darwin and modern science-第21章

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nd　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　latent　character　may　reappear；　reproducing　thereby　a　character　which　was　once　prominent　in　more　or　less　remote　ancestors。　　The　third　and　most　interesting　case　is　that　of　the　production　of　quite　new　characters　which　never　existed　in　the　ancestors。　　Upon　this　progressive　mutability　the　main　development　of　the　animal　and　vegetable　kingdom　evidently　depends。　　In　contrast　to　this；　the　two　other　cases　are　called　retrogressive　and　degressive　mutability。　　In　nature　retrogressive　mutability　plays　a　large　part；　in　agriculture　and　in　horticulture　it　gives　rise　to　numerous　varieties；　which　have　in　the　past　been　preserved；　either　on　account　of　their　usefulness　or　beauty；　or　simply　as　fancy…types。　　In　fact　the　possession　of　numbers　of　varieties　may　be　considered　as　the　main　character　of　domesticated　animals　and　cultivated　plants。

In　the　case　of　retrogressive　and　degressive　mutability　the　internal　cause　is　at　once　apparent；　for　it　is　this　which　causes　the　disappearance　or　reappearance　of　some　character。　　With　progressive　mutations　the　case　is　not　so　simple；　since　the　new　character　must　first　be　produced　and　then　displayed。　　These　two　processes　are　theoretically　different；　but　they　may　occur　together　or　after　long　intervals。　　The　production　of　the　new　character　I　call　premutation；　and　the　displaying　mutation。　　Both　of　course　must　have　their　external　as　well　as　their　internal　causes；　as　I　have　repeatedly　pointed　out　in　my　work　on　the　Mutation　Theory。　　（〃Die　Mutationstheorie〃；　2　vols。；　Leipzig；　1901。）

It　is　probable　that　nutrition　plays　as　important　a　part　among　the　external　causes　of　mutability　as　it　does　among　those　of　fluctuating　variability。　　Observations　in　support　of　this　view；　however；　are　too　scanty　to　allow　of　a　definite　judgment。　　Darwin　assumed　an　accumulative　influence　of　external　causes　in　the　case　of　the　production　of　new　varieties　or　species。　　The　accumulation　might　be　limited　to　the　life…time　of　a　single　individual；　or　embrace　that　of　two　or　more　generations。　　In　the　end　a　degree　of　instability　in　the　equilibrium　of　one　or　more　characters　might　be　attained；　great　enough　for　a　character　to　give　way　under　a　small　shock　produced　by　changed　conditions　of　life。　　The　character　would　then　be　thrown　over　from　the　old　state　of　equilibrium　into　a　new　one。

Characters　which　happen　to　be　in　this　state　of　unstable　equilibrium　are　called　mutable。　　They　may　be　either　latent　or　active；　being　in　the　former　case　derived　from　old　active　ones　or　produced　as　new　ones　（by　the　process；　designated　premutation）。　　They　may　be　inherited　in　this　mutable　condition　during　a　long　series　of　generations。　　I　have　shown　that　in　the　case　of　the　evening　primrose　of　Lamarck　this　state　of　mutability　must　have　existed　for　at　least　half　a　century；　for　this　species　was　introduced　from　Texas　into　England　about　the　year　1860；　and　since　then　all　the　strains　derived　from　its　first　distribution　over　the　several　countries　of　Europe　show　the　same　phenomena　in　producing　new　forms。　　The　production　of　the　dwarf　evening　primrose；　or　Oenothera　nanella；　is　assumed　to　be　due　to　one　of　the　factors；　which　determines　the　tall　stature　of　the　parent　form；　becoming　latent；　this　would；　therefore；　afford　an　example　of　retrogressive　mutation。　　Most　of　the　other　types　of　my　new　mutants；　on　the　other　hand；　seem　to　be　due　to　progressive　mutability。

The　external　causes　of　this　curious　period　of　mutability　are　as　yet　wholly　unknown　and　can　hardly　be　guessed　at；　since　the　origin　of　the　Oenothera　Lamarckiana　is　veiled　in　mystery。　　The　seeds；　introduced　into　England　about　1860；　were　said　to　have　come　from　Texas；　but　whether　from　wild　or　from　cultivated　plants　we　do　not　know。　　Nor　has　the　species　been　recorded　as　having　been　observed　in　the　wild　condition。　　This；　however；　is　nothing　peculiar。　　The　European　types　of　Oenothera　biennis　and　O。　muricata　are　in　the　same　condition。　　The　first　is　said　to　have　been　introduced　from　Virginia；　and　the　second　from　Canada；　but　both　probably　from　plants　cultivated　in　the　gardens　of　these　countries。　　Whether　the　same　elementary　species　are　still　growing　on　those　spots　is　unknown；　mainly　because　the　different　sub…species　of　the　species　mentioned　have　not　been　systematically　studied　and　distinguished。

The　origin　of　new　species；　which　is　in　part　the　effect　of　mutability；　is；　however；　due　mainly　to　natural　selection。　　Mutability　provides　the　new　characters　and　new　elementary　species。　　Natural　selection；　on　the　other　hand；　decides　what　is　to　live　and　what　to　die。　　Mutability　seems　to　be　free；　and　not　restricted　to　previously　determined　lines。　　Selection；　however；　may　take　place　along　the　same　main　lines　in　the　course　of　long　geological　epochs；　thus　directing　the　development　of　large　branches　of　the　animal　and　vegetable　kingdom。　　In　natural　selection　it　is　evident　that　nutrition　and　environment　are　the　main　factors。　　But　it　is　probable　that；　while　nutrition　may　be　one　of　the　main　causes　of　mutability；　environment　may　play　the　chief　part　in　the　decisions　ascribed　to　natural　selection。　　Relations　to　neighbouring　plants　and　to　injurious　or　useful　animals；　have　been　considered　the　most　important　determining　factors　ever　since　the　time　when　Darwin　pointed　out　their　prevailing　influence。

From　this　discussion　of　the　main　causes　of　variability　we　may　derive　the　proposition　that　the　study　of　every　phenomenon　in　the　field　of　heredity；　of　variability；　and　of　the　origin　of　new　species　will　have　to　be　considered　from　two　standpoints；　on　one　hand　we　have　the　internal　causes；　on　the　other　the　external　ones。　　Sometimes　the　first　are　more　easily　detected；　in　other　cases　the　latter　are　more　accessible　to　investigation。　　But　the　complete　elucidation　of　any　phenomenon　of　life　must　always　combine　the　study　of　the　influence　of　internal　with　that　of　external　causes。

III。　　POLYMORPHIC　VARIABILITY　IN　CEREALS。

One　of　the　propositions　of　Darwin's　theory　of　the　struggle　for　life　maintains　that　the　largest　amount　of　life　can　be　supported　on　any　area；　by　great　diversification　or　divergence　in　the　structure　and　constitution　of　its　inhabitants。　　Every　meadow　and　every　forest　affords　a　pro