darwin and modern science-第65部分

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ken　because　it　is　always　represented　by　a　certain　definite　state；　whether　we　are　dealing　with　a　single　cell　or　with　a　small　group　of　cells。　　These　have　been　produced　from　pre…existing　cells　and　they　in　turn　from　others；　the　problem　is　constantly　pushed　back　through　a　succession　of　generations　until　it　becomes　identified　with　that　of　the　origin　of　species。

A　way；　however；　is　opened　for　investigation；　experience　teaches　us　that　this　inner　world　is　not　a　constant　factor：　　on　the　contrary；　it　appears　to　be　very　variable。　　The　dependence　of　VARIABLE　INTERNAL　on　VARIABLE　EXTERNAL　conditions　gives　us　the　key　with　which　research　may　open　the　door。　　In　the　lower　plants　this　dependence　is　at　once　apparent；　each　cell　is　directly　subject　to　external　influences。　　In　the　higher　plants　with　their　different　organs；　these　influences　were　transmitted　to　cells　in　course　of　development　along　exceedingly　complex　lines。　　In　the　case　of　the　growing…point　of　a　bud；　which　is　capable　of　producing　a　complete　plant；　direct　influences　play　a　much　less　important　part　than　those　exerted　through　other　organs；　particularly　through　the　roots　and　leaves；　which　are　essential　in　nutrition。　　These　correlations；　as　we　may　call　them；　are　of　the　greatest　importance　as　aids　to　an　understanding　of　form…production。　　When　a　bud　is　produced　on　a　particular　part　of　a　plant；　it　undergoes　definite　internal　modifications　induced　by　the　influence　of　other　organs；　the　activity　of　which　is　governed　by　the　environment；　and　as　the　result　of　this　it　develops　along　a　certain　direction；　it　may；　for　example；　become　a　flower。　　The　particular　direction　of　development　is　determined　before　the　rudiment　is　differentiated　and　is　exerted　so　strongly　that　further　development　ensues　without　interruption；　even　though　the　external　conditions　vary　considerably　and　exert　a　positively　inimical　influence：　　this　produces　the　impression　that　development　proceeds　entirely　independently　of　the　outer　world。　　The　widespread　belief　that　such　independence　exists　is　very　premature　and　at　all　events　unproven。

The　state　of　the　young　rudiment　is　the　outcome　of　previous　influences　of　the　external　world　communicated　through　other　organs。　　Experiments　show　that　in　certain　cases；　if　the　efficiency　of　roots　and　leaves　as　organs　concerned　with　nutrition　is　interfered　with；　the　production　of　flowers　is　affected；　and　their　characters；　which　are　normally　very　constant；　undergo　far…reaching　modifications。　　To　find　the　right　moment　at　which　to　make　the　necessary　alteration　in　the　environment　is　indeed　difficult　and　in　many　cases　not　yet　possible。　　This　is　especially　the　case　with　fertilised　eggs；　which　in　a　higher　degree　than　buds　have　acquired；　through　parental　influences；　an　apparently　fixed　internal　organisation；　and　this　seems　to　have　pre…determined　their　development。　　It　is；　however；　highly　probable　that　it　will　be　possible；　by　influencing　the　parents；　to　alter　the　internal　organisation　and　to　switch　off　development　on　to　other　lines。

Having　made　these　general　observations　I　will　now　cite　a　few　of　the　many　facts　at　our　disposal；　in　order　to　illustrate　the　methods　and　aim　of　the　experimental　methods　of　research。　　As　a　matter　of　convenience　I　will　deal　separately　with　modification　of　development　and　with　modification　of　single　organs。

I。　　EFFECT　OF　ENVIRONMENT　UPON　THE　COURSE　OF　DEVELOPMENT。

Every　plant；　whether　an　alga　or　a　flowering　plant　passes；　under　natural　conditions；　through　a　series　of　developmental　stages　characteristic　of　each　species；　and　these　consist　in　a　regular　sequence　of　definite　forms。　　It　is　impossible　to　form　an　opinion　from　mere　observation　and　description　as　to　what　inner　changes　are　essential　for　the　production　of　the　several　forms。　　We　must　endeavour　to　influence　the　inner　factors　by　known　external　conditions　in　such　a　way　that　the　individual　stages　in　development　are　separately　controlled　and　the　order　of　their　sequence　determined　at　will　by　experimental　treatment。　　Such　control　over　the　course　of　development　may　be　gained　with　special　certainty　in　the　case　of　the　lower　organisms。

With　these　it　is　practicable　to　control　the　principal　conditions　of　cultivation　and　to　vary　them　in　various　ways。　　By　this　means　it　has　been　demonstrated　that　each　developmental　stage　depends　upon　special　external　conditions；　and　in　cases　where　our　knowledge　is　sufficient；　a　particular　stage　may　be　obtained　at　will。　　In　the　Green　Algae　（See　Klebs；　〃Die　Bedingung　der　Fortpflanzung。。。〃；　Jena；　1896；　also　〃Jahrb。　fur　Wiss。　Bot。〃　1898　and　1900；　〃Probleme　der　Entwickelung；　III。〃　　〃Biol。　Centralbl。〃　1904；　page　452。）；　as　in　the　case　of　Fungi；　we　may　classify　the　stages　of　development　into　purely　vegetative　growth　（growth；　cell…division；　branching）；　asexual　reproduction　（formation　of　zoospores；　conidia）　and　sexual　processes　（formation　of　male　and　female　sexual　organs）。　　By　modifying　the　external　conditions　it　is　possible　to　induce　algae　or　fungi　（Vaucheria；　Saprolegnia）　to　grow　continuously　for　several　years　or；　in　the　course　of　a　few　days；　to　die　after　an　enormous　production　of　asexual　or　sexual　cells。　　In　some　instances　even　an　almost　complete　stoppage　of　growth　may　be　caused；　reproductive　cells　being　scarcely　formed　before　the　organism　is　again　compelled　to　resort　to　reproduction。　　Thus　the　sequence　of　the　different　stages　in　development　can　be　modified　as　we　may　desire。

The　result　of　a　more　thorough　investigation　of　the　determining　conditions　appears　to　produce　at　first　sight　a　confused　impression　of　all　sorts　of　possibilities。　　Even　closely　allied　species　exhibit　differences　in　regard　to　the　connection　between　their　development　and　external　conditions。　　It　is　especially　noteworthy　that　the　same　form　in　development　may　be　produced　as　the　result　of　very　different　alterations　in　the　environment。　　At　the　same　time　we　can　undoubtedly　detect　a　certain　unity　in　the　multiplicity　of　the　individual　phenomena。

If　we　compare　the　factors　essential　for　the　different　stages　in　development；　we　see　that　the　question　always　resolves　itself　into　one　of　modification　of　similar　conditions　common　to　all　life…processes。　　We　should　rather　have　inferred　that　there　exist　specific　external　stimuli　for　each　developmental　stage；　for　instance；　certain　chemical　agencies。　　Experiments　hitherto　made　support　the　conclusion　that　QUANTITATIVE　alterations　in　the　general　conditions　of　life　produce　different　types　of　development。　　An　alga　or　a　fungus　grows　so　long　as　all　the　conditions　of　nutrition　remain　at　a　certain　optimum　for　growth。　　In　order　to　bring　about　asexual　reproduction；　e。g。　the　formation　of　zoospores；　it　is　sometimes　necessary　to　increase　the　degree　of　intensity　of　external　factors；　sometimes；　on　the　other　hand；　these　must　be　reduced　in　intensity。　　In　the　case　of　many　algae　a　decrease　in　light…intensity　or　in　the　amount　of　salts　in　the　culture　solution；　or　in　the　temperature；　induces　asexual　reproduction；　while　in　others；　on　the　contrary；　an　increase　in　regard　to　each　of　these　factors　is　required　to　produce　the　same　result。　　This　holds　good　for　the　quantitative　variations　which　induce　sexual　reproduction　in　algae。　　The　controlling　factor　is　found　to　be　a　reduction　in　the　supply　of　nutritive　salts　and　the　exposure　of　the　plants　to　prolonged　illumination　or；　better　still；　an　increase　in　the　intensity　of　the　light；　the　efficiency　of　illumination　depending　on　the　consequent　formation　of　organic　substances　such　as　carbohydrates。

The　quantitative　alterations　of　external　conditions　may　be　spoken　of　as　releasing　stimuli。　　They　produce；　in　the　complex　equilibrium　of　the　cell；　quantitative　modifications　in　the　arrangement　and　distribution　of　mass；　by　means　of　which　other　chemical　processes　are　at　once　set　in　motion；　and　finally　a　new　condition　of　equilibrium　is　attained。　　But　the　commonly　expressed　view　that　the　environment　can　as　a　rule　act　only　as　a　releasing　agent　is　incorrect；　because　it　overlooks　an　essential　point。　　The　power　of　a　cell　to　receive　stimuli　is　only　acquired　as　the　result　of　previous　nutrition；　which　has　produced　a　definite　condition　of　concentration　of　different　substances。　　Quantities　are　in　this　case　the　determining　factors。　　The　distribution　of　quantities　is　especially　important　in　the　sexual　reproduction　of　algae；　for　which　a　vigorous　production　of　the　materials　formed　during　carbon…assimilation　appears　to　be　essential。

In　the　Flowering　plants；　on　the　other　hand；　for　reasons　already　mentioned；　the　whole　problem　is　more　complicated。　　Investigations　on　changes　in　the　course　of　development　of　fertilised　eggs　have　hitherto　been　unsuccessful；　the　difficulty　of　influencing　egg…cells　deeply　immersed　in　tissue　constitutes　a　serious　obstacle。　　Other　parts　of　plants　are；　however；　convenient　objects　of　experiment；　e。g。　the　growing　apices　of　buds　which　serve　as　cuttings　for　reproductive　purposes；　or　buds　on　tubers；　runners；　rhizomes；　etc。　　A　growing　apex　consists　of　cells　capable　of　division　in　which；　as　in　egg…cells；　a　complete　series　of　latent　possibilities　of　development　is　embodied。　　Which　of　these　possibilities　becomes　effective　depends　upon　the　action　of　the　outer　world　transmitted　by　organs　concerned　with　nutrition。

Of　the　different　stages　which　a　flowering　plant　passes　through　in　the　course　of　its　development　we　will　deal　only　with　one　in　order　to　show　that；　in　spite　of　its　great　complexity；　the　problem　is；　in　essentials；　equally　open　to　attack　in　the　higher　plants　and　in　the　simplest　organisms。　　The　most　important　stage　in　the　life　of　a　flowering　plant　is　the　transition　from　purely　vegetative　growth　to　sexual　reproductionthat　is；　the　production　of　flowers。　　In　certain　cases　it　can　be　demonstrated　that　there　is　no　internal　cause；　dependent　simply　on　the　specific　structure；　which　compels　a　plant　to　produce　its　flowers　after　a　definite　period　of　vegetative　growth。　　（Klebs；　〃Willkurliche　Entwickelungsanderungen〃；　Jena　1903；　see　also　〃Probleme　der　Entwickelung〃；　I。　II。　　〃Centralbl。〃　1904。）

One　extreme　case；　that　of　exceptionally　early　flowering；　has　been　observed　in　nature　and　more　often　in　cultivation。　　A　number　of　plants　under　certain　conditions　are　able　to　flower　soon　after　germination。　　（Cf。　numerous　records　of　this　kind　by　Diels；　〃Jugendformen　und　Bluten〃；　Berlin；　1906。）　　This　shortening　of　the　period　of　developmen