darwin and modern science-第25节

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rsal　validity　for　all　organic　beings。　　It　must　hold　true　for　all　characters　in　man；　physical　as　well　as　intellectual　and　moral　qualities；　it　must　hold　true　for　the　plant　kingdom　as　well　as　for　the　animal　kingdom；　in　short；　it　must　include　the　whole　living　world。

Quetelet's　law　may　be　most　easily　studied　in　those　cases　where　the　variability　relates　to　measure；　number　and　weight；　and　a　vast　number　of　facts　have　since　confirmed　its　exactness　and　its　validity　for　all　kinds　of　organisms；　organs　and　qualities。　　But　if　we　examine　it　more　closely；　we　find　that　it　includes　just　those　minute　variations；　which；　as　Darwin　repeatedly　pointed　out；　have　often　no　significance　for　the　origin　of　species。　　In　the　phenomena；　described　by　Quetelet's　law　nothing　〃happens　to　arise〃；　all　is　governed　by　the　common　law；　which　states　that　small　deviations　from　the　mean　type　are　frequent；　but　that　larger　aberrations　are　rare；　the　rarer　as　they　are　larger。　　Any　degree　of　variation　will　be　found　to　occur；　if　only　the　number　of　individuals　studied　is　large　enough：　　it　is　even　possible　to　calculate　before　hand；　how　many　specimens　must　be　compared　in　order　to　find　a　previously　fixed　degree　of　deviation。

The　variations；　which　from　time　to　time　happen　to　appear；　are　evidently　not　governed　by　this　law。　　They　cannot；　as　yet；　be　produced　at　will：　　no　sowings　of　thousands　or　even　of　millions　of　plants　will　induce　them；　although　by　such　means　the　chance　of　their　occurring　will　obviously　be　increased。　　But　they　are　known　to　occur；　and　to　occur　suddenly　and　abruptly。　　They　have　been　observed　especially　in　horticulture；　where　they　are　ranged　in　the　large　and　ill…defined　group　called　sports。　　Korschinsky　has　collected　all　the　evidence　which　horticultural　literature　affords　on　this　point。　　（S。　Korschinsky；　〃Heterogenesis　und　Evolution〃；　〃Flora〃；　Vol。　LXXXIX。　pages　240…363；　1901。）　　Several　cases　of　the　first　appearance　of　a　horticultural　novelty　have　been　recorded：　　this　has　always　happened　in　the　same　way；　it　appeared　suddenly　and　unexpectedly　without　any　definite　relation　to　previously　existing　variability。　　Dwarf　types　are　one　of　the　commonest　and　most　favourite　varieties　of　flowering　plants；　they　are　not　originated　by　a　repeated　selection　of　the　smallest　specimens；　but　appear　at　once；　without　intermediates　and　without　any　previous　indication。　　In　many　instances　they　are　only　about　half　the　height　of　the　original　type；　thus　constituting　obvious　novelties。　　So　it　is　in　other　cases　described　by　Korschinsky：　　these　sports　or　mutations　are　now　recognised　to　be　the　main　source　of　varieties　of　horticultural　plants。

As　already　stated；　I　do　not　pretend　that　the　production　of　horticultural　novelties　is　the　prototype　of　the　origin　of　new　species　in　nature。　　I　assume　that　they　are；　as　a　rule；　derived　from　the　parent　species　by　the　loss　of　some　organ　or　quality；　whereas　the　main　lines　of　the　evolution　of　the　animal　and　vegetable　kingdom　are　of　course　determined　by　progressive　changes。　　Darwin　himself　has　often　pointed　out　this　difference。　　But　the　saltatory　origin　of　horticultural　novelties　is　as　yet　the　simplest　parallel　for　natural　mutations；　since　it　relates　to　forms　and　phenomena；　best　known　to　the　general　student　of　evolution。

The　point　which　I　wish　to　insist　upon　is　this。　　The　difference　between　small　and　ever　present　fluctuations　and　rare　and　more　sudden　variations　was　clear　to　Darwin；　although　the　facts　known　at　his　time　were　too　meagre　to　enable　a　sharp　line　to　be　drawn　between　these　two　great　classes　of　variability。　　Since　Darwin's　time　evidence；　which　proves　the　correctness　of　his　view；　has　accumulated　with　increasing　rapidity。　　Fluctuations　constitute　one　type；　they　are　never　absent　and　follow　the　law　of　chance；　but　they　do　not　afford　the　material　from　which　to　build　new　species。　　Mutations；　on　the　other　hand；　only　happen　to　occur　from　time　to　time。　　They　do　not　necessarily　produce　greater　changes　than　fluctuations；　but　such　as　may　become；　or　rather　are　from　their　very　nature；　constant。　　It　is　this　constancy　which　is　the　mark　of　specific　characters；　and　on　this　basis　every　new　specific　character　may　be　assumed　to　have　arisen　by　mutation。

Some　authors　have　tried　to　show　that　the　theory　of　mutation　is　opposed　to　Darwin's　views。　　But　this　is　erroneous。　　On　the　contrary；　it　is　in　fullest　harmony　with　the　great　principle　laid　down　by　Darwin。　　In　order　to　be　acted　upon　by　that　complex　of　environmental　forces；　which　Darwin　has　called　natural　selection；　the　changes　must　obviously　first　be　there。　　The　manner　in　which　they　are　produced　is　of　secondary　importance　and　has　hardly　any　bearing　on　the　theory　of　descent　with　modification。　　（〃Life　and　Letters〃　II。　125。）　　　A　critical　survey　of　all　the　facts　of　variability　of　plants　in　nature　as　well　as　under　cultivation　has　led　me　to　the　conviction；　that　Darwin　was　right　in　stating　that　those　rare　beneficial　variations；　which　from　time　to　time　happen　to　arise；the　now　so…called　mutationsare　the　real　source　of　progress　in　the　whole　realm　of　the　organic　world。

II。　　EXTERNAL　AND　INTERNAL　CAUSES　OF　VARIABILITY。

All　phenomena　of　animal　and　plant　life　are　governed　by　two　sets　of　causes；　one　of　these　is　external；　the　other　internal。　　As　a　rule　the　internal　causes　determine　the　nature　of　a　phenomenonwhat　an　organism　can　do　and　what　it　cannot　do。　　The　external　causes；　on　the　other　hand；　decide　when　a　certain　variation　will　occur；　and　to　what　extent　its　features　may　be　developed。

As　a　very　clear　and　wholly　typical　instance　I　cite　the　cocks…combs　（Celosia）。　　This　race　is　distinguished　from　allied　forms　by　its　faculty　of　producing　the　well…known　broad　and　much　twisted　combs。　　Every　single　individual　possesses　this　power；　but　all　individuals　do　not　exhibit　it　in　its　most　complete　form。　　In　some　cases　this　faculty　may　not　be　exhibited　at　the　top　of　the　main　stem；　although　developed　in　lateral　branches：　　in　others　it　begins　too　late　for　full　development。　　Much　depends　upon　nourishment　and　cultivation；　but　almost　always　the　horticulturist　has　to　single　out　the　best　individuals　and　to　reject　those　which　do　not　come　up　to　the　standard。

The　internal　causes　are　of　a　historical　nature。　　The　external　ones　may　be　defined　as　nourishment　and　environment。　　In　some　cases　nutrition　is　the　main　factor；　as；　for　instance；　in　fluctuating　variability；　but　in　natural　selection　environment　usually　plays　the　larger　part。

The　internal　or　historical　causes　are　constant　during　the　life…time　of　a　species；　using　the　term　species　in　its　most　limited　sense；　as　designating　the　so…called　elementary　species　or　the　units　out　of　which　the　ordinary　species　are　built　up。　　These　historical　causes　are　simply　the　specific　characters；　since　in　the　origin　of　a　species　one　or　more　of　these　must　have　been　changed；　thus　producing　the　characters　of　the　new　type。　　These　changes　must；　of　course；　also　be　due　partly　to　internal　and　partly　to　external　causes。

In　contrast　to　these　changes　of　the　internal　causes；　the　ordinary　variability　which　is　exhibited　during　the　life…time　of　a　species　is　called　fluctuating　variability。　　The　name　mutations　or　mutating　variability　is　then　given　to　the　changes　in　the　specific　characters。　　It　is　desirable　to　consider　these　two　main　divisions　of　variability　separately。

In　the　case　of　fluctuations　the　internal　causes；　as　well　as　the　external　ones；　are　often　apparent。　　The　specific　characters　may　be　designated　as　the　mean　about　which　the　observed　forms　vary。　　Almost　every　character　may　be　developed　to　a　greater　or　a　less　degree；　but　the　variations　of　the　single　characters　producing　a　small　deviation　from　the　mean　are　usually　the　commonest。　　The　limits　of　these　fluctuations　may　be　called　wide　or　narrow；　according　to　the　way　we　look　at　them；　but　in　numerous　cases　the　extreme　on　the　favoured　side　hardly　surpasses　double　the　value　of　that　on　the　other　side。　　The　degree　of　this　development；　for　every　individual　and　for　every　organ；　is　dependent　mainly　on　nutrition。　　Better　nourishment　or　an　increased　supply　of　food　produces　a　higher　development；　only　it　is　not　always　easy　to　determine　which　direction　is　the　fuller　and　which　is　the　poorer　one。　　The　differences　among　individuals　grown　from　different　seeds　are　described　as　examples　of　individual　variability；　but　those　which　may　be　observed　on　the　same　plant；　or　on　cuttings；　bulbs　or　roots　derived　from　one　individual　are　referred　to　as　cases　of　partial　variability。　　Partial　variability；　therefore；　determines　the　differences　among　the　flowers；　fruits；　leaves　or　branches　of　one　individual：　　in　the　main；　it　follows　the　same　laws　as　individual　variability；　but　the　position　of　a　branch　on　a　plant　also　determines　its　strength；　and　the　part　it　may　take　in　the　nourishment　of　the　whole。　　Composite　flowers　and　umbels　therefore　have；　as　a　rule；　fewer　rays　on　weak　branches　than　on　the　strong　main　ones。　　The　number　of　carpels　in　the　fruits　of　poppies　becomes　very　small　on　the　weak　lateral　branches；　which　are　produced　towards　the　autumn；　as　well　as　on　crowded；　and　therefore　on　weakened　individuals。　　Double　flowers　follow　the　same　rule；　and　numerous　other　instances　could　easily　be　adduced。

Mutating　variability　occurs　along　three　main　lines。　　Either　a　character　may　disappear；　or；　as　we　now　say；　become　latent；　or　a　laten