darwin and modern science-第83章

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the　separation　of　an　organ　from　its　normal　connections；　the　factors　concerned　are　more　comprehensible。　　A　detached　leaf；　e。g。；　is　at　once　cut　off　from　a　supply　of　water　and　salts；　and　is　deprived　of　the　means　of　getting　rid　of　organic　substances　which　it　produces；　the　result　is　a　considerable　alteration　in　the　degree　of　concentration。　　No　experimental　investigation　on　these　lines　has　yet　been　made。　　Our　ignorance　has　often　led　to　the　view　that　we　are　dealing　with　a　force　whose　specific　quality　is　the　restitution　of　the　parts　lost　by　operation；　the　proof；　therefore；　that　in　certain　cases　a　similar　production　of　new　roots　or　buds　may　be　induced　without　previous　injury　and　simply　by　a　change　in　external　conditions　assumes　an　importance。　　（Klebs；　〃Willkurliche　Entwickelung〃；　page　100；　also；　〃Probleme　der　Entwickelung〃；　〃Biol。　Centralbl。〃　1904；　page　610。）

A　specially　striking　phenomenon　of　regeneration；　exhibited　also　by　uninjured　plants；　is　afforded　by　polarity；　which　was　discovered　by　Vochting。　　（See　the　classic　work　of　Vochting；　〃Ueber　Organbildung　im　Pflanzenreich〃；　I。　Bonn；　1888；　also　〃Bot。　Zeit。　1906；　page　101；　cf。　Goebel；　〃Experimentelle　Morphologie〃；　Leipzig　and　Berlin；　1908；　Section　V；　Polaritat。）　　It　is　found；　for　example；　that　roots　are　formed　from　the　base　of　a　detached　piece　of　stem　and　shoots　from　the　apex。　　Within　the　limits　of　this　essay　it　is　impossible　to　go　into　this　difficult　question；　it　is；　however；　important　from　the　point　of　view　of　our　general　survey　to　emphasise　the　fact　that　the　physiological　distinctions　between　base　and　apex　of　pieces　of　stem　are　only　of　a　quantitative　kind；　that　is；　they　consist　in　the　inhibition　of　certain　phenomena　or　in　favouring　them。　　As　a　matter　of　fact　roots　may　be　produced　from　the　apices　of　willows　and　cuttings　of　other　plants；　the　distinction　is　thus　obliterated　under　the　influence　of　environment。　　The　fixed　polarity　of　cuttings　from　full　grown　stems　cannot　be　destroyed；　it　is　the　expression　of　previous　development。　　Vochting　speaks　of　polarity　as　a　fixed　inherited　character。　　This　is　an　unconvincing　conclusion；　as　nothing　can　be　deduced　from　our　present　knowledge　as　to　the　causes　which　led　up　to　polarity。　　We　know　that　the　fertilised　egg；　like　the　embryo；　is　fixed　at　one　end　by　which　it　hangs　freely　in　the　embryo…sac　and　afterwards　in　the　endosperm。　　From　the　first；　therefore；　the　two　ends　have　different　natures；　and　these　are　revealed　in　the　differentiation　into　root…apex　and　stem…apex。　　A　definite　direction　in　the　flow　of　food…substances　is　correlated　with　this　arrangement；　and　this　eventually　leads　to　a　polarity　in　the　tissues。　　This　view　requires　experimental　proof；　which　in　the　case　of　the　egg…cells　of　flowering　plants　hardly　appears　possible；　but　it　derives　considerable　support　from　the　fact　that　in　herbaceous　plants；　e。g。　Sempervivum　（Klebs；　〃Variationen　der　Bluten〃；　〃Jahrb。　Wiss。　Bot。〃　1905；　page　260。）；　rosettes　or　flower…shoots　are　formed　in　response　to　external　conditions　at　the　base；　in　the　middle；　or　at　the　apex　of　the　stem；　so　that　polarity　as　it　occurs　under　normal　conditions　cannot　be　the　result　of　unalterable　hereditary　factors。　　On　the　other　hand；　the　lower　plants　should　furnish　decisive　evidence　on　this　question；　and　the　experiments　of　Stahl；　Winkler；　Kniep；　and　others　indicate　the　right　method　of　attacking　the　problem。

The　relation　of　leaf…form　to　environment　has　often　been　investigated　and　is　well　known。　　The　leaves　of　bog　and　water　plants　（Cf。Goebel；　loc。　cit。　chapter　II。；　also　Gluck；　〃Untersuchungen　uber　Wasser…　und　Sumpfgewachse〃；　Jena；　Vols。　I。…II。　1905…06。）　afford　the　most　striking　examples　of　modifications：　　according　as　they　are　grown　in　water；　moist　or　dry　air；　the　form　of　the　species　characteristic　of　the　particular　habitat　is　produced；　since　the　stems　are　also　modified。　　To　the　same　group　of　phenomena　belongs　the　modification　of　the　forms　of　leaves　and　stems　in　plants　on　transplantation　from　the　plains　to　the　mountains　（Bonnier；　〃Recherches　sur　l'Anatomie　experimentale　des　Vegetaux〃；　Corbeil；　1895。）　or　vice　versa。　　Such　variations　are　by　no　means　isolated　examples。　　All　plants　exhibit　a　definite　alteration　in　form　as　the　result　of　prolonged　cultivation　in　moist　or　dry　air；　in　strong　or　feeble　light；　or　in　darkness；　or　in　salt　solutions　of　different　composition　and　strength。

Every　individual　which　is　exposed　to　definite　combinations　of　external　factors　exhibits　eventually　the　same　type　of　modification。　　This　is　the　type　of　variation　which　Darwin　termed　〃definite。〃　　It　is　easy　to　realise　that　indefinite　or　fluctuating　variations　belong　essentially　to　the　same　class　of　phenomena；　both　are　reactions　to　changes　in　environment。　　In　the　production　of　individual　variations　two　different　influences　undoubtedly　cooperate。　　One　set　of　variations　is　caused　by　different　external　conditions；　during　the　production；　either　of　sexual　cells　or　of　vegetative　primordia；　another　set　is　the　result　of　varying　external　conditions　during　the　development　of　the　embryo　into　an　adult　plant。　　The　two　sets　of　influences　cannot　as　yet　be　sharply　differentiated。　　If；　for　purposes　of　vegetative　reproduction；　we　select　pieces　of　the　same　parent…plant　of　a　pure　species；　the　second　type　of　variation　predominates。　　Individual　fluctuations　depend　essentially　in　such　cases　on　small　variations　in　environment　during　development。

These　relations　must　be　borne　in　mind　if　we　wish　to　understand　the　results　of　statistical　methods。　　Since　the　work　of　Quetelet；　Galton；　and　others　the　statistical　examination　of　individual　differences　in　animals　and　plants　has　become　a　special　science；　which　is　primarily　based　on　the　consideration　that　the　application　of　the　theory　of　probability　renders　possible　mathematical　statement　and　control　of　the　results。　　The　facts　show　that　any　character；　size　of　leaf；　length　of　stem；　the　number　of　members　in　a　flower；　etc。　do　not　vary　haphazard　but　in　a　very　regular　manner。　　In　most　cases　it　is　found　that　there　is　a　value　which　occurs　most　commonly；　the　average　or　medium　value；　from　which　the　larger　and　smaller　deviations；　the　so…called　plus　and　minus　variations　fall　away　in　a　continuous　series　and　end　in　a　limiting　value。　　In　the　simpler　cases　a　falling　off　occurs　equally　on　both　sides　of　the　curve；　the　curve　constructed　from　such　data　agrees　very　closely　with　the　Gaussian　curve　of　error。　　In　more　complicated　cases　irregular　curves　of　different　kinds　are　obtained　which　may　be　calculated　on　certain　suppositions。

The　regular　fluctuations　about　a　mean　according　to　the　rule　of　probability　is　often　attributed　to　some　law　underlying　variability。　　（de　Vries；　〃Mutationstheorie〃；　Vol。　I。　page　35；　Leipzig；　1901。）　　But　there　is　no　such　law　which　compels　a　plant　to　vary　in　a　particular　manner。　　Every　experimental　investigation　shows；　as　we　have　already　remarked；　that　the　fluctuation　of　characters　depends　on　fluctuation　in　the　external　factors。　　The　applicability　of　the　method　of　probability　follows　from　the　fact　that　the　numerous　individuals　of　a　species　are　influenced　by　a　limited　number　of　variable　conditions。　　（Klebs；　〃Willkurl。　Ent。〃　Jena；　1903；　page　141。）　　As　each　of　these　conditions　includes　within　certain　limits　all　possible　values　and　exhibits　all　possible　combinations；　it　follows　that；　according　to　the　rules　of　probability；　there　must　be　a　mean　value；　about　which　the　larger　and　smaller　deviations　are　distributed。　　Any　character　will　be　found　to　have　the　mean　value　which　corresponds　with　that　combination　of　determining　factors　which　occurs　most　frequently。　　Deviations　towards　plus　and　minus　values　will　be　correspondingly　produced　by　rarer　conditions。

A　conclusion　of　fundamental　importance　may　be　drawn　from　this　conception；　which　is；　to　a　certain　extent；　supported　by　experimental　investigation。　　（Klebs；　〃Studien　uber　Variation〃；　〃Arch。　fur　Entw。〃　1907。）　　There　is　no　normal　curve　for　a　particular　CHARACTER；　there　is　only　a　curve　for　the　varying　combinations　of　conditions　occurring　in　nature　or　under　cultivation。　　Under　other　conditions　entirely　different　curves　may　be　obtained　with　other　variants　as　a　mean　value。　　If；　for　example；　under　ordinary　conditions　the　number　10　is　the　most　frequent　variant　for　the　stamens　of　Sedum　spectabile；　in　special　circumstances　（r