darwin and modern science-第71节

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l　exposed　to　certain　conditions　during　several　generations　are　modified　in　the　same　manner。〃　　Indefinite　variation　is　much　more　general　and　a　more　important　factor　in　the　production　of　new　species；　as　a　result　of　this；　single　individuals　are　distinguished　from　one　another　by　〃slight〃　differences；　first　in　one　then　in　another　character。　　There　may　also　occur；　though　this　is　very　rare；　more　marked　modifications；　〃variations　which　seem　to　us　in　our　ignorance　to　arise　spontaneously。〃　　（〃Origin　of　Species〃　（6th　edition）；　page　421。）　　The　selection　theory　demands　the　further　postulate　that　such　changes；　〃whether　extremely　slight　or　strongly　marked；〃　are　inherited。　　Darwin　was　no　nearer　to　an　experimental　proof　of　this　assumption　than　to　the　discovery　of　the　actual　cause　of　variability。　　It　was　not　until　the　later　years　of　his　life　that　Darwin　was　occupied　with　the　〃perplexing　problem。。。what　causes　almost　every　cultivated　plant　to　vary〃　（〃Life　and　Letters〃；　Vol。　III。　page　342。）：　　he　began　to　make　experiments　on　the　influence　of　the　soil；　but　these　were　soon　given　up。

In　the　course　of　the　violent　controversy　which　was　the　outcome　of　Darwin's　work　the　fundamental　principles　of　his　teaching　were　not　advanced　by　any　decisive　observations。　　Among　the　supporters　and　opponents；　Nageli　（Nageli；　〃Theorie　der　Abstammungslehre〃；　Munich；　1884；　cf。　Chapter　III。）　was　one　of　the　few　who　sought　to　obtain　proofs　by　experimental　methods。　　His　extensive　cultural　experiments　with　alpine　Hieracia　led　him　to　form　the　opinion　that　the　changes　which　are　induced　by　an　alteration　in　the　food…supply；　in　climate　or　in　habitat；　are　not　inherited　and　are　therefore　of　no　importance　from　the　point　of　view　of　the　production　of　species。　　And　yet　Nageli　did　attribute　an　important　influence　to　the　external　world；　he　believed　that　adaptations　of　plants　arise　as　reactions　to　continuous　stimuli；　which　supply　a　need　and　are　therefore　useful。　　These　opinions；　which　recall　the　teleological　aspect　of　Lamarckism；　are　entirely　unsupported　by　proof。　　While　other　far…reaching　attempts　at　an　explanation　of　the　theory　of　descent　were　formulated　both　in　Nageli's　time　and　afterwards；　some　in　support　of；　others　in　opposition　to　Darwin；　the　necessity　of　investigating；　from　different　standpoints；　the　underlying　causes；　variability　and　heredity；　was　more　and　more　realised。　　To　this　category　belong　the　statistical　investigations　undertaken　by　Quetelet　and　Galton；　the　researches　into　hybridisation；　to　which　an　impetus　was　given　by　the　re…　discovery　of　the　Mendelian　law　of　segregation；　as　also　by　the　culture　experiments　on　mutating　species　following　the　work　of　de　Vries；　and　lastly　the　consideration　of　the　question　how　far　variation　and　heredity　are　governed　by　external　influences。　　These　latter　problems；　which　are　concerned　in　general　with　the　causes　of　form…production　and　form…　modification；　may　be　treated　in　a　short　summary　which　falls　under　two　heads；　one　having　reference　to　the　conditions　of　form…production　in　single　species；　the　other　being　concerned　with　the　conditions　governing　the　transformation　of　species。

I。　THE　INFLUENCE　OF　EXTERNAL　CONDITIONS　ON　FORM…PRODUCTION　IN　SINGLE　SPECIES。

The　members　of　plants；　which　we　express　by　the　terms　stem；　leaf；　flower；　etc。　are　capable　of　modification　within　certain　limits；　since　Lamarck's　time　this　power　of　modification　has　been　brought　more　or　less　into　relation　with　the　environment。　　We　are　concerned　not　only　with　the　question　of　experimental　demonstration　of　this　relationship；　but；　more　generally；　with　an　examination　of　the　origin　of　forms；　the　sequences　of　stages　in　development　that　are　governed　by　recognisable　causes。　　We　have　to　consider　the　general　problem；　to　study　the　conditions　of　all　typical　as　well　as　of　atypic　forms；　in　other　words；　to　found　a　physiology　of　form。

If　we　survey　the　endless　variety　of　plant…forms　and　consider　the　highly　complex　and　still　little　known　processes　in　the　interior　of　cells；　and　if　we　remember　that　the　whole　of　this　branch　of　investigation　came　into　existence　only　a　few　decades　ago；　we　are　able　to　grasp　the　fact　that　a　satisfactory　explanation　of　the　factors　determining　form　cannot　be　discovered　all　at　once。　　The　goal　is　still　far　away。　　We　are　not　concerned　now　with　the　controversial　question；　whether；　on　the　whole；　the　fundamental　processes　in　the　development　of　form　can　be　recognised　by　physiological　means。　　A　belief　in　the　possibility　of　this　can　in　any　case　do　no　harm。　　What　we　may　and　must　attempt　is　thisto　discover　points　of　attack　on　one　side　or　another；　which　may　enable　us　by　means　of　experimental　methods　to　come　into　closer　touch　with　these　elusive　and　difficult　problems。　　While　we　are　forced　to　admit　that　there　is　at　present　much　that　is　insoluble　there　remains　an　inexhaustible　supply　of　problems　capable　of　solution。

The　object　of　our　investigations　is　the　species；　but　as　regards　the　question；　what　is　a　species；　science　of　to…day　takes　up　a　position　different　from　that　of　Darwin。　　For　him　it　was　the　Linnean　species　which　illustrates　variation：　　we　now　know；　thanks　to　the　work　of　Jordan；　de　Bary；　and　particularly　to　that　of　de　Vries　（de　Vries；　〃Die　Mutationstheorie〃；　Leipzig；　1901；　Vol。　I。　page　33。）；　that　the　Linnean　species　consists　of　a　large　or　small　number　of　entities；　elementary　species。　　In　experimental　investigation　it　is　essential　that　observations　be　made　on　a　pure　species；　or；　as　Johannsen　（Johannsen；　〃Ueber　Erblichkeit　in　Populationen　und　reinen　Linien〃；　Jena；　1903。）　says；　on　a　pure　〃line。〃　　What　has　long　been　recognised　as　necessary　in　the　investigation　of　fungi；　bacteria　and　algae　must　also　be　insisted　on　in　the　case　of　flowering　plants；　we　must　start　with　a　single　individual　which　is　reproduced　vegetatively　or　by　strict　self…fertilisation。　　In　dioecious　plants　we　must　aim　at　the　reproduction　of　brothers　and　sisters。

We　may　at　the　outset　take　it　for　granted　that　a　pure　species　remains　the　same　under　similar　external　conditions；　it　varies　as　these　vary。　　IT　IS　CHARACTERISTIC　OF　A　SPECIES　THAT　IT　ALWAYS　EXHIBITS　A　CONSTANT　RELATION　TO　A　PARTICULAR　ENVIRONMENT。　　In　the　case　of　two　different　species；　e。g。　the　hay　and　anthrax　bacilli　or　two　varieties　of　Campanula　with　blue　and　white　flowers　respectively；　a　similar　environment　produces　a　constant　difference。　　The　cause　of　this　is　a　mystery。

According　to　the　modern　standpoint；　the　living　cell　is　a　complex　chemico…　physical　system　which　is　regarded　as　a　dynamical　system　of　equilibrium；　a　conception　suggested　by　Herbert　Spencer　and　which　has　acquired　a　constantly　increasing　importance　in　the　light　of　modern　developments　in　physical　chemistry。　　The　various　chemical　compounds；　proteids；　carbohydrates；　fats；　the　whole　series　of　different　ferments；　etc。　occur　in　the　cell　in　a　definite　physical　arrangement。　　The　two　systems　of　two　species　must　as　a　matter　of　fact　possess　a　constant　difference；　which　it　is　necessary　to　define　by　a　special　term。　　We　say；　therefore；　that　the　SPECIFIC　STRUCTURE　is　different。

By　way　of　illustrating　this　provisionally；　we　may　assume　that　the　proteids　of　the　two　species　possess　a　constant　chemical　difference。　　This　conception　of　specific　structure　is　specially　important　in　its　bearing　on　a　further　treatment　of　the　subject。　　In　the　original　cell；　eventually　also　in　every　cell　of　a　plant；　the　characters　which　afterwards　become　apparent　must　exist　somewhere；　they　are　integral　parts　of　the　capabilities　or　potentialities　of　specific　structure。　　Thus　not　only　the　characters　which　are　exhibited　under　ordinary　conditions　in　nature；　but　also　many　others　which　become　apparent　only　under　special　conditions　（In　this　connection　I　leave　out　of　account；　as　before；　the　idea　of　material　carriers　of　heredity　which　since　the　publication　of　Darwin's　Pangenesis　hypothesis　has　been　frequently　suggested。　　See　my　remarks　in　〃Variationen　der　Bluten〃；　〃Pringsheim's　Jahrb。　Wiss。　Bot。〃　1905；　page　298；　also　Detto；　〃Biol。　Centralbl。〃　1907；　page　81；　〃Die　Erklarbarkeit　der　Ontogenese　durch　materielle　Anlagen〃。）；　are　to　be　included　as　such　potentialities　in　cells；　the　conception　of　specific　structure　includes　the　WHOLE　OF　THE　POTENTIALITIES　OF　A　SPECIES；　specific　structure　comprises　that　which　we　must　always　assume　without　being　able　to　explain　it。

A　relatively　simple　substance；　such　as　oxalate　of　lime；　is　known　under　a　great　number　of　different　crystalline　forms　belonging　to　different　systems　（Compare　Kohl's　work　on　〃Anatomisch…phys。　Untersuchungen　uber　Kalksalze〃；　etc。　Marburg；　1889。）；　these　may　occur　as　single　crystals；　concretions　or　as　concentric　sphaerites。　　The　power　to　assume　this　variety　of　form　is　in　some　way　inherent　in　the　molecular　structure；　though　we　cannot；　even　in　this　case；　explain　the　necessary　connection　between　structure　and　crystalline　form。　　These　potentialities　can　only　become　operative　under　the　influence　of　external　conditions；　their　stimulation　into　activity　depends　on　the　degree　of　concentration　of　the　various　solutions；　on　the　nature　of　the　particular　calcium　salt；　on　the　acid　or　alkaline　reactions。　　Broadly　speaking；　the　plant　cell　behaves　in　a　similar　way。　　The　manifestation　of　each　form；　which　is　inherent　as　a　potentiality　in　the　specific　structure；　is　ultimately　to　be　referred　to　external　conditions。

An　i