darwin and modern science-第67部分

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e　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　（red　light）　this　is　replaced　by　the　number　5。　　The　more　accurately　we　know　the　conditions　for　a　particular　form　or　number；　and　are　able　to　reproduce　it　by　experiment；　the　nearer　we　are　to　achieving　our　aim　of　rendering　a　particular　variation　impossible　or　of　making　it　dominant。

In　addition　to　the　individual　variations　of　a　species；　more　pronounced　fluctuations　occur　relatively　rarely　and　sporadically　which　are　spoken　of　as　〃single　variations；〃　or　if　specially　striking　as　abnormalities　or　monstrosities。　　These　forms　have　long　attracted　the　attention　of　morphologists；　a　large　number　of　observations　of　this　kind　are　given　in　the　handbooks　of　Masters　（Masters；　〃Vegetable　Teratology〃；　London；　1869。）　and　Penzig　（Penzig；　〃Pflanzen…Teratologie；　Vols　I。　and　II。　Genua；　1890…94。）　　These　variations；　which　used　to　be　regarded　as　curiosities；　have　now　assumed　considerable　importance　in　connection　with　the　causes　of　form…　development。　　They　also　possess　special　interest　in　relation　to　the　question　of　heredity；　a　subject　which　does　not　at　present　concern　us；　as　such　deviations　from　normal　development　undoubtedly　arise　as　individual　variations　induced　by　the　influence　of　environment。

Abnormal　developments　of　all　kinds　in　stems；　leaves；　and　flowers；　may　be　produced　by　parasites；　insects；　or　fungi。　　They　may　also　be　induced　by　injury；　as　Blaringhem　（Blaringhem；　〃Mutation　et　traumatismes〃；　Paris；　1907。）　has　more　particularly　demonstrated；　which；　by　cutting　away　the　leading　shoots　of　branches　in　an　early　stage　of　development；　caused　fasciation；　torsion；　anomalous　flowers；　etc。　　The　experiments　of　Blaringhem　point　to　the　probability　that　disturbances　in　the　conditions　of　food…supply　consequent　on　injury　are　the　cause　of　the　production　of　monstrosities。　　This　is　certainly　the　case　in　my　experiments　with　species　of　Sempervivum　（Klebs；　〃Kunstliche　Metamorphosen〃；　Stuttgart；　1906。）；　individuals；　which　at　first　formed　normal　flowers；　produced　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　th