darwin and modern science-第22章

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ability　of　other　butterflies　to　be　eaten；are　certain；　and　its　consequencesthe　existence　of　mimetic　palatable　formsconfirm　it　in　the　most　convincing　manner。　　Of　the　many　cases　now　known　I　select　one；　which　is　especially　remarkable；　and　which　has　been　thoroughly　investigated；　Papilio　dardanus　（merope）；　a　large；　beautiful；　diurnal　butterfly　which　ranges　from　Abyssinia　throughout　the　whole　of　Africa　to　the　south　coast　of　Cape　Colony。

The　males　of　this　form　are　everywhere　ALMOST　the　same　in　colour　and　in　form　of　wings；　save　for　a　few　variations　in　the　sparse　black　markings　on　the　pale　yellow　ground。　　But　the　females　occur　in　several　quite　different　forms　and　colourings；　and　one　of　these　only；　the　Abyssinian　form；　is　like　the　male；　while　the　other　three　or　four　are　MIMETIC；　that　is　to　say；　they　copy　a　butterfly　of　quite　a　different　family　the　Danaids；　which　are　among　the　IMMUNE　forms。　　In　each　region　the　females　have　thus　copied　two　or　three　different　immune　species。　　There　is　much　that　is　interesting　to　be　said　in　regard　to　these　species；　but　it　would　be　out　of　keeping　with　the　general　tenor　of　this　paper　to　give　details　of　this　very　complicated　case　of　polymorphism　in　P。　dardanus。　　Anyone　who　is　interested　in　the　matter　will　find　a　full　and　exact　statement　of　the　case　in　as　far　as　we　know　it；　in　Poulton's　〃Essays　on　Evolution〃　（pages　373…375）。　（Professor　Poulton　has　corrected　some　wrong　descriptions　which　I　had　unfortunately　overlooked　in　the　Plates　of　my　book　〃Vortrage　uber　Descendenztheorie〃；　and　which　refer　to　Papilio　dardanus　（merope）。　　These　mistakes　are　of　no　importance　as　far　as　and　understanding　of　the　mimicry…theory　is　concerned；　but　I　hope　shortly　to　be　able　to　correct　them　in　a　later　edition。）　　I　need　only　add　that　three　different　mimetic　female　forms　have　been　reared　from　the　eggs　of　a　single　female　in　South　Africa。　　The　resemblance　of　these　forms　to　their　immune　models　goes　so　far　that　even　the　details　of　the　LOCAL　forms　of　the　models　are　copied　by　the　mimetic　species。

It　remains　to　be　said　that　in　Madagascar　a　butterfly；　Papilio　meriones；　occurs；　of　which　both　sexes　are　very　similar　in　form　and　markings　to　the　non…mimetic　male　of　P。　dardanus；　so　that　it　probably　represents　the　ancestor　of　this　latter　species。

In　face　of　such　facts　as　these　every　attempt　at　another　explanation　must　fail。　　Similarly　all　the　other　details　of　the　case　fulfil　the　preliminary　postulates　of　selection；　and　leave　no　room　for　any　other　interpretation。　　That　the　males　do　not　take　on　the　protective　colouring　is　easily　explained；　because　they　are　in　general　more　numerous；　and　the　females　are　more　important　for　the　preservation　of　the　species；　and　must　also　live　longer　in　order　to　deposit　their　eggs。　　We　find　the　same　state　of　things　in　many　other　species；　and　in　one　case　（Elymnias　undularis）　in　which　the　male　is　also　mimetically　coloured；　it　copies　quite　a　differently　coloured　immune　species　from　the　model　followed　by　the　female。　　This　is　quite　intelligible　when　we　consider　that　if　there　were　TOO　MANY　false　immune　types；　the　birds　would　soon　discover　that　there　were　palatable　individuals　among　those　with　unpalatable　warning　colours。　　Hence　the　imitation　of　different　immune　species　by　Papilio　dardanus！

I　regret　that　lack　of　space　prevents　my　bringing　forward　more　examples　of　mimicry　and　discussing　them　fully。　　But　from　the　case　of　Papilio　dardanus　alone　there　is　much　to　be　learnt　which　is　of　the　highest　importance　for　our　understanding　of　transformations。　　It　shows　us　chiefly　what　I　once　called；　somewhat　strongly　perhaps；　THE　OMNIPOTENCE　OF　NATURAL　SELECTION　in　answer　to　an　opponent　who　had　spoken　of　its　〃inadequacy。〃　　We　here　see　that　one　and　the　same　species　is　capable　of　producing　four　or　five　different　patterns　of　colouring　and　marking；　thus　the　colouring　and　marking　are　not；　as　has　often　been　supposed；　a　necessary　outcome　of　the　specific　nature　of　the　species；　but　a　true　adaptation；　which　cannot　arise　as　a　direct　effect　of　climatic　conditions；　but　solely　through　what　I　may　call　the　sorting　out　of　the　variations　produced　by　the　species；　according　to　their　utility。　　That　caterpillars　may　be　either　green　or　brown　is　already　something　more　than　could　have　been　expected　according　to　the　old　conception　of　species；　but　that　one　and　the　same　butterfly　should　be　now　pale　yellow；　with　black；　now　red　with　black　and　pure　white；　now　deep　black　with　large；　pure　white　spots；　and　again　black　with　a　large　ochreous…yellow　spot；　and　many　small　white　and　yellow　spots；　that　in　one　sub…species　it　may　be　tailed　like　the　ancestral　form；　and　in　another　tailless　like　its　Danaid　model；all　this　shows　a　far…reaching　capacity　for　variation　and　adaptation　that　wide　never　have　expected　if　we　did　not　see　the　facts　before　us。　　How　it　is　possible　that　the　primary　colour…variations　should　thus　be　intensified　and　combined　remains　a　puzzle　even　now；　we　are　reminded　of　the　modern　three…colour　printing；perhaps　similar　combinations　of　the　primary　colours　take　place　in　this　case；　in　any　case　the　direction　of　these　primary　variations　is　determined　by　the　artist　whom　we　know　as　natural　selection；　for　there　is　no　other　conceivable　way　in　which　the　model　could　affect　the　butterfly　that　is　becoming　more　and　more　like　it。　　The　same　climate　surrounds　all　four　forms　of　female；　they　are　subject　to　the　same　conditions　of　nutrition。　　Moreover；　Papilio　dardanus　is　by　no　means　the　only　species　of　butterfly　which　exhibits　different　kinds　of　colour…pattern　on　its　wings。　　Many　species　of　the　Asiatic　genus　Elymnias　have　on　the　upper　surface　a　very　good　imitation　of　an　immune　Euploeine　（Danainae）；　often　with　a　steel…blue　ground…colour；　while　the　under　surface　is　well　concealed　when　the　butterfly　is　at　rest；　thus　there　are　two　kinds　of　protective　coloration　each　with　a　different　meaning！　　The　same　thing　may　be　observed　in　many　non…mimetic　butterflies；　for　instance　in　all　our　species　of　Vanessa；　in　which　the　under　side　shows　a　grey…brown　or　brownish…black　protective　coloration；　but　we　do　not　yet　know　with　certainty　what　may　be　the　biological　significance　of　the　gaily　coloured　upper　surface。

In　general　it　may　be　said　that　mimetic　butterflies　are　comparatively　rare　species；　but　there　are　exceptions；　for　instance　Limenitis　archippus　in　North　America；　of　which　the　immune　model　（Danaida　plexippus）　also　occurs　in　enormous　numbers。

In　another　mimicry…category　the　imitators　are　often　more　numerous　than　the　models；　namely　in　the　case　of　the　imitation　of　DANGEROUS　INSECTS　by　harmless　species。　　Bees　and　wasps　are　dreaded　for　their　sting；　and　they　are　copied　by　harmless　flies　of　the　genera　Eristalis　and　Syrphus；　and　these　mimics　often　occur　in　swarms　about　flowering　plants　without　damage　to　themselves　or　to　their　models；　they　are　feared　and　are　therefore　left　unmolested。

In　regard　also　to　the　FAITHFULNESS　OF　THE　COPY　the　facts　are　quite　in　harmony　with　the　theory；　according　to　which　the　resemblance　must　have　arisen　and　increased　BY　DEGREES。　　We　can　recognise　this　in　many　cases；　for　even　now　the　mimetic　species　show　very　VARYING　DEGREES　OF　RESEMBLANCE　to　their　immune　model。　　If　we　compare；　for　instance；　the　many　different　imitators　of　Danaida　chrysippus　we　find　that；　with　their　brownish…yellow　ground…colour；　and　the　position　and　size；　and　more　or　less　sharp　limitation　of　their　clear　marginal　spots；　they　have　reached　very　different　degrees　of　nearness　to　their　model。　　Or　compare　the　female　of　Elymnias　undularis　with　its　model　Danaida　genutia；　there　is　a　general　resemblance；　but　the　marking　of　the　Danaida　is　very　roughly　imitated　in　Elymnias。

Another　fact　that　bears　out　the　theory　of　mimicry　is；　that　even　when　the　resemblance　in　colour…pattern　is　very　great；　the　WING…VENATION；　　which　is　so　constant；　and　so　important　in　determining　the　systematic　position　of　butterflies；　is　never　affected　by　the　variation。　　The　pursuers　of　the　butterfly　have　no　time　to　trouble　about　entomological　intricacies。

I　must　not　pass　over　a　discovery　of　Poulton's　which　is　of　great　theoretical　importancethat　mimetic　butterflies　may　reach　the　same　effect　by　very　different　means。　　（〃Journ。　Linn。　Soc。　London　（Zool。）〃；　Vol。　XXVI。　1898；　pages　598…602。）　　Thus　the　glass…like　transparency　of　the　wing　of　a　certain　Ithomiine　（Methona）　a