darwin and modern science-第75节

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duced　a　great　variety　of　abnormalities　as　the　result　of　changes　in　nutrition；　we　may　call　to　mind　the　fact　that　the　formation　of　inflorescences　occurs　normally　when　a　vigorous　production　of　organic　compounds；　such　as　starch；　sugar；　etc。　follows　a　diminution　in　the　supply　of　mineral　salts。　　On　the　other　hand；　the　development　of　inflorescences　is　entirely　suppressed　if；　at　a　suitable　moment　before　the　actual　foundations　have　been　laid；　water　and　mineral　salts　are　supplied　to　the　roots。　　If；　during　the　week　when　the　inflorescence　has　just　been　laid　down　and　is　growing　very　slowly；　the　supply　of　water　and　salts　is　increased；　the　internal　conditions　of　the　cells　are　essentially　changed。　　At　a　later　stage；　after　the　elongation　of　the　inflorescence；　rosettes　of　leaves　are　produced　instead　of　flowers；　and　structures　intermediate　between　the　two　kinds　of　organs；　a　number　of　peculiar　plant…forms　are　thus　obtained　（Cf。　Lotsy；　〃Vorlesungen　uber　Deszendenztheorien〃；　Vol。　II。　pl。　3；　Jena；　1908。）　　Abnormalities　in　the　greatest　variety　are　produced　in　flowers　by　varying　the　time　at　which　the　stimulus　is　applied；　and　by　the　cooperation　of　other　factors　such　as　temperature；　darkness；　etc。　　In　number　and　arrangement　the　several　floral　members　vary　within　wide　limits；　sepals；　petals；　stamens；　and　carpels　are　altered　in　form　and　colour；　a　transformation　of　stamens　to　carpels　and　from　carpels　to　stamens　occurs　in　varying　degrees。　　The　majority　of　the　deviations　observed　had　not　previously　been　seen　either　under　natural　conditions　or　in　cultivation；　they　were　first　brought　to　light　through　the　influence　of　external　factors。

Such　transformations　of　flowers　become　apparent　at　a　time；　which　is　separated　by　about　two　months　from　the　period　at　which　the　particular　cause　began　to　act。　　There　is；　therefore；　no　close　connection　between　the　appearance　of　the　modifications　and　the　external　conditions　which　prevail　at　the　moment。　　When　we　are　ignorant　of　the　causes　which　are　operative　so　long　before　the　results　are　seen；　we　gain　the　impression　that　such　variations　as　occur　are　spontaneous　or　autonomous　expressions　of　the　inner　nature　of　the　plant。　　It　is　much　more　likely　that；　as　in　Sempervivum；　they　were　originally　produced　by　an　external　stimulus　which　had　previously　reached　the　sexual　cells　or　the　young　embryo。　　In　any　case　abnormalities　of　this　kind　appear　to　be　of　a　special　type　as　compared　with　ordinary　fluctuating　variations。　　Darwin　pointed　out　this　difference；　Bateson　（Bateson；　〃Materials　for　the　study　of　Variation〃；　London；　1894；　page　5。）　has　attempted　to　make　the　distinction　sharper；　at　the　same　time　emphasising　its　importance　in　heredity。

Bateson　applies　the　term　CONTINUOUS　to　small　variations　connected　with　one　another　by　transitional　stages；　while　those　which　are　more　striking　and　characterised　from　the　first　by　a　certain　completeness；　he　names　DISCONTINUOUS。　　He　drew　attention　to　a　great　difficulty　which　stands　in　the　way　of　Lamarck's　hypothesis；　as　also　of　Darwin's　view。　　〃According　to　both　theories；　specific　diversity　of　form　is　consequent　upon　diversity　of　environment；　and　diversity　of　environment　is　thus　the　ultimate　measure　of　diversity　of　specific　form。　　Here　then　we　meet　the　difficulty　that　diverse　environments　often　shade　into　each　other　insensibly　and　form　a　continuous　series；　whereas　the　Specific　Forms　of　life　which　are　subject　to　them　on　the　whole　form　a　Discontinuous　Series。〃　　This　difficulty　is；　however；　not　of　fundamental　importance　as　well　authenticated　facts　have　been　adduced　showing　that　by　alteration　of　the　environment　discontinuous　variations；　such　as　alterations　in　the　number　and　form　of　members　of　a　flower；　may　be　produced。　　We　can　as　yet　no　more　explain　how　this　happens　than　we　can　explain　the　existence　of　continuous　variations。　　We　can　only　assert　that　both　kinds　of　variation　arise　in　response　to　quantitative　alterations　in　external　conditions。　　The　question　as　to　which　kind　of　variation　is　produced　depends　on　the　greater　or　less　degree　of　alteration；　it　is　correlated　with　the　state　of　the　particular　cells　at　the　moment。

In　this　short　sketch　it　is　only　possible　to　deal　superficially　with　a　small　part　of　the　subject。　　It　has　been　clearly　shown　that　in　view　of　the　general　dependence　of　development　on　the　factors　of　the　environment　a　number　of　problems　are　ready　for　experimental　treatment。　　One　must；　however；　not　forget　that　the　science　of　the　physiology　of　form　has　not　progressed　beyond　its　initial　stages。　　Just　now　our　first　duty　is　to　demonstrate　the　dependence　on　external　factors　in　as　many　forms　of　plants　as　possible；　in　order　to　obtain　a　more　thorough　control　of　all　the　different　plant…forms。　The　problem　is　not　only　to　produce　at　will　（and　independently　of　their　normal　mode　of　life）　forms　which　occur　in　nature；　but　also　to　stimulate　into　operation　potentialities　which　necessarily　lie　dormant　under　the　conditions　which　prevail　in　nature。　　The　constitution　of　a　species　is　much　richer　in　possibilities　of　development　than　would　appear　to　be　the　case　under　normal　conditions。　　It　remains　for　man　to　stimulate　into　activity　all　the　potentialities。

But　the　control　of　plant…form　is　only　a　preliminary　stepthe　foundation　stones　on　which　to　erect　a　coherent　scientific　structure。　　We　must　discover　what　are　the　internal　processes　in　the　cell　produced　by　external　factors；　which　as　a　necessary　consequence　result　in　the　appearance　of　a　definite　form。　　We　are　here　brought　into　contact　with　the　most　obscure　problem　of　life。　　Progress　can　only　be　made　pari　passu　with　progress　in　physics　and　chemistry；　and　with　the　growth　of　our　knowledge　of　nutrition；　growth；　etc。

Let　us　take　one　of　the　simplest　casesan　alteration　in　form。　　A　cylindrical　cell　of　the　alga　Stigeoclonium　assumes；　as　Livingstone　（Livingstone；　〃On　the　nature　of　the　stimulus　which　causes　the　change　of　form；　etc。〃　　〃Botanical　Gazette〃；　XXX。　1900；　also　XXXII。　1901。）　has　shown；　a　spherical　form　when　the　osmotic　pressure　of　the　culture　fluid　is　increased；　or　a　spore　of　Mucor；　which；　in　a　sugar　solution　grows　into　a　branched　filament；　in　the　presence　of　a　small　quantity　of　acid　（hydrogen　ions）　becomes　a　comparatively　large　sphere。　　（Ritter；　〃Ueber　Kugelhefe；　etc。〃　　〃Ber。　bot。　Gesell。〃　Berlin；　XXV。　page　255；　1907。）　　In　both　cases　there　has　undoubtedly　been　an　alteration　in　the　osmotic　pressure　of　the　cell…sap；　but　this　does　not　suffice　to　explain　the　alteration　in　form；　since　the　unknown　alterations；　which　are　induced　in　the　protoplasm；　must　in　their　turn　influence　the　cell…membrane。　　In　the　case　of　the　very　much　more　complex　alterations　in　form；　such　as　we　encounter　in　the　course　of　development　of　plants；　there　do　not　appear　to　be　any　clues　which　lead　us　to　a　deeper　insight　into　the　phenomena。　　Nevertheless　we　continue　the　attempt；　seeking　with　the　help　of　any　available　hypothesis　for　points　of　attack；　which　may　enable　us　to　acquire　a　more　complete　mastery　of　physiological　methods。　　To　quote　a　single　example；　I　may　put　the　question；　what　internal　changes　produce　a　transition　from　vegetative　growth　to　sexual　reproduction？

The　facts；　which　are　as　clearly　established　from　the　lower　as　for　the　higher　plants；　teach　us　that　quantitative　alteration　in　the　environment　produces　such　a　transition。　　This　suggests　the　conclusion　that　quantitative　internal　changes　in　the　cells；　and　with　them　disturbances　in　the　degree　of　concentration；　are　induced；　through　which　the　chemical　reactions　are　led　in　the　direction　of　sexual　reproduction。　　An　increase　in　the　production　of　organic　substances　in　the　presence　of　light；　chiefly　of　the　carbohydrates；　with　a　simultaneous　decrease　in　the　amount　of　inorganic　salts　and　water；　are　the　cause　of　the　disturbance　and　at　the　same　time　of　the　alteration　in　the　direction　of　development。　　Possibly　indeed　mineral　salts　as　such　are　not　in　question；　but　only　in　the　form　of　other　organic　combinations；　particularly　proteid　material；　so　that　we　are　concerned　with　an　alteration　in　the　relation　of　the　carbohydrates　and　proteids。　　The　difficulties　of　such　researches　are　very　great　because　the　methods　are　not　yet　sufficiently　exact　to　demonstrate　the　frequently　small　quantitative　differences　in　chemical　composition。　　Questions　relating　to　the　enzymes；　which　are　of　the　greatest　importance　in　all　these　life…processes；　are　especially　complicated。　　In　any　case　it　is　the　necessary　result　of　such　an　hypothesis　that　we　must　employ　chemical　methods　of　investigation　in　dealing　with　problems　connected　with　the　physiology　of　form。

II。　　INFLUENCE　OF　ENVIRONMENT　ON　THE　TRANSFORMATION　OF　SPECIES。

The　study　of　the　physiology　of　form…development　in　a　pure　species　has　already　yielded　results　and　makes　slow　but　sure　progress。　　The　physiology　of　the　possibility　of　the　transformation　of　one　species　into　another　is　based；　as　yet；　rather　on　pious　hope　than　on　accomplished　fact。　　From　the　first　it　appeared　to　be　hopeless　to　investigate　physiologically　the　origin　of　Linnean　species　and　at　the　same　time　that　of　the　natural　system；　an　aim　which　Darwin　had　before　him　in　his　enduring　work。　　The　historical　sequence　of　events；　of　which　an　organism　is　the　expression；　can　only　be　treated　hypothetically　with　the　help　of　facts　supplied　by　comparative　morphology；　the　history　of　development；　geographical