a history of science-4-第21章

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〃On　the　other　hand；　we　observe　that　even　the　cotyledons　which　are　most　like　a　leaf　when　compared　with　the　following　leaves　of　the　stalk　are　always　more　undeveloped　or　less　developed。　This　is　chiefly　noticeable　in　their　margin　which　is　extremely　simple　and　shows　few　traces　of　indentation。

〃A　few　or　many　of　the　next　following　leaves　are　often　already　present　in　the　seed；　and　lie　enclosed　between　the　cotyledons；　in　their　folded　state　they　are　known　by　the　name　of　plumules。　Their　form；　as　compared　with　the　cotyledons　and　the　following　leaves；　varies　in　different　plants。　　Their　chief　point　of　variance；　however；　from　the　cotyledons　is　that　they　are　flat；　delicate；　and　formed　like　real　leaves　generally。　They　are　wholly　green；　rest　on　a　visible　node；　and　can　no　longer　deny　their　relationship　to　the　following　leaves　of　the　stalk；　to　which；　however；　they　are　usually　still　inferior；　in　so　far　as　that　their　margin　is　not　completely　developed。

〃The　further　development；　however；　goes　on　ceaselessly　in　the　leaf；　from　node　to　node；　its　midrib　is　elongated；　and　more　or　less　additional　ribs　stretch　out　from　this　towards　the　sides。　The　leaves　now　appear　notched；　deeply　indented；　or　composed　of　several　small　leaves；　in　which　last　case　they　seem　to　form　complete　little　branches。　　The　date…palm　furnishes　a　striking　example　of　such　a　successive　transformation　of　the　simplest　leaf　form。　A　midrib　is　elongated　through　a　succession　of　several　leaves；　the　single　fan…shaped　leaf　becomes　torn　and　diverted；　and　a　very　complicated　leaf　is　developed；　which　rivals　a　branch　in　form。

〃The　transition　to　inflorescence　takes　place　more　or　less　rapidly。　In　the　latter　case　we　usually　observe　that　the　leaves　of　the　stalk　loose　their　different　external　divisions；　and；　on　the　other　hand；　spread　out　more　or　less　in　their　lower　parts　where　they　are　attached　to　the　stalk。　If　the　transition　takes　place　rapidly；　the　stalk；　suddenly　become　thinner　and　more　elongated　since　the　node　of　the　last…developed　leaf；　shoots　up　and　collects　several　leaves　around　an　axis　at　its　end。

〃That　the　petals　of　the　calyx　are　precisely　the　same　organs　which　have　hitherto　appeared　as　leaves　on　the　stalk；　but　now　stand　grouped　about　a　common　centre　in　an　often　very　different　form；　can；　as　it　seems　to　me；　be　most　clearly　demonstrated。　　Already　in　connection　with　the　cotyledons　above；　we　noticed　a　similar　working　of　nature。　The　first　species；　while　they　are　developing　out　of　the　seed…kernel；　display　a　radiate　crown　of　unmistakable　needles；　and　in　the　first　childhood　of　these　plants　we　see　already　indicated　that　force　of　nature　whereby　when　they　are　older　their　flowering　and　fruit…giving　state　will　be　produced。

〃We　see　this　force　of　nature；　which　collects　several　leaves　around　an　axis；　produce　a　still　closer　union　and　make　these　approximated；　modified　leaves　still　more　unrecognizable　by　joining　them　together　either　wholly　or　partially。　　The　bell…shaped　or　so…called　one…petalled　calices　represent　these　cloudy　connected　leaves；　which；　being　more　or　less　indented　from　above；　or　divided；　plainly　show　their　origin。

〃We　can　observe　the　transition　from　the　calyx　to　the　corolla　in　more　than　one　instance；　for；　although　the　color　of　the　calyx　is　still　usually　green；　and　like　the　color　of　the　leaves　of　the　stalk；　it　nevertheless　often　varies　in　one　or　another　of　its　partsat　the　tips；　the　margins；　the　back；　or　even；　the　inward　sidewhile　the　outer　still　remains　on　green。

〃The　relationship　of　the　corolla　to　the　leaves　of　the　stalk　is　shown　in　more　than　one　way；　since　on　the　stalks　of　some　plants　appear　leaves　which　are　already　more　or　less　colored　long　before　they　approach　inflorescence；　others　are　fully　colored　when　near　inflorescence。　　Nature　also　goes　over　at　once　to　the　corolla；　sometimes　by　skipping　over　the　organs　of　the　calyx；　and　in　such　a　case　we　likewise　have　an　opportunity　to　observe　that　leaves　of　the　stalk　become　transformed　into　petals。　Thus　on　the　stalk　of　tulips；　for　instance；　there　sometimes　appears　an　almost　completely　developed　and　colored　petal。　Even　more　remarkable　is　the　case　when　such　a　leaf；　half　green　and　half　of　it　belonging　to　the　stalk；　remains　attached　to　the　latter；　while　another　colored　part　is　raised　with　the　corolla；　and　the　leaf　is　thus　torn　in　two。

〃The　relationship　between　the　petals　and　stamens　is　very　close。　In　some　instances　nature　makes　the　transition　regulare。g。；　among　the　Canna　and　several　plants　of　the　same　family。　　A　true；　little…modified　petal　is　drawn　together　on　its　upper　margin；　and　produces　a　pollen　sac；　while　the　rest　of　the　petal　takes　the　place　of　the　stamen。　In　double　flowers　we　can　observe　this　transition　in　all　its　stages。　In　several　kinds　of　roses；　within　the　fully　developed　and　colored　petals　there　appear　other　ones　which　are　drawn　together　in　the　middle　or　on　the　side。　　This　drawing　together　is　produced　by　a　small　weal；　which　appears　as　a　more　or　less　complete　pollen　sac；　and　in　the　same　proportion　the　leaf　approaches　the　simple　form　of　a　stamen。

〃The　pistil　in　many　cases　looks　almost　like　a　stamen　without　anthers；　and　the　relationship　between　the　formation　of　the　two　is　much　closer　than　between　the　other　parts。　　In　retrograde　fashion　nature　often　produces　cases　where　the　style　and　stigma　（Narben）　become　retransformed　into　petalsthat　is；　the　Ranunculus　Asiaticus　becomes　double　by　transforming　the　stigma　and　style　of　the　fruit…receptacle　into　real　petals；　while　the　stamens　are　often　found　unchanged　immediately　behind　the　corolla。

〃In　the　seed　receptacles；　in　spite　of　their　formation；　of　their　special　object；　and　of　their　method　of　being　joined　together；　we　cannot　fail　to　recognize　the　leaf　form。　　Thus；　for　instance；　the　pod　would　be　a　simple　leaf　folded　and　grown　together　on　its　margin；　the　siliqua　would　consist　of　more　leaves　folded　over　another；　the　compound　receptacles　would　be　explained　as　being　several　leaves　which；　being　united　above　one　centre；　keep　their　inward　parts　separate　and　are　joined　on　their　margins。　We　can　convince　ourselves　of　this　by　actual　sight　when　such　composite　capsules　fall　apart　after　becoming　ripe；　because　then　every　part　displays　an　opened　pod。〃＇1＇


The　theory　thus　elaborated　of　the　metamorphosis　of　parts　was　presently　given　greater　generality　through　extension　to　the　animal　kingdom；　in　the　doctrine　which　Goethe　and　Oken　advanced　independently；　that　the　vertebrate　skull　is　essentially　a　modified　and　developed　vertebra。　These　were　conceptions　worthy　of　a　poetimpossible；　indeed；　for　any　mind　that　had　not　the　poetic　faculty　of　correlation。　　But　in　this　case　the　poet's　vision　was　prophetic　of　a　future　view　of　the　most　prosaic　science。　The　doctrine　of　metamorphosis　of　parts　soon　came　to　be　regarded　as　of　fundamental　importance。

But　the　doctrine　had　implications　that　few　of　its　early　advocates　realized。　If　all　the　parts　of　a　flowersepal；　petal；　stamen；　pistil；　with　their　countless　deviations　of　contour　and　colorare　but　modifications　of　the　leaf；　such　modification　implies　a　marvellous　differentiation　and　development。　To　assert　that　a　stamen　is　a　metamorphosed　leaf　means；　if　it　means　anything；　that　in　the　long　sweep　of　time　the　leaf　has　by　slow　or　sudden　gradations　changed　its　character　through　successive　generations；　until　the　offspring；　so　to　speak；　of　a　true　leaf　has　become　a　stamen。　　But　if　such　a　metamorphosis　as　this　is　possibleif　the　seemingly　wide　gap　between　leaf　and　stamen　may　be　spanned　by　the　modification　of　a　line　of　organismswhere　does　the　possibility　of　modification　of　organic　type　find　its　bounds？　　Why　may　not　the　modification　of　parts　go　on　along　devious　lines　until　the　remote　descendants　of　an　organism　are　utterly　unlike　that　organism？　　Why　may　we　not　thus　account　for　the　development　of　various　species　of　beings　all　sprung　from　one　parent　stock？　　That；　too；　is　a　poet's　dream；　but　is　it　only　a　dream？　Goethe　thought　not。　　Out　of　his　studies　of　metamorphosis　of　parts　there　grew　in　his　mind　the　belief　that　the　multitudinous　species　of　plants　and　animals　about　us　have　been　evolved　from　fewer　and　fewer　earlier　parent　types；　like　twigs　of　a　giant　tree　drawing　their　nurture　from　the　same　primal　root。　It　was　a　bold　and　revolutionary　thought；　and　the　world　regarded　it　as　but　the　vagary　of　a　poet。


ERASMUS　DARWIN

Just　at　the　time　when　this　thought　was　taking　form　in　Goethe's　brain；　the　same　idea　was　germinating　in　the　mind　of　another　philosopher；　an　Englishman　of　international　fame；　Dr。　Erasmus　Darwin；　who；　while　he　lived；　enjoyed　the　widest　popularity　as　a　poet；　the　rhymed　couplets　of　his　Botanic　Garden　being　quoted　everywhere　with　admiration。　And　posterity　repudiating　the　verse　which　makes　the　body　of　the　book；　yet　grants　permanent　value　to　the　book　itself；　because；　forsooth；　its　copious　explanatory　foot…notes　furnish　an　outline　of　the　status　of　almost　every　department　of　science　of　the　time。

But　even　though　he　lacked　the　highest　art　of　the　versifier；　Darwin　had；　beyond　peradventure；　the　imagination　of　a　poet　coupled　with　profound　scientific　knowledge；　and　it　was　his　poetic　insight；　correlating　organisms　seemingly　diverse　in　structure　and　imbuing　the　lowliest　flower　with　a　vital　personality；　which　led　him　to　suspect　that　there　are　no　lines　of　demarcation　in　nature。　〃Can　it　be；〃　he　queries；　〃that　one　form　of　organism　has　developed　from　another；　that　different　species　are　really　but　modified　descendants　of　one　parent　stock？〃　　The　alluring　thought　nestled　in　his　mind　and　was　nurtured　there；　and　grew　in　a　fixed　belief；　which　was　given　fuller　expression　in　his　Zoonomia　and　in　the　posthumous　Temple　of　Nature。

Here　is　his　rendering　of　the　idea　as　versified　in　the　Temple　of　Nature：

　〃Organic　life　beneath　the　shoreless　waves　　　Was　born；　and　nursed　in　Ocean's　pearly　caves；　　　First　forms　minute；　unseen　by　spheric　glass；　　　Move　on　the　mud；　or　pierce　the　watery　mass；　　　These；　as　successive　generations　bloom；　　　New　powers　acquire　and　larger　limbs　assume；　　　Whence　countless　groups　of　vegetation　spring；　　　And　breathing　realms　of　fin；　and　feet；　and　wing。

　〃Thus　the　tall　Oak；　the　giant　of　the　wood；　　　Which　bears　Britannia's　thunders　on　the　flood；　　　The　Whale；　unmeasured　monster　of　the　main；　　　The　lordly　lion；　monarch　of　the　plain；　　　The　eagle；　soaring　in　the　realms　of　air；　　　Whose　eye；　undazzled；　drinks　the　solar　glare；　　　Imperious　man；　who　rules　the　bestial　crowd；　　　Of　language；　reason；　and　reflection　proud；　　　With　brow　erect；　who　scorns　this　earthy　sod；　　　And　styles　himself　the　image　of　his　God　　　Arose　from　rudiments　of　form　and　sense；　　　An　embryon　point　or　microscopic　ens！〃＇2＇


Here；　clearly　enough；　is　the　idea　of　evolution。　　But　in　that　day　there　was　little　proof　forthcoming　of　its　validity　that　could　satisfy　any　one　but　a　poet；　and　when　Erasmus　Darwin　died；　in　1802；　the　idea　of　transmutation　of　species　was　still　but　an　unsubstantiated　dream。

It　was　a　dream；　however；　which　was　not　confined　to　Goethe　and　Darwin。　Even　earlier　the　idea　had　come　more　or　less　vaguely　to　another　great　dreamerand　workerof　Germany；　Immanuel　Kant；　and　to　several　great　Frenchmen；　including　De　Maillet；　Maupertuis；　Robinet；　and　the　famous　naturalist　Buffona　man　who　had　the　imagination　of　a　poet；　though　his　message　was　couched　in　most　artistic　prose。　　Not　long　after　the　middle　of　the　eighteenth　century　Buffon　had　put　forward　the