darwin and modern science-第31章

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ed　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　doubt　and　in　a　degree　hitherto　unsuspected；　but　from　the　standpoint　of　the　selectionist　it　was　a　failure。　　Fortunately　there　were；　however；　one　or　two　exceptions。　　A　few　lots　showed　a　perfect　uniformity　in　regard　to　all　the　stalks　and　ears：　　these　were　small　families。　　This　fact　suggested　the　idea　that　each　might　have　been　derived　from　a　single　ear。　　During　the　selection　in　the　previous　summer；　Nilsson　had　tried　to　find　as　many　ears　as　possible　of　each　new　type　which　he　recognised　in　his　fields。　　But　the　variability　of　his　crops　was　so　great；　that　he　was　rarely　able　to　include　more　than　two　or　three　ears　in　the　same　group；　and；　in　a　few　cases；　he　found　only　one　representative　of　the　supposed　type。　　It　might；　therefore；　be　possible　that　those　small　uniform　plots　were　the　direct　progeny　of　ears；　the　grains　of　which　had　not　been　mixed　with　those　from　other　ears　before　sowing。　　Exact　records　had；　of　course；　been　kept　of　the　chosen　samples；　and　the　number　of　ears　had　been　noted　in　each　case。　　It　was；　therefore；　possible　to　answer　the　question　and　it　was　found　that　those　plots　alone　were　uniform　on　which　the　kernels　of　one　single　ear　only　had　been　sown。　　Nilsson　concluded　that　the　mixture　of　two　or　more　ears　in　a　single　sowing　might　be　the　cause　of　the　lack　of　uniformity　in　the　progeny。　　Apparently　similar　ears　might　be　different　in　their　progeny。

Once　discovered；　this　fact　was　elevated　to　the　rank　of　a　leading　principle　and　tested　on　as　large　a　scale　as　possible。　　The　fields　were　again　carefully　investigated　and　every　single　ear；　which　showed　a　distinct　divergence　from　the　main　type　in　one　character　or　another；　was　selected。　　A　thousand　samples　were　chosen；　but　this　time　each　sample　consisted　of　one　ear　only。　　Next　year；　the　result　corresponded　to　the　expectation。　　Uniformity　prevailed　almost　everywhere；　only　a　few　lots　showed　a　discrepancy；　which　might　be　ascribed　to　the　accidental　selection　of　hybrid　ears。　　It　was　now　clear　that　the　progeny　of　single　ears　was；　as　a　rule；　pure；　whereas　that　of　mixed　ears　was　impure。　　The　single…ear　selection　or　single…ear　sowing；　which　had　fallen　into　discredit　in　Germany　and　elsewhere　in　Europe；　was　rediscovered。　　It　proved　to　be　the　only　trustworthy　principle　of　selection。　　Once　isolated；　such　single…parent　races　are　constant　from　seed　and　remain　true　to　their　type。　　No　further　selection　is　needed；　they　have　simply　to　be　multiplied　and　their　real　value　tested。

Patrick　Shirreff；　in　his　early　experiments；　Le　Couteur；　Hays　and　others　had　observed　the　rare　occurrence　of　exceptionally　good　yielders　and　the　value　of　their　isolation　to　the　agriculturist。　　The　possibility　of　error　in　the　choice　of　such　striking　specimens　and　the　necessity　of　judging　their　value　by　their　progeny　were　also　known　to　these　investigators；　but　they　had　not　the　slightest　idea　of　all　the　possibilities　suggested　by　their　principle。　　Nilsson；　who　is　a　botanist　as　well　as　an　agriculturist；　discovered　that；　besides　these　exceptionably　good　yielders；　every　variety　of　a　cereal　consists　of　hundreds　of　different　types；　which　find　the　best　conditions　for　success　when　grown　together；　but　which；　after　isolation；　prove　to　be　constant。　　Their　preference　for　mixed　growth　is　so　definite；　that　once　isolated；　their　claims　on　manure　and　treatment　are　found　to　be　much　higher　than　those　of　the　original　mixed　variety。　　Moreover；　the　greatest　care　is　necessary　to　enable　them　to　retain　their　purity；　and　as　soon　as　they　are　left　to　themselves　they　begin　to　deteriorate　through　accidental　crosses　and　admixtures　and　rapidly　return　to　the　mixed　condition。

Reverting　now　to　Darwin's　discussion　of　the　variability　of　cereals；　we　may　conclude　that　subsequent　investigation　has　proved　it　to　be　exactly　of　the　kind　which　he　describes。　　The　only　difference　is　that　in　reality　it　reaches　a　degree；　quite　unexpected　by　Darwin　and　his　contemporaries。　　But　it　is　polymorphic　variability　in　the　strictest　sense　of　the　word。　　How　the　single　constituents　of　a　variety　originate　we　do　not　see。　　We　may　assume；　and　there　can　hardly　be　a　doubt　about　the　truth　of　the　assumption；　that　a　new　character；　once　produced；　will　slowly　but　surely　be　combined　through　accidental　crosses　with　a　large　number　of　previously　existing　types；　and　so　will　tend　to　double　the　number　of　the　constituents　of　the　variety。　　But　whether　it　first　appears　suddenly　or　whether　it　is　only　slowly　evolved　we　cannot　determine。　　It　would；　of　course；　be　impossible　to　observe　either　process　in　such　a　mixture。　　Only　cultures　of　pure　races；　of　single…parent　races　as　we　have　called　them；　can　afford　an　opportunity　for　this　kind　of　observation。　　In　the　fields　of　Svalof　new　and　unexpected　qualities　have　recently　been　seen；　from　time　to　time；　to　appear　suddenly。　　These　characters　are　as　distinct　as　the　older　ones　and　appear　to　be　constant　from　the　moment　of　their　origin。

Darwin　has　repeatedly　insisted　that　man　does　not　cause　variability。　　He　simply　selects　the　variations　given　to　him　by　the　hand　of　nature。　　He　may　repeat　this　process　in　order　to　accumulate　different　new　characters　in　the　same　family；　thus　producing　varieties　of　a　higher　order。　　This　process　of　accumulation　would；　if　continued　for　a　longer　time；　lead　to　the　augmentation　of　the　slight　differences　characteristic　of　varieties　into　the　greater　differences　characteristic　of　species　and　genera。　　It　is　in　this　way　that　horticultural　and　agricultural　experience　contribute　to　the　problem　of　the　conversion　of　varieties　into　species；　and　to　the　explanation　of　the　admirable　adaptations　of　each　organism　to　its　complex　conditions　of　life。　　In　the　long　run　new　forms；　distinguished　from　their　allies　by　quite　a　number　of　new　characters；　would；　by　the　extermination　of　the　older　intermediates；　become　distinct　species。

Thus　we　see　that　the　theory　of　the　origin　of　species　by　means　of　natural　selection　is　quite　independent　of　the　question；　how　the　variations　to　be　selected　arise。　　They　may　arise　slowly；　from　simple　fluctuations；　or　suddenly；　by　mutations；　in　both　cases　natural　selection　will　take　hold　of　them；　will　multiply　them　if　they　are　beneficial；　and　in　the　cours