darwin and modern science-第20节

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rds　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）　and　its　Pierine　mimic　（Dismorphia　orise）　depends　on　a　diminution　in　the　size　of　the　scales；　in　the　Danaine　genus　Ituna　it　is　due　to　the　fewness　of　the　scales；　and　in　a　third　imitator；　a　moth　（Castnia　linus　var。　heliconoides）　the　glass…like　appearance　of　the　wing　is　due　neither　to　diminution　nor　to　absence　of　scales；　but　to　their　absolute　colourlessness　and　transparency；　and　to　the　fact　that　they　stand　upright。　In　another　moth　mimic　（Anthomyza）　the　arrangement　of　the　transparent　scales　is　normal。　　Thus　it　is　not　some　unknown　external　influence　that　has　brought　about　the　transparency　of　the　wing　in　these　five　forms；　as　has　sometimes　been　supposed。　　Nor　is　it　a　hypothetical　INTERNAL　evolutionary　tendency；　for　all　three　vary　in　a　different　manner。　　The　cause　of　this　agreement　can　only　lie　in　selection；　which　preserves　and　intensifies　in　each　species　the　favourable　variations　that　present　themselves。　　The　great　faithfulness　of　the　copy　is　astonishing　in　these　cases；　for　it　is　not　THE　WHOLE　wing　which　is　transparent；　certain　markings　are　black　in　colour；　and　these　contrast　sharply　with　the　glass…like　ground。　　It　is　obvious　that　the　pursuers　of　these　butterflies　must　be　very　sharp…sighted；　for　otherwise　the　agreement　between　the　species　could　never　have　been　pushed　so　far。　　The　less　the　enemies　see　and　observe；　the　more　defective　must　the　imitation　be；　and　if　they　had　been　blind；　no　visible　resemblance　between　the　species　which　required　protection　could　ever　have　arisen。

A　seemingly　irreconcilable　contradiction　to　the　mimicry　theory　is　presented　in　the　following　cases；　which　were　known　to　Bates；　who；　however；　never　succeeded　in　bringing　them　into　line　with　the　principle　of　mimicry。

In　South　America　there　are；　as　we　have　already　said；　many　mimics　of　the　immune　Ithomiinae　（or　as　Bates　called　them　Heliconidae）。　　Among　these　there　occur　not　merely　species　which　are　edible；　and　thus　require　the　protection　of　a　disguise；　but　others　which　are　rejected　on　account　of　their　unpalatableness。　　How　could　the　Ithomiine　dress　have　developed　in　their　case；　and　of　what　use　is　it；　since　the　species　would　in　any　case　be　immune？　In　Eastern　Brazil；　for　instance；　there　are　four　butterflies；　which　bear　a　most　confusing　resemblance　to　one　another　in　colour；　marking；　and　form　of　wing；　and　all　four　are　unpalatable　to　birds。　　They　belong　to　four　different　genera　and　three　sub…families；　and　we　have　to　inquire：　　Whence　came　this　resemblance　and　what　end　does　it　serve？　　For　a　long　time　no　satisfactory　answer　could　be　found；　but　Fritz　Muller　（In　〃Kosmos〃；　1879；　page　100。）；　　seventeen　years　after　Bates；　offered　a　solution　to　the　riddle；　when　he　pointed　out　that　young　birds　could　not　have　an　instinctive　knowledge　of　the　unpalatableness　of　the　Ithomiines；　but　must　learn　by　experience　which　species　were　edible　and　which　inedible。　　Thus　each　young　bird　must　have　tasted　at　least　one　individual　of　each　inedible　species　and　discovered　its　unpalatability；　before　it　learnt　to　avoid；　and　thus　to　spare　the　species。　　But　if　the　four　species　resemble　each　other　very　closely　the　bird　will　regard　them　all　as　of　the　same　kind；　and　avoid　them　all。　　Thus　there　developed　a　process　of　selection　which　resulted　in　the　survival　of　the　Ithomiine…like　individuals；　and　in　so　great　an　increase　of　resemblance　between　the　four　species；　that　they　are　difficult　to　distinguish　one　from　another　even　in　a　collection。　　The　advantage　for　the　four　species；　living　side　by　side　as　they　do　e。g。　in　Bahia；　lies　in　the　fact　that　only　one　individual　from　the　MIMICRY…RING　（〃inedible　association〃）　need　be　tasted　by　a　young　bird；　instead　of　at　least　four　individuals；　as　would　otherwise　be　the　case。　　As　the　number　of　young　birds　is　great；　this　makes　a　considerable　difference　in　the　ratio　of　elimination。

These　interesting　mimicry…rings　（trusts）；　which　have　much　significance　for　the　theory；　have　been　the　subject　of　numerous　and　careful　investigations；　and　at　least　their　essential　features　are　now　fully　established。　　Muller　took　for　granted；　without　making　any　investigations；　that　young　birds　only　lea