darwin and modern science-第17章

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98；　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　learn　by　experience　to　distinguish　between　different　kinds　of　victims。　　But　Lloyd　Morgan's　（〃Habit　and　Instinct〃；　London；　1896。）　experiments　with　young　birds　proved　that　this　is　really　the　case；　and　at　the　same　time　furnished　an　additional　argument　against　the　LAMARCKIAN　PRINCIPLE。

In　addition　to　the　mimicry…rings　first　observed　in　South　America；　others　have　been　described　from　Tropical　India　by　Moore；　and　by　Poulton　and　Dixey　from　Africa；　and　we　may　expect　to　learn　many　more　interesting　facts　in　this　connection。　　Here　again　the　preliminary　postulates　of　the　theory　are　satisfied。　　And　how　much　more　that　would　lead　to　the　same　conclusion　might　be　added！

As　in　the　case　of　mimicry　many　species　have　come　to　resemble　one　another　through　processes　of　selection；　so　we　know　whole　classes　of　phenomena　in　which　plants　and　animals　have　become　adapted　to　one　another；　and　have　thus　been　modified　to　a　considerable　degree。　　I　refer　particularly　to　the　relation　between　flowers　and　insects；　but　as　there　is　an　article　on　〃The　Biology　of　Flowers〃　in　this　volume；　I　need　not　discuss　the　subject；　but　will　confine　myself　to　pointing　out　the　significance　of　these　remarkable　cases　for　the　theory　of　selection。　　Darwin　has　shown　that　the　originally　inconspicuous　blossoms　of　the　phanerogams　were　transformed　into　flowers　through　the　visits　of　insects；　and　that；　conversely；　several　large　orders　of　insects　have　been　gradually　modified　by　their　association　with　flowers；　especially　as　regards　the　parts　of　their　body　actively　concerned。　　Bees　and　butterflies　in　particular　have　become　what　they　are　through　their　relation　to　flowers。　　In　this　case　again　all　that　is　apparently　contradictory　to　the　theory　can；　on　closer　investigation；　be　beautifully　interpreted　in　corroboration　of　it。　　Selection　can　give　rise　only　to　what　is　of　use　to　the　organism　actually　concerned；　never　to　what　is　of　use　to　some　other　organism；　and　we　must　therefore　expect　to　find　that　in　flowers　only　characters　of　use　to　THEMSELVES　have　arisen；　never　characters　which　are　of　use　to　insects　only；　and　conversely　that　in　the　insects　characters　useful　to　them　and　not　merely　to　the　plants　would　have　originated。　　For　a　long　time　it　seemed　as　if　an　exception　to　this　rule　existed　in　the　case　of　the　fertilisation　of　the　yucca　blossoms　by　a　little　moth；　Pronuba　yuccasella。　　This　little　moth　has　a　sickle…shaped　appendage　to　its　mouth…parts　which　occurs　in　no　other　Lepidopteron；　and　which　is　used　for　pushing　the　yellow　pollen　into　the　opening　of　the　pistil；　thus　fertilising　the　flower。　　Thus　it　appears　as　if　a　new　structure；　which　is　useful　only　to　the　plant；　has　arisen　in　the　insect。　　But　the　difficulty　is　solved　as　soon　as　we　learn　that　the　moth　lays　its　eggs　in　the　fruit…buds　of　the　Yucca；　and　that　the　larvae；　when　they　emerge；　feed　on　the　developing　seeds。　　In　effecting　the　fertilisation　of　the　flower　the　moth　is　at　the　same　time　making　provision　for　its　own　offspring；　since　it　is　only　after　fertilisation　that　the　seeds　begin　to　develop。　　There　is　thus　nothing　to　prevent　our　referring　this　structural　adaptation　in　Pronuba　yuccasella　to　processes　of　selection；　which　have　gradually　transformed　the　maxillary　palps　of　the　female　into　the　sickle…shaped　instrument　for　collecting　the　pollen；　and　which　have　at　the　same　time　developed　in　the　insect　the　instinct　to　press　the　pollen　into　the　pistil。

In　this　domain；　then；　the　theory　of　selection　finds　nothing　but　corroboration；　and　it　would　be　impossible　to　substitute　for　it　any　other　explanation；　which；　now　that　the　facts　are　so　well　known；　could　be　regarded　as　a　serious　rival　to　it。　　That　selection　is　a　factor；　and　a　very　powerful　factor　in　the　evolution　of　organisms；　can　no　longer　be　doubted。　　Even　although　we　cannot　bring　forward　formal　proofs　of　it　IN　DETAIL；　cannot　calculate　definitely　the　size　of　the　variations　which　present　themselves；　and　their　selection…value；　cannot；　in　short；　reduce　the　whole　process　to　a　mathematical　formula；　yet　we　must　assume　selection；　because　it　is　the　only　possible　explanation　applicable　to　whole　classes　of　phenomena；　and　because；　on　the　other　hand；　it　is　made　up　of　factors　which　we　know　can　be　proved　actually　to　exist；　and　which；　IF　they　exist；　must　of　logical　necessity　cooperate　in　the　manner　required　by　the　theory。　　WE　MUST　ACCEPT　IT　BECAUSE　THE　PHENOMENA　OF　EVOLUTION　AND　ADAPTATION　MUST　HAVE　A　NATURAL　BASIS；　AND　BECAUSE　IT　IS　THE　ONLY　POSSIBLE　EXPLANATION　OF　THEM。　　（This　has　been　discussed　in　many　of　my　earlier　works。　　See　for　instance　〃The　All…　Sufficiency　of　Natural　Selection；　a　reply　to　Herbert　Spencer〃；　London；　1893。）

Many　people　are　willing　to　admit　that　selection　explains　adaptations；　but　they　maintain　that　only　a　part　of　the　phenomena　are　thus　explained；　because　everything　does　not　depend　upon　adaptation。　　They　regard　adaptation　as；　so　to　speak；　a　special　effort　on　the　part　of　Nature；　which　she　keeps　in　readiness　to　meet　particularly　difficult　claims　of　the　external　world　on　organisms。　　But　if　we　look　at　the　matter　more　carefully　we　shall　find　that　adaptations　are　by　no　means　exceptional；　but　that　they　are　present　everywhere　in　such　enormous　numbers；　that　it　would　be　difficult　in　regard　to　any　structure　whatever；　to　prove　that　adaptation　had　NOT　played　a　part　in　its　evolution。

How　often　has　the　senseless　objection　been　urged　against　selection　that　it　can　create　nothing；　it　can　only　reject。　　It　is　true　that　it　cannot　create　either　the　living　substance　or　the　variations　of　it；　both　must　be　given。　　But　in　rejecting　one　thing　it　preserves　another；　intensifies　it；　combines　it；　and　in　this　way　CREATES　what　is　new。　　EVERYTHING　in　organisms　depends　on　adaptation；　that　is　to　say；　everything　must　be　admitted　through　the　narrow　door　of　selection；　otherwise　it　can　take　no　part　in　the　building　up　of　the　whole。　　But；　it　is　asked；　what　of　the　direct　effect　of　external　conditions；　temperature；　nutrition；　climate　and　the　like？　　Undoubtedly　these　can　give　rise　to　variations；　but　they　too　must　pass　through　the　door　of　selection；　and　if　they　cannot　do　this　they　are　rejected；　eliminated　from　the　constitution　of　the　species。

It　may；　perhaps；　be　objected　that　such　external　influences　are　often　of　a　compelling　power；　and　that　every　animal　MUST　submit　to　them；　and　that　thus　selection　has　no　choice　and　can　neither　select　nor　reject。　　There　may　be　such　cases；　let　us　assume　for　instance　that　the　effect　of　the　cold　of　the　Arctic　regions　was　to　make　all　the　mammals　become　black；　the　result　would　be　that　they　would　all　be　eliminated　by　selection；　and　that　no　mammals　would　be　able　to　live　there　at　all。　　But　in　most　cases　a　certain　percentage　of　animals　resists　these　strong　influences；　and　thus　selection　secures　a　foothold　on　which　to　work；　eliminating　the　unfavourable　variation；　and　establishing　a　useful　colouring；　consistent　with　what　is　required　for　the　maintenance　of　the　species。

Everything　depends　upon　adaptation！　　We　have　spoken　much　of　adaptation　in　colouring；　in　connection　with　the　examples　brought　into　prominence　by　Darwin；　because　these　are　conspicuous；　easily　verified；　and　at　the　same　time　convincing　for　the　theory　of　selection。