darwin and modern science-第8章

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sp　which　protects　it。　　Without　processes　of　selection　we　should　be　obliged　to　assume　a　〃pre…established　harmony〃　after　the　famous　Leibnitzian　model；　by　means　of　which　the　clock　of　the　evolution　of　organisms　is　so　regulated　as　to　strike　in　exact　synchronism　with　that　of　the　history　of　the　earth！　　All　forms　of　life　are　strictly　adapted　to　the　conditions　of　their　life；　and　can　persist　under　these　conditions　alone。

There　must　therefore　be　an　intrinsic　connection　between　the　conditions　and　the　structural　adaptations　of　the　organism；　and；　SINCE　THE　CONDITIONS　OF　LIFE　CANNOT　BE　DETERMINED　BY　THE　ANIMAL　ITSELF；　THE　ADAPTATIONS　MUST　BE　CALLED　FORTH　BY　THE　CONDITIONS。

The　selection　theory　teaches　us　how　this　is　conceivable；　since　it　enables　us　to　understand　that　there　is　a　continual　production　of　what　is　non…　purposive　as　well　as　of　what　is　purposive；　but　the　purposive　alone　survives；　while　the　non…purposive　perishes　in　the　very　act　of　arising。　　This　is　the　old　wisdom　taught　long　ago　by　Empedocles。

II。　　THE　LAMARCKIAN　PRINCIPLE。

Lamarck；　as　is　well　known；　formulated　a　definite　theory　of　evolution　at　the　beginning　of　the　nineteenth　century；　exactly　fifty　years　before　the　Darwin…　Wallace　principle　of　selection　was　given　to　the　world。　　This　brilliant　investigator　also　endeavoured　to　support　his　theory　by　demonstrating　forces　which　might　have　brought　about　the　transformations　of　the　organic　world　in　the　course　of　the　ages。　　In　addition　to　other　factors；　he　laid　special　emphasis　on　the　increased　or　diminished　use　of　the　parts　of　the　body；　assuming　that　the　strengthening　or　weakening　which　takes　place　from　this　cause　during　the　individual　life；　could　be　handed　on　to　the　offspring；　and　thus　intensified　and　raised　to　the　rank　of　a　specific　character。　　Darwin　also　regarded　this　LAMARCKIAN　PRINCIPLE；　as　it　is　now　generally　called；　as　a　factor　in　evolution；　but　he　was　not　fully　convinced　of　the　transmissibility　of　acquired　characters。

As　I　have　here　to　deal　only　with　the　theory　of　selection；　I　need　not　discuss　the　Lamarckian　hypothesis；　but　I　must　express　my　opinion　that　there　is　room　for　much　doubt　as　to　the　cooperation　of　this　principle　in　evolution。　　Not　only　is　it　difficult　to　imagine　how　the　transmission　of　functional　modifications　could　take　place；　but；　up　to　the　present　time；　notwithstanding　the　endeavours　of　many　excellent　investigators；　not　a　single　actual　proof　of　such　inheritance　has　been　brought　forward。　　Semon's　experiments　on　plants　are；　according　to　the　botanist　Pfeffer；　not　to　be　relied　on；　and　even　the　recent；　beautiful　experiments　made　by　Dr　Kammerer　on　salamanders；　cannot；　as　I　hope　to　show　elsewhere；　be　regarded　as　proof；　if　only　because　they　do　not　deal　at　all　with　functional　modifications；　that　is；　with　modifications　brought　about　by　use；　and　it　is　to　these　ALONE　that　the　Lamarckian　principle　refers。

III。　　OBJECTIONS　TO　THE　THEORY　OF　SELECTION。

（a）　　Saltatory　evolution。

The　Darwinian　doctrine　of　evolution　depends　essentially　on　THE　CUMULATIVE　AUGMENTATION　of　minute　variations　in　the　direction　of　utility。　　But　can　such　minute　variations；　which　are　undoubtedly　continually　appearing　among　the　individuals　of　the　same　species；　possess　any　selection…value；　can　they　determine　which　individuals　are　to　survive；　and　which　are　to　succumb；　can　they　be　increased　by　natural　selection　till　they　attain　to　the　highest　development　of　a　purposive　variation？

To　many　this　seems　so　improbable　that　they　have　urged　a　theory　of　evolution　by　leaps　from　species　to　species。　　Kolliker；　in　1872；　compared　the　evolution　of　species　with　the　processes　which　we　can　observe　in　the　individual　life　in　cases　of　alternation　of　generations。　　But　a　polyp　only　gives　rise　to　a　medusa　because　it　has　itself　arisen　from　one；　and　there　can　be　no　question　of　a　medusa　ever　having　arisen　suddenly　and　de　novo　from　a　polyp…bud；　if　only　because　both　forms　are　adapted　in　their　structure　as　a　whole；　and　in　every　detail　to　the　conditions　of　their　life。　　A　sudden　origin；　in　a　natural　way；　of　numerous　adaptations　is　inconceivable。　　Even　the　degeneration　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　do