darwin and modern science-第23章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


iment　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　course　of　time　accumulate　them；　so　as　to　produce　that　great　diversity　of　organic　life；　which　we　so　highly　admire。

Darwin　has　left　the　decision　of　this　difficult　and　obviously　subordinate　point　to　his　followers。　　But　in　his　Pangenesis　hypothesis　he　has　given　us　the　clue　for　a　close　study　and　ultimate　elucidation　of　the　subject　under　discussion。


V。　　HEREDITY　AND　VARIATION　IN　MODERN　LIGHTS。

By　W。　BATESON；　M。A。；　F。R。S。

Professor　of　Biology　in　the　University　of　Cambridge。

Darwin's　work　has　the　property　of　greatness　in　that　it　may　be　admired　from　more　aspects　than　one。　　For　some　the　perception　of　the　principle　of　Natural　Selection　stands　out　as　his　most　wonderful　achievement　to　which　all　the　rest　is　subordinate。　　Others；　among　whom　I　would　range　myself；　look　up　to　him　rather　as　the　first　who　plainly　distinguished；　collected；　and　comprehensively　studied　that　new　class　of　evidence　from　which　hereafter　a　true　understanding　of　the　process　of　Evolution　may　be　developed。　　We　each　prefer　our　own　standpoint　of　admiration；　but　I　think　that　it　will　be　in　their　wider　aspect　that　his　labours　will　most　command　the　veneration　of　posterity。

A　treatise　written　to　advance　knowledge　may　be　read　in　two　moods。　　The　reader　may　keep　his　mind　passive；　willing　merely　to　receive　the　impress　of　the　writer's　thought；　or　he　may　read　with　his　attention　strained　and　alert；　asking　at　every　instant　how　the　new　knowledge　can　be　used　in　a　further　advance；　watching　continually　for　fresh　footholds　by　which　to　climb　higher　still。　　Of　Shelley　it　has　been　said　that　he　was　a　poet　for　poets：　　so　Darwin　was　a　naturalist　for　naturalists。　　It　is　when　his　writings　are　used　in　the　critical　and　more　exacting　spirit　with　which　we　test　the　outfit　for　our　own　enterprise　that　we　learn　their　full　value　and　strength。　　Whether　we　glance　back　and　compare　his　performance　with　the　efforts　of　his　predecessors；　or　look　forward　along　the　course　which　modern　research　is　disclosing；　we　shall　honour　most　in　him　not　the　rounded　merit　of　finite　accomplishment；　but　the　creative　power　by　which　he　inaugurated　a　line　of　discovery　endless　in　variety　and　extension。　　Let　us　attempt　thus　to　see　his　work　in　true　perspective　between　the　past　from　which　it　grew；　and　the　present　which　is　its　consequence。　　Darwin　attacked　the　problem　of　Evolution　by　reference　to　facts　of　three　classes：　　Variation；　Heredity；　Natural　Selection。　　His　work　was　not　as　the　laity　suppose；　a　sudden　and　unheralded　revelation；　but　the　first　fruit　of　a　long　and　hitherto　barren　controversy。　The　occurrence　of　variation　from　type；　and　the　hereditary　transmission　of　such　variation　had　of　course　been　long　familiar　to　practical　men；　and　inferences　as　to　the　possible　bearing　of　those　phenomena　on　the　nature　of　specific　difference　had　been　from　time　to　time　drawn　by　naturalists。　　Maupertuis；　for　example；　wrote　〃Ce　qui　nous　reste　a　examiner；　c'est　comment　d'un　seul　individu；　il　a　pu　naitre　tant　d'especes　si　differentes。〃　　And　again　〃La　Nature　contient　le　fonds　de　toutes　ces　varietes：　　mais　le　hasard　ou　l'art　les　mettent　en　oeuvre。　　C'est　ainsi　que　ceux　dont　l'industrie　s'applique　a　satisfaire　le　gout　des　curieux；　sont；　pour　ainsi　dire；　creatures　d'especes　nouvelles。〃　　（〃Venus　Physique；　contenant　deux　Dissertations；　l'une　sur　l'origine　des　Hommes　et　des　Animaux：　　Et　l'autre　sur　l'origine　des　Noirs〃　La　Haye；　1746；　pages　124　and　129。　　For　an　introduction　to　the　writings　of　Maupertuis　I　am　indebted　to　an　article　by　Professor　Lovejoy　in　〃Popular　Sci。　Monthly〃；　1902。）

Such　passages；　of　which　many　（though　few　so　emphatic）　can　be　found　in　eighteenth　century　writers；　indicate　a　true　perception　of　the　mode　of　Evolution。　　The　speculations　hinted　at　by　Buffon　（For　the　fullest　account　of　the　views　of　these　pioneers　of　Evolution；　see　the　works　of　Samuel　Butler；　especially　〃Evolution；　Old　and　New〃　（2nd　edition）　1882。　　Butler's　claims　on　behalf　of　Buffon　have　met　with　some　acceptance；　but　after　reading　what　Butler　has　said；　and　a　considerable　part　of　Buffon's　own　works；　the　word　〃hinted〃　seems　to　me　a　sufficiently　correct　description　of　the　part　he　played。　　It　is　interesting　to　note　that　in　the　chapter　on　the　Ass；　which　contains　some　of　his　evolutionary　passages；　there　is　a　reference　to　〃plusieurs　idees　tres…elevees　sur　la　generation〃　contained　in　the　Letters　of　Maupertuis。）；　developed　by　Erasmus　Darwin；　and　independently　proclaimed　above　all　by　Lamarck；　gave　to　the　doctrine　of　descent　a　wide　renown。　　The　uniformitarian　teaching　which　Lyell　deduced　from　geological　observation　had　gained　acceptance。　　The　facts　of　geographical　distribution　（See　especially　W。　Lawrence；　〃Lectures　on　Physiology〃；　London；　1823；　pages　213　f。）　had　been　shown　to　be　obviously　inconsistent　with　the　Mosaic　legend。　　Prichard；　and　Lawrence；　following　the　example　of　Blumenbach；　had　successfully　demonstrated　that　the　races　of　Man　could　be　regarded　as　different　forms　of　one　species；　contrary　to　the　opinion　up　till　then　received。　　These　treatises　all　begin；　it　is　true；　with　a　profound　obeisance　to　the　sons　of　Noah；　but　that　performed；　they　continue　on　strictly　modern　lines。