darwin and modern science-第10节

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on　of　a　medusoid　from　a　free…swimming　animal　to　a　mere　brood…　sac　（gonophore）　is　not　sudden　and　saltatory；　but　occurs　by　imperceptible　modifications　throughout　hundreds　of　years；　as　we　can　learn　from　the　numerous　stages　of　the　process　of　degeneration　persisting　at　the　same　time　in　different　species。

If；　then；　the　degeneration　to　a　simple　brood…sac　takes　place　only　by　very　slow　transitions；　each　stage　of　which　may　last　for　centuries；　how　could　the　much　more　complex　ASCENDING　evolution　possibly　have　taken　place　by　sudden　leaps？　　I　regard　this　argument　as　capable　of　further　extension；　for　wherever　in　nature　we　come　upon　degeneration；　it　is　taking　place　by　minute　steps　and　with　a　slowness　that　makes　it　not　directly　perceptible；　and　I　believe　that　this　in　itself　justifies　us　in　concluding　that　THE　SAME　MUST　BE　TRUE　OF　ASCENDING　evolution。　　But　in　the　latter　case　the　goal　can　seldom　be　distinctly　recognised　while　in　cases　of　degeneration　the　starting…point　of　the　process　can　often　be　inferred；　because　several　nearly　related　species　may　represent　different　stages。

In　recent　years　Bateson　in　particular　has　championed　the　idea　of　saltatory；　or　so…called　discontinuous　evolution；　and　has　collected　a　number　of　cases　in　which　more　or　less　marked　variations　have　suddenly　appeared。　　These　are　taken　for　the　most　part　from　among　domesticated　animals　which　have　been　bred　and　crossed　for　a　long　time；　and　it　is　hardly　to　be　wondered　at　that　their　much　mixed　and　much　influenced　germ…plasm　should；　under　certain　conditions；　give　rise　to　remarkable　phenomena；　often　indeed　producing　forms　which　are　strongly　suggestive　of　monstrosities；　and　which　would　undoubtedly　not　survive　in　free　nature；　unprotected　by　man。　　I　should　regard　such　cases　as　due　to　an　intensified　germinal　selectionthough　this　is　to　anticipate　a　littleand　from　this　point　of　view　it　cannot　be　denied　that　they　have　a　special　interest。　　But　they　seem　to　me　to　have　no　significance　as　far　as　the　transformation　of　species　is　concerned；　if　only　because　of　the　extreme　rarity　of　their　occurrence。

There　are；　however；　many　variations　which　have　appeared　in　a　sudden　and　saltatory　manner；　and　some　of　these　Darwin　pointed　out　and　discussed　in　detail：　　the　copper　beech；　the　weeping　trees；　the　oak　with　〃fern…like　leaves；〃　certain　garden…flowers；　etc。　　But　none　of　them　have　persisted　in　free　nature；　or　evolved　into　permanent　types。

On　the　other　hand；　wherever　enduring　types　have　arisen；　we　find　traces　of　a　gradual　origin　by　successive　stages；　even　if；　at　first　sight；　their　origin　may　appear　to　have　been　sudden。　　This　is　the　case　with　SEASONAL　DIMORPHISM；　the　first　known　cases　of　which　exhibited　marked　differences　between　the　two　generations；　the　winter　and　the　summer　brood。　　Take　for　instance　the　much　discussed　and　studied　form　Vanessa　（Araschnia）　levana…prorsa。　　Here　the　differences　between　the　two　forms　are　so　great　and　so　apparently　disconnected；　that　one　might　almost　believe　it　to　be　a　sudden　mutation；　were　it　not　that　old　transition…stages　can　be　called　forth　by　particular　temperatures；　and　we　know　other　butterflies；　as　for　instance　our　Garden　Whites；　in　which　the　differences　between　the　two　generations　are　not　nearly　so　marked；　indeed；　they　are　so　little　apparent　that　they　are　scarcely　likely　to　be　noticed　except　by　experts。　　Thus　here　again　there　are　small　initial　steps；　some　of　which；　indeed；　must　be　regarded　as　adaptations；　such　as　the　green…sprinkled　or　lightly　tinted　under…surface　which　gives　them　a　deceptive　resemblance　to　parsley　or　to　Cardamine　leaves。

Even　if　saltatory　variations　do　occur；　we　cannot　assume　that　these　HAVE　EVER　LED　TO　FORMS　WHICH　ARE　CAPABLE　OF　SURVIVAL　UNDER　THE　CONDITIONS　OF　WILD　LIFE。　　Experience　has　shown　that　in　plants　which　have　suddenly　varied　the　power　of　persistence　is　diminished。　　Korschinksky　attributes　to　them　weaknesses　of　organisation　in　general；　〃they　bloom　late；　ripen　few　of　their　seeds；　and　show　great　sensitiveness　to　cold。〃　　These　are　not　the　characters　which　make　for　success　in　the　struggle　for　existence。

We　must　briefly　refer　here　to　the　viewsmuch　discussed　in　the　last　decade　of　H。　de　Vries；　who　believes　that　the　roots　of　transformation　must　be　sought　for　in　SALTATORY　VARIATIONS　ARISING　FROM　INTERNAL　CAUSES；　and　distinguishes　such　MUTATIONS；　as　he　has　called　them；　from　ordinary　individual　variations；　in　that　they　breed　true；　that　is；　with　strict　inbreeding　they　are　handed　on　pure　to　the　next　generation。　　I　have　elsewhere　endeavoured　to　point　out　the　weaknesses　of　this　theory　（〃Vortrage　uber　Descendenztheorie〃；　Jena；　1904；　II。　269。　　English　Translation　London；　1904；　II。　page　317。）；　and　I　am　the　less　inclined　to　return　to　it　here　that　it　now　appears　（See　Poulton；　〃Essays　on　Evolution〃；　Oxford；　1908；　pages　xix…xxii。）　that　the　far…reaching　conclusions　drawn　by　de　Vries　from　his　observations　on　the　Evening　Primrose；　Oenothera　lamarckiana；　rest　upon　a　very　insecure　foundation。　　The　plant　from　which　de　Vries　saw　numerous　〃species〃his　〃mutations〃arise　was　not；　as　he　assumed；　a　WILD　SPECIES　that　had　been　introduced　to　Europe　from　America；　but　was　probably　a　hybrid　form　which　was　first　discovered　in　the　Jardin　des　Plantes　in　Paris；　and　which　does　not　appear　to　exist　anywhere　in　America　as　a　wild　species。

This　gives　a　severe　shock　to　the　〃Mutation　theory；〃　for　the　other　ACTUALLY　WILD　species　with　which　de　Vries　experimented　showed　no　〃mutations〃　but　yielded　only　negative　results。

Thus　we　come　to　the　conclusion　that　Darwin　（〃Origin　of　Species〃　（6th　edition）；　pages　176　et　seq。）　was　right　in　regarding　transformations　as　taking　place　by　minute　steps；　which；　if　useful；　are　augmented　in　the　course　of　innumerable　generations；　because　their　possessors　more　frequently　survive　in　the　struggle　for　existence。

（b）　　SELECTION…VALUE　OF　THE　INITIAL　STEPS。

Is　it　possible　that　the　significant　deviations　which　we　know　as　〃individual　variations〃　can　form　the　beginning　of　a　process　of　selection？　　Can　they　decide　which　is　to　perish　and　which　to　survive？　　To　use　a　phrase　of　Romanes；　can　they　have　SELECTION…VALUE？

Darwin　himself　answered　this　question；　and　brought　together　many　excellent　examples　to　show　that　differences；　apparently　insignificant　because　very　small；　might　be　of　decisive　importance　for　the　life　of　the　possessor。　　But　it　is　by　no　means　enough　to　bring　forward　cases　of　this　kind；　for　the　question　is　not　merely　whether　finished　adaptations　have　selection…value；　but　whether　the　first　beginnings　of　these；　and　whether　the　small；　I　might　almost　say　minimal　increments；　which　have　led　up　from　these　beginnings　to　the　perfect　adaptation；　have　also　had　selection…value。　　To　this　question　even　one　who；　like　myself；　has　been　for　many　years　a　convinced　adherent　of　the　theory　of　selection；　can　only　reply：　　WE　MUST　ASSUME　SO；　BUT　WE　CANNOT　PROVE　IT　IN　ANY　CASE。　　It　is　not　upon　demonstrative　evidence　that　we　rely　when　we　champion　the　doctrine　of　selection　as　a　scientific　truth；　we　base　our　argument　on　quite　other　grounds。　　Undoubtedly　there　are　many　apparently　insignificant　features；　which　can　nevertheless　be　shown　to　be　adaptations　for　instance；　the　thickness　of　the　basin…shaped　shell　of　the　limpets　that　live　among　the　breakers　on　the　shore。　　There　can　be　no　doubt　that　the　thickness　of　these　shells；　combined　with　their　flat　form；　protects　the　animals　from　the　force　of　the　waves　breaking　upon　them；but　how　have　they　become　so　thick？　　What　proportion　of　thickness　was　sufficient　to　decide　that　of　two　variants　of　a　limpet　one　should　survive；　the　other　be　eliminated？　　We　can　say　nothing　more　than　that　we　infer　from　the　present　state　of　the　shell；　that　it　must　have　varied　in　regard　to　differences　in　shell…thickness；　and　that　these　differences　must　have　had　selection…value；　no　proof　therefore；　but　an　assumption　which　we　must　show　to　be　convincing。

For　a　long　time　the　marvellously　complex　RADIATE　and　LATTICE…WORK　skeletons　of　Radiolarians　were　regarded　as　a　mere　outflow　of　〃Nature's　infinite　wealth　of　form；〃　as　an　instance　of　a　purely　morphological　character　with　no　biological　significance。　　But　recent　investigations　have　shown　that　these；　too；　have　an　adaptive　significance　（Hacker）。　　The　same　thing　has　been　shown　by　Schutt　in　regard　to　the　lowly　unicellular　plants；　the　Peridineae；　which　abound　alike　on　the　surface　of　the　ocean　and　in　its　depths。　　It　has　been　shown　that　the　long　skeletal　processes　which　grow　out　from　these　organisms　have　significance　not　merely　as　a　supporting　skeleton；　but　also　as　an　extension　of　the　superficial　area；　which　increases　the　contact　with　the　water…particles；　and　prevents　the　floating　organisms　from　sinking。　　It　has　been　established　that　the　processes　are　considerably　shorter　in　the　colder　layers　of　the　ocean；　and　that　they　may　be　twelve　times　as　long　（Chun；　〃Reise　der　Valdivia〃；　Leipzig；　1904。）　in　the　warmer　layers；　thus　corresponding　to　the　greater　or　smaller　amount　of　friction　which　takes　place　in　the　denser　and　less　dense　layers　of　the　water。

The　Peridineae　of　the　warmer　ocean　layers　have　thus　become　long…rayed；　those　of　the　colder　layers　short…rayed；　not　through　the　direct　effect　of　friction　on　the　protoplasm；　but　through　processes　of　selection；　which　favoured　the　longer　rays　in　warm　water；　since　they　kept　the　organism　afloat；　while　thos