darwin and modern science-第22章

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e　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；　however；　are　strictly　inherited；　and　this　fact　makes　it　very　probable　that　the　less　obvious　constituents　of　the　mixtures　in　ordinary　fields　must　be　constant　and　pure　as　long　as　they　do　not　intercross。　　Natural　crossing　is　in　most　cereals　a　phenomenon　of　rare　occurrence；　common　enough　to　admit　of　the　production　of　all　possible　hybrid　combinations；　but　requiring　the　lapse　of　a　long　series　of　years　to　reach　its　full　effect。

Darwin　laid　great　stress　on　this　high　amount　of　variability　in　the　plants　of　the　same　variety；　and　illustrated　it　by　the　experience　of　Colonel　Le　Couteur　（〃On　the　Varieties；　Properties；　and　Classification　of　Wheat〃；　Jersey；　1837。）　on　his　farm　on　the　isle　of　Jersey；　who　cultivated　upwards　of　150　varieties　of　wheat；　which　he　claimed　were　as　pure　as　those　of　any　other　agriculturalist。　　But　Professor　La　Gasca　of　Madrid；　who　visited　him；　drew　attention　to　aberrant　ears；　and　pointed　out；　that　some　of　them　might　be　better　yielders　than　the　majority　of　plants　in　the　crop；　whilst　others　might　be　poor　types。　　Thence　he　concluded　that　the　isolation　of　the　better　ones　might　be　a　means　of　increasing　his　crops。　　Le　Couteur　seems　to　have　considered　the　constancy　of　such　smaller　types　after　isolation　as　absolutely　probable；　since　he　did　not　even　discuss　the　possibility　of　their　being　variable　or　of　their　yielding　a　changeable　or　mixed　progeny。　　This　curious　fact　proves　that　he　considered　the　types；　discovered　in　his　fields　by　La　Gasca　to　be　of　the　same　kind　as　his　other　varieties；　which　until　that　time　he　had　relied　upon　as　being　pure　and　uniform。　　Thus　we　see；　that　for　him；　the　variability　of　cereals　was　what　we　now　call　polymorphy。　　He　looked　through　his　fields　for　useful　aberrations；　and　collected　twenty…three　new　types　of　wheat。　　He　was；　moreover；　clear　about　one　point；　which；　on　being　rediscovered　after　half　a　century；　has　become　the　starting…point　for　the　new　Swedish　principle　of　selecting　agricultural　plants。　　It　was　the　principle　of　single…ear　sowing；　instead　of　mixing　the　grains　of　all　the　selected　ears　together。　　By　sowing　each　ear　on　a　separate　plot　he　intended　not　only　to　multiply　them；　but　also　to　compare　their　value。　　This　comparison　ultimately　led　him　to　the　choice　of　some　few　valuable　sorts；　one　of　which；　the　〃Bellevue　de　Talavera；〃　still　holds　its　place　among　the　prominent　sorts　of　wheat　cultivated　in　France。　　This　variety　seems　to　be　really　a　uniform　type；　a　quality　very　useful　under　favourable　conditions　of　cultivation；　but　which　seems　to　have　destroyed　its　capacity　for　further　improvement　by　selection。

The　principle　of　single…ear　sowing；　with　a　view　to　obtain　pure　and　uniform　strains　without　further　selection；　has；　until　a　few　years　ago；　been　almost　entirely　lost　sight　of。　　Only　a　very　few　agriculturists　have　applied　it：　　among　these　are　Patrick　Shirreff　（〃Die　Verbesserung　der　Getreide…Arten〃；　translated　by　R。　Hesse；　Halle；　1880。）　in　Scotland　and　Willet　M。　Hays　（〃Wheat；　varieties；　breeding；　cultivation〃；　Univ。　Minnesota；　Agricultural　Experimental　Station；　Bull。　no。　62；　1899。）　in　Minnesota。　　Patrick　Shirreff　observed　the　fact；　that　in　large　fields　of　cereals；　single　plants　may　from　time　to　time　be　found　with　larger　ears；　which　justify　the　expectation　of　a　far　greater　yield。　　In　the　course　of　about　twenty…five　years　he　isolated　in　this　way　two　varieties　of　wheat　and　two　of　oats。　　He　simply　multiplied　them　as　fast　as　possible；　without　any　selection；　and　put　them　on　the　market。

Hays　was　struck　by　the　fact　that　the　yield　of　wheat　in　Minnesota　was　far　beneath　that　in　the　neighbouring　States。　　The　local　varieties　were　Fife　and　Blue　Stem。　　They　gave　him；　on　inspection；　some　better　specimens；　〃phenomenal　yielders〃　as　he　called　them。　　These　were　simply　isolated　and　propagated；　and；　after　comparison　with　the　parent…variety　and　with　some　other　selected　strains　of　less　value；　were　judged　to　be　of　sufficient　importance　to　be　tested　by　cultivation　all　over　the　State　of　Minnesota。　　They　have　since　almost　supplanted　the　original　types；　at　least　in　most　parts　of　the　State；　with　the　result　that　the　total　yield　of　wheat　in　Minnesota　is　said　to　have　been　increased　by　about　a　million　dollars　yearly。

Definite　progress　in　the　method　of　single…ear　sowing　has；　however；　been　made　only　recently。　　It　had　been　foreshadowed　by　Patrick　Shirreff；　who　after　the　production　of　the　four　varieties　already　mentioned；　tried　to　carry　out　his　work　on　a　larger　scale；　by　including　numerous　minor　deviations　from　the　main　type。　　He　found　by　doing　so　that　the　chances　of　obtaining　a　better　form　were　sufficiently　increased　to　justify　the　trial。　　But　it　was　Nilsson　who　discovered　the　almost　inexhaustible　polymorphy　of　cereals　and　other　agricultural　crops　and　made　it　the　starting…point　for　a　new　and　entirely　trustworthy　method　of　the　highest　utility。　　By　this　means　he　has　produced　during　the　last　fifteen　years　a　number　of　new　and　valuable　races；　which　have　already　supplanted　the　old　types　on　numerous　farms　in　Sweden　and　which　are　now　being　introduced　on　a　large　scale　into　Germany　and　other　European　countries。

It　is　now　twenty　years　since　the　station　at　Svalof　was　founded。　　During　the　first　period　of　its　work；　embracing　about　five　years；　selection　was　practised　on　the　principle　which　was　then　generally　used　in　Germany。　　In　order　to　improve　a　race　a　sample　of　the　best　ears　was　carefully　selected　from　the　best　fields　of　the　variety。　　These　ears　were　considered　as　representatives　of　the　type　under　cultivation；　and　it　was　assumed　that　by　sowing　their　grains　on　a　small　plot　a　family　could　be　obtained；　which　could　afterwards　be　improved　by　a　continuous　selection。　　Differences　between　the　collected　ears　were　either　not　observed　or　disregarded。　　At　Svalof　this　method　of　selection　was　practised　on　a　far　larger　scale　than　on　any　German　farm；　and　the　result　was；　broadly　speaking；　the　same。　　This　may　be　stated　in　the　following　words：　　improvement　in　a　few　cases；　failure　in　all　the　others。　　Some　few　varieties　could　be　improved　and　yielded　excellent　new　types；　some　of　which　have　since　been　introduced　into　Swedish　agriculture　and　are　now　prominent　races　in　the　southern　and　middle　parts　of　the　country。　　But　the　station　had　definite　aims；　and　among　them　was　the　improvement　of　the　Chevalier　barley。　　This；　in　Middle　Sweden；　is　a　fine　brewer's　barley；　but　liable　to　failure　during　unfavourable　summers　on　account　of　its　slender　stems。　　It　was　selected　with　a　view　of　giving　it　stiffer　stems；　but　in　spite　of　all　the　care　and　work　bestowed　upon　it　no　satisfactory　result　was　obtained。

This　experience；　combined　with　a　number　of　analogous　failures；　could　not　fail　to　throw　doubt　upon　the　whole　method。　　It　was　evident　that　good　results　were　only　exceptions；　and　that　in　most　cases　the　principle　was　not　one　that　could　be　relied　upon。　　The　exceptions　might　be　due　to　unknown　causes；　and　not　to　the　validity　of　the　method；　it　became　therefore　of　much　more　interest　to　search　for　the　causes　than　to　continue　the　work　along　these　lines。

In　the　year　1892　a　number　of　different　varieties　of　cereals　were　cultivated　on　a　large　scale　and　a　selection　was　again　made　from　them。　　About　two　hundred　samples　of　ears　were　chosen；　each　apparently　constituting　a　different　type。　　Their　seeds　were　sown　on　separate　plots　and　manured　and　treated　as　much　as　possible　in　the　same　manner。　　The　plots　were　small　and　arranged　in　rows　so　as　to　facilitate　the　comparison　of　allied　types。　　During　the　whole　period　of　growth　and　during　the　ripening　of　the　ears　the　plots　were　carefully　studied　and　compared：　　they　were　harvested　separately；　ears　and　kernels　were　counted　and　weighed；　and　notes　were　made　concerning　layering；　rust　and　other　cereal　pests。

The　result　of　this　experiment　was；　in　the　main；　no　distinct　improvement。　　Nilsson　was　especially　struck　by　the　fact　that　the　plots；　which　should　represent　distinct　types；　were　far　from　uniform。　　Many　of　them　were　as　multiform　as　the　fields　from　which　the　parent…ears　were　taken。　　Others　showed　variability　in　a　less　degree；　but　in　almost　all　of　them　it　was　clear　that　a　pure　race　had　not　been　obtained。　　The　experiment　was　a　fair　one；　inasmuch　as　it　demonstrated　the　polymorphic　variability　of　cereals　beyond　all