darwin and modern science-第26节

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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　proof　of　this　thesis。　　The　numerical　proportion　of　the　different　species　of　the　flora　is　always　changing　according　to　external　influences。　　Thus；　in　a　given　meadow；　some　species　will　flower　abundantly　in　one　year　and　then　almost　disappear；　until；　after　a　series　of　years；　circumstances　allow　them　again　to　multiply　rapidly。　　Other　species；　which　have　taken　their　places；　will　then　become　rare。　　It　follows　from　this　principle；　that　notwithstanding　the　constantly　changing　conditions；　a　suitable　selection　from　the　constituents　of　a　meadow　will　ensure　a　continued　high　production。　　But；　although　the　principle　is　quite　clear；　artificial　selection　has；　as　yet；　done　very　little　towards　reaching　a　really　high　standard。

The　same　holds　good　for　cereals。　　In　ordinary　circumstances　a　field　will　give　a　greater　yield；　if　the　crop　grown　consists　of　a　number　of　sufficiently　differing　types。　　Hence　it　happens　that　almost　all　older　varieties　of　wheat　are　mixtures　of　more　or　less　diverging　forms。　　In　the　same　variety　the　numerical　composition　will　vary　from　year　to　year；　and　in　oats　this　may；　in　bad　years；　go　so　far　as　to　destroy　more　than　half　of　the　harvest；　the　wind…oats　（Avena　fatua）；　which　scatter　their　grain　to　the　winds　as　soon　as　it　ripens；　increasing　so　rapidly　that　they　assume　the　dominant　place。　　A　severe　winter；　a　cold　spring　and　other　extreme　conditions　of　life　will　destroy　one　form　more　completely　than　another；　and　it　is　evident　that　great　changes　in　the　numerical　composition　of　the　mixture　may　thus　be　brought　about。

This　mixed　condition　of　the　common　varieties　of　cereals　was　well　known　to　Darwin。　　For　him　it　constituted　one　of　the　many　types　of　variability。　　It　is　of　that　peculiar　nature　to　which；　in　describing　other　groups；　he　applies　the　term　polymorphy。　　It　does　not　imply　that　the　single　constituents　of　the　varieties　are　at　present　really　changing　their　characters。　　On　the　other　hand；　it　does　not　exclude　the　possibility　of　such　changes。　　It　simply　states　that　observation　shows　the　existence　of　different　forms；　how　these　have　originated　is　a　question　which　it　does　not　deal　with。　　In　his　well…　known　discussion　of　the　variability　of　cereals；　Darwin　is　mainly　concerned　with　the　question；　whether　under　cultivation　they　have　undergone　great　changes　or　only　small　ones。　　The　decision　ultimately　depends　on　the　question；　how　many　forms　have　originally　been　taken　into　cultivation。　　Assuming　five　or　six　initial　species；　the　variability　must　be　assumed　to　have　been　very　large；　but　on　the　assumption　that　there　were　between　ten　and　fifteen　types；　the　necessary　range　of　variability　is　obviously　much　smaller。　　But　in　regard　to　this　point；　we　are　of　course　entirely　without　historical　data。

Few　of　the　varieties　of　wheat　show　conspicuous　differences；　although　their　number　is　great。　　If　we　compare　the　differentiating　characters　of　the　smaller　types　of　cereals　with　those　of　ordinary　wild　species；　even　within　the　same　genus　or　family；　they　are　obviously　much　less　marked。　　All　these　small　characters；　howeve