthat　the　external　world　is　the　determining　factor。　　Lamarck　（Lamarck；　〃Philosophie　zoologique〃；　pages　223…227。　　Paris；　1809。）　endeavoured；　more　especially；　to　demonstrate　from　the　behaviour　of　plants　that　changes　in　environment　induce　change　in　form　which　eventually　leads　to　the　production　of　new　species。　　In　the　case　of　animals；　Lamarck　adopted　the　teleological　view　that　alterations　in　the　environment　first　lead　to　alterations　in　the　needs　of　the　organisms；　which；　as　the　result　of　a　kind　of　conscious　effort　of　will；　induce　useful　modifications　and　even　the　development　of　new　organs。　　His　work　has　not　exercised　any　influence　on　the　progress　of　science：　　Darwin　himself　confessed　in　regard　to　Lamarck's　work　〃I　got　not　a　fact　or　idea　from　it。〃　　（〃Life　and　Letters〃；　Vol。　II。　page　215。）

On　a　mass　of　incomparably　richer　and　more　essential　data　Darwin　based　his　view　of　the　descent　of　organisms　and　gained　for　it　general　acceptance；　as　an　explanation　of　modification　he　elaborated　the　ingeniously　conceived　selection　theory。　　The　question　of　special　interest　in　this　connection；　namely　what　is　the　importance　of　the　influence　of　the　environment；　Darwin　always　answered　with　some　hesitation　and　caution；　indeed　with　a　certain　amount　of　indecision。

The　fundamental　principle　underlying　his　theory　is　that　of　general　variability　as　a　whole；　the　nature　and　extent　of　which；　especially　in　cultivated　organisms；　are　fully　dealt　with　in　his　well…known　book。　　（Darwin；　〃The　variation　of　Animals　and　Plants　under　domestication〃；　2　vols。；　edition　1；　1868；　edition　2；　1875；　popular　edition　1905。）　　In　regard　to　the　question　as　to　the　cause　of　variability　Darwin　adopts　a　consistently　mechanical　view。　　He　says：　　〃These　several　considerations　alone　render　it　probable　that　variability　of　every　kind　is　directly　or　indirectly　caused　by　changed　conditions　of　life。　　Or；　to　put　the　case　under　another　point　of　view；　if　it　were　possible　to　expose　all　the　individuals　of　a　species　during　many　generations　to　absolutely　uniform　conditions　of　life；　there　would　be　no　variability。〃　　（〃The　variation　of　Animals　and　Plants〃　（2nd　edition）；　Vol。　II。　page　242。）　　Darwin　did　not　draw　further　conclusions　from　this　general　principle。

Variations　produced　in　organisms　by　the　environment　are　distinguished　by　Darwin　as　〃the　definite〃　and　〃the　indefinite。〃　　（Ibid。　II。　page　260。　　See　also　〃Origin　of　Species〃　（6th　edition）；　page　6。）　　The　first　occur　〃when　all　or　nearly　all　the　offspring　of　an　individual　exposed　to　certain　conditions　during　several　generations　are　modified　in　the　same　manner。〃　　Indefi