darwin and modern science-第19节

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due　to　the　fact　that　the　scales　are　absent　on　a　circular　or　oval　patch　so　that　the　colourless　wing…membrane　lies　bare；　and　one　can　look　through　the　spot　as　through　a　window。　　Whether　the　bird　which　is　seeking　or　pursuing　the　butterflies　takes　these　holes　for　dewdrops；　or　for　the　work　of　a　devouring　insect；　does　not　affect　the　question；　the　mirror…like　spot　undoubtedly　increases　the　general　deceptiveness；　for　the　same　thing　occurs　in　many　leaf…butterflies；　though　not　in　all；　and　in　some　cases　it　is　replaced　in　quite　a　peculiar　manner。　　In　one　species　of　Anaea　（A。　divina）；　the　resting　butterfly　looks　exactly　like　a　leaf　out　of　the　outer　edge　of　which　a　large　semicircular　piece　has　been　eaten；　possibly　by　a　caterpillar；　but　if　we　look　more　closely　it　is　obvious　that　there　is　no　part　of　the　wing　absent；　and　that　the　semicircular　piece　is　of　a　clear；　pale　yellow　colour；　while　the　rest　of　the　wing　is　of　a　strongly　contrasted　dark　brown。

But　the　deceptive　resemblance　may　be　caused　in　quite　a　different　manner。　　I　have　often　speculated　as　to　what　advantage　the　brilliant　white　C　could　give　to　the　otherwise　dusky…coloured　〃Comma　butterfly〃　（Grapta　C。　album）。　　Poulton's　recent　observations　（〃Proc。　Ent。　Soc〃。；　London；　May　6；　1903。）　have　shown　that　this　represents　the　imitation　of　a　crack　such　as　is　often　seen　in　dry　leaves；　and　is　very　conspicuous　because　the　light　shines　through　it。

The　utility　obviously　lies　in　presenting　to　the　bird　the　very　familiar　picture　of　a　broken　leaf　with　a　clear　shining　slit；　and　we　may　conclude；　from　the　imitation　of　such　small　details；　that　the　birds　are　very　sharp　observers　and　that　the　smallest　deviation　from　the　usual　arrests　their　attention　and　incites　them　to　closer　investigation。　　It　is　obvious　that　such　detailedwe　might　almost　say　such　subtledeceptive　resemblances　could　only　have　come　about　in　the　course　of　long　ages　through　the　acquirement　from　time　to　time　of　something　new　which　heightened　the　already　existing　resemblance。

In　face　of　facts　like　these　there　can　be　no　question　of　chance；　and　no　one　has　succeeded　so　far　in　finding　any　other　explanation　to　replace　that　by　selection。　　For　the　rest；　the　apparent　leaves　are　by　no　means　perfect　copies　of　a　leaf；　many　of　them　only　represent　the　torn　or　broken　piece；　or　the　half　or　two…thirds　of　a　leaf；　but　then　the　leaves　themselves　frequently　do　not　present　themselves　to　the　eye　as　a　whole；　but　partially　concealed　among　other　leaves。　　Even　those　butterflies　which；　like　the　species　of　Kallima　and　Anaea；　represent　the　whole　of　a　leaf　with　stalk；　ribs；　apex；　and　the　whole　breadth；　are　not　actual　copies　which　would　satisfy　a　botanist；　there　is　often　much　wanting。　　In　Kallima　the　lateral　ribs　of　the　leaf　are　never　all　included　in　the　markings；　there　are　only　two　or　three　on　the　left　side　and　at　most　four　or　five　on　the　right；　and　in　many　individuals　these　are　rather　obscure；　while　in　others　they　are　comparatively　distinct。　　This　furnishes　us　with　fresh　evidence　in　favour　of　their　origin　through　processes　of　selection；　for　a　botanically　perfect　picture　could　not　arise　in　this　way；　there　could　only　be　a　fixing　of　such　details　as　heightened　the　deceptive　resemblance。

Our　postulate　of　origin　through　selection　also　enables　us　to　understand　why　the　leaf…imitation　is　on　the　lower　surface　of　the　wing　in　the　diurnal　Lepidoptera；　and　on　the　upper　surface　in　the　nocturnal　forms；　corresponding　to　the　attitude　of　the　wings　in　the　resting　position　of　the　two　groups。

The　strongest　of　all　proofs　of　the　theory；　however；　is　afforded　by　cases　of　true　〃mimicry；〃　those　adaptations　discovered　by　Bates　in　1861；　consisting　in　the　imitation　of　one　species　by　another；　which　becomes　more　and　more　like　its　model。　　The　model　is　always　a　species　that　enjoys　some　special　protection　from　enemies；　whether　because　it　is　unpleasant　to　taste；　or　because　it　is　in　some　way　dangerous。

It　is　chiefly　among　insects　and　especially　among　butterflies　that　we　find　the　greatest　number　of　such　cases。　　Several　of　these　have　been　minutely　studied；　and　every　detail　has　been　investigated；　so　that　it　is　difficult　to　understand　how　there　can　still　be　disbelief　in　regard　to　them。　　If　the　many　and　exact　observations　which　have　been　carefully　collected　and　critically　discussed；　for　instance　by　Poulton　（〃Essays　on　Evolution〃；　1889…1907；　Oxford；　1908；　passim；　e。g。　page　269。）　were　thoroughly　studied；　the　arguments　which　are　still　frequently　urged　against　mimicry　would　be　found　untenable；　we　can　hardly　hope　to　find　more　convincing　proof　of　the　actuality　of　the　processes　of　selection　than　these　cases　put　into　our　hands。　　The　preliminary　postulates　of　the　theory　of　mimicry　have　been　disputed；　for　instance；　that　diurnal　butterflies　are　persecuted　and　eaten　by　birds；　but　observations　specially　directed　towards　this　point　in　India；　Africa；　America　and　Europe　have　placed　it　beyond　all　doubt。　　If　it　were　necessary　I　could　myself　furnish　an　account　of　my　own　observations　on　this　point。

In　the　same　way　it　has　been　established　by　experiment　and　observation　in　the　field　that　in　all　the　great　regions　of　distribution　there　are　butterflies　which　are　rejected　by　birds　and　lizards；　their　chief　enemies；　on　account　of　their　unpleasant　smell　or　taste。　　These　butterflies　are　usually　gaily　and　conspicuously　coloured　and　thusas　Wallace　first　interpreted　itare　furnished　with　an　easily　recognisable　sign：　　a　sign　of　unpalatableness　or　WARNING　COLOURS。　　If　they　were　not　thus　recognisable　easily　and　from　a　distance；　they　would　frequently　be　pecked　at　by　birds；　and　then　rejected　because　of　their　unpleasant　taste；　but　as　it　is；　the　insect…eaters　recognise　them　at　once　as　unpalatable　booty　and　ignore　them。　Such　IMMUNE　（The　expression　does　not　refer　to　all　the　enemies　of　this　butterfly；　against　ichneumon…flies；　for　instance；　their　unpleasant　smell　usually　gives　no　protection。）　species；　wherever　they　occur；　are　imitated　by　other　palatable　species；　which　thus　acquire　a　certain　degree　of　protection。

It　is　true　that　this　explanation　of　the　bright；　conspicuous　colours　is　only　a　hypothesis；　but　its　foundations；unpalatableness；　and　the　liability　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　c