darwin and modern science-第91节

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w　more　clearness　of　mind。〃　　（〃More　Letters〃；　I。　page　175。）

Even　before　the　〃Origin〃　appeared　Darwin　anticipated　the　first　results　upon　the　mind　of　naturalists。　　He　wrote　to　Asa　Gray；　Dec。　21；　1859：　　〃I　have　made　up　my　mind　to　be　well　abused；　but　I　think　it　of　importance　that　my　notions　should　be　read　by　intelligent　men；　accustomed　to　scientific　argument；　though　NOT　naturalists。　　It　may　seem　absurd；　but　I　think　such　men　will　drag　after　them　those　naturalists　who　have　too　firmly　fixed　in　their　heads　that　a　species　is　an　entity。〃　　（〃Life　and　Letters〃　II。　page　245。）

Mimicry　was　not　only　one　of　the　first　great　departments　of　zoological　knowledge　to　be　studied　under　the　inspiration　of　natural　Selection；　it　is　still　and　will　always　remain　one　of　the　most　interesting　and　important　of　subjects　in　relation　to　this　theory　as　well　as　to　evolution。　　In　mimicry　we　investigate　the　effect　of　environment　in　its　simplest　form：　　we　trace　the　effects　of　the　pattern　of　a　single　species　upon　that　of　another　far　removed　from　it　in　the　scale　of　classification。　　When　there　is　reason　to　believe　that　the　model　is　an　invader　from　another　region　and　has　only　recently　become　an　element　in　the　environment　of　the　species　native　to　its　second　home；　the　problem　gains　a　special　interest　and　fascination。　　Although　we　are　chiefly　dealing　with　the　fleeting　and　changeable　element　of　colour　we　expect　to　find　and　we　do　find　evidence　of　a　comparatively　rapid　evolution。　　The　invasion　of　a　fresh　model　is　for　certain　species　an　unusually　sudden　change　in　the　forces　of　the　environment　and　in　some　instances　we　have　grounds　for　the　belief　that　the　mimetic　response　has　not　been　long　delayed。

MIMICRY　AND　SEX。

Ever　since　Wallace's　classical　memoir　on　mimicry　in　the　Malayan　Swallowtail　butterflies；　those　naturalists　who　have　written　on　the　subject　have　followed　his　interpretation　of　the　marked　prevalence　of　mimetic　resemblance　in　the　female　sex　as　compared　with　the　male。　　They　have　believed　with　Wallace　that　the　greater　dangers　of　the　female；　with　slower　flight　and　often　alighting　for　oviposition；　have　been　in　part　met　by　the　high　development　of　this　special　mode　of　protection。　　The　fact　cannot　be　doubted。　　It　is　extremely　common　for　a　non…mimetic　male　to　be　accompanied　by　a　beautifully　mimetic　female　and　often　by　two　or　three　different　forms　of　female；　each　mimicking　a　different　model。　　The　male　of　a　polymorphic　mimetic　female　is；　in　fact；　usually　non…mimetic　（e。g。　Papilio　dardanus　=　merope）；　or　if　a　mimic　（e。g。　the　Nymphaline　genus　Euripus）；　resembles　a　very　different　model。　　On　the　other　hand　a　non…mimetic　female　accompanied　by　a　mimetic　male　is　excessively　rare。　　An　example　is　afforded　by　the　Oriental　Nymphaline；　Cethosia；　in　which　the　males　of　some　species　are　rough　mimics　of　the　brown　Danaines。　　In　some　of　the　orb…weaving　spiders　the　males　mimic　ants；　while　the　much　larger　females　are　non…mimetic。　　When　both　sexes　mimic；　it　is　very　common　in　butterflies　and　is　also　known　in　moths；　for　the　females　to　be　better　and　often　far　better　mimics　than　the　males。

Although　still　believing　that　Wallace's　hypothesis　in　large　part　accounts　for　the　facts　briefly　summarised　above；　the　present　writer　has　recently　been　led　to　doubt　whether　it　offers　a　complete　explanation。　　Mimicry　in　the　male；　even　though　less　beneficial　to　the　species　than　mimicry　in　the　female；　would　still　surely　be　advantageous。　　Why　then　is　it　so　often　entirely　restricted　to　the　female？　　While　the　attempt　to　find　an　answer　to　this　question　was　haunting　me；　I　re…read　a　letter　written　by　Darwin　to　Wallace；　April　15；　1868；　containing　the　following　sentences：　　〃When　female　butterflies　are　more　brilliant　than　their　males　you　believe　that　they　have　in　most　cases；　or　in　all　cases；　been　rendered　brilliant　so　as　to　mimic　some　other　species；　and　thus　escape　danger。　　But　can　you　account　for　the　males　not　having　been　rendered　equally　brilliant　and　equally　protected？　　Although　it　may　be　most　for　the　welfare　of　the　species　that　the　female　should　be　protected；　yet　it　would　be　some　advantage；　certainly　no　disadvantage；　for　the　unfortunate　male　to　enjoy　an　equal　immunity　from　danger。　　For　my　part；　I　should　say　that　the　female　alone　had　happened　to　vary　in　the　right　manner；　and　that　the　beneficial　variations　had　been　transmitted　to　the　same　sex　alone。　　Believing　in　this；　I　can　see　no　improbability　（but　from　analogy　of　domestic　animals　a　strong　probability）　that　variations　leading　to　beauty　must　often　have　occurred　in　the　males　alone；　and　been　transmitted　to　that　sex　alone。　　Thus　I　should　account　in　many　cases　for　the　greater　beauty　of　the　male　over　the　female；　without　the　need　of　the　protective　principle。〃　　（〃More　Letters〃；　II。　pages　73；　74。　　On　the　same　subject〃the　gay…coloured　females　of　Pieris〃　（Perrhybris　（Mylothris）　pyrrha　of　Brazil）；　Darwin　wrote　to　Wallace；　May　5；　1868；　as　follows：　　〃I　believe　I　quite　follow　you　in　believing　that　the　colours　are　wholly　due　to　mimicry；　and　I　further　believe　that　the　male　is　not　brilliant　from　not　having　received　through　inheritance　colour　from　the　female；　and　from　not　himself　having　varied；　in　short；　that　he　has　not　been　influenced　by　selection。〃　　It　should　be　noted　that　the　male　of　this　species　does　exhibit　a　mimetic　pattern　on　the　under　surface。　　〃More　Letters〃　II。　page　78。）

The　consideration　of　the　facts　of　mimicry　thus　led　Darwin　to　the　conclusion　that　the　female　happens　to　vary　in　the　right　manner　more　commonly　than　the　male；　while　the　secondary　sexual　characters　of　males　supported　the　conviction　〃that　from　some　unknown　cause　such　characters　（viz。　new　characters　arising　in　one　sex　and　transmitted　to　it　alone）　apparently　appear　oftener　in　the　male　than　in　the　female。〃　　（Letter　from　Darwin　to　Wallace；　May　5；　1867；　〃More　Letters〃；　II。　Page　61。）

Comparing　these　conflicting　arguments　we　are　led　to　believe　that　the　first　is　the　stronger。　　Mimicry　in　the　male　would　be　no　disadvantage　but　an　advantage；　and　when　it　appears　would　be　and　is　taken　advantage　of　by　selection。　　The　secondary　sexual　characters　of　males　would　be　no　advantage　but　a　disadvantage　to　females；　and；　as　Wallace　thinks；　are　withheld　from　this　sex　by　selection。　　It　is　indeed　possible　that　mimicry　has　been　hindered　and　often　prevented　from　passing　to　the　males　by　sexual　selection。　We　know　that　Darwin　was　much　impressed　（〃Descent　of　Man〃；　page　325。）　by　Thomas　Belt's　daring　and　brilliant　suggestion　that　the　white　patches　which　exist；　although　ordinarily　concealed；　on　the　wings　of　mimetic　males　of　certain　Pierinae　（Dismorphia）；　have　been　preserved　by　preferential　mating。　He　supposed　this　result　to　have　been　brought　about　by　the　females　exhibiting　a　deep…seated　preference　for　males　that　displayed　the　chief　ancestral　colour；　inherited　from　periods　before　any　mimetic　pattern　had　been　evolved　in　the　species。　　But　it　has　always　appeared　to　me　that　Belt's　deeply　interesting　suggestion　requires　much　solid　evidence　and　repeated　confirmation　before　it　can　be　accepted　as　a　valid　interpretation　of　the　facts。　　In　the　present　state　of　our　knowledge；　at　any　rate　of　insects　and　especially　of　Lepidoptera；　it　is　probable　that　the　female　is　more　apt　to　vary　than　the　male　and　that　an　important　element　in　the　interpretation　of　prevalent　female　mimicry　is　provided　by　this　fact。

In　order　adequately　to　discuss　the　question　of　mimicry　and　sex　it　would　be　necessary　to　analyse　the　whole　of　the　facts；　so　far　as　they　are　known　in　butterflies。　　On　the　present　occasion　it　is　only　possible　to　state　the　inferences　which　have　been　drawn　from　general　impressions；inferences　which　it　is　believed　will　be　sustained　by　future　inquiry。

（1）　　Mimicry　may　occasionally　arise　in　one　sex　because　the　differences　which　distinguish　it　from　the　other　sex　happen　to　be　such　as　to　afford　a　starting…point　for　the　resemblance。　　Here　the　male　is　at　no　disadvantage　as　compared　with　the　female；　and　the　rarity　of　mimicry　in　the　male　alone　（e。g。　Cethosia）　is　evidence　that　the　great　predominance　of　female　mimicry　is　not　to　be　thus　explained。

（2）　　The　tendency　of　the　female　to　dimorphism　and　polymorphism　has　been　of　great　importance　in　determining　this　predominance。　　Thus　if　the　female　appear　in　two　different　forms　and　the　male　in　only　one　it　will　be　twice　as　probable　that　she　will　happen　to　possess　a　sufficient　foundation　for　the　evolution　of　mimicry。

（3）　　The　appearance　of　melanic　or　partially　melanic　forms　in　the　female　has　been　of　very　great　service；　providing　as　it　does　a　change　of　ground…colour。　Thus　the　mimicry　of　the　black　generally　red…marked　American　〃Aristolochia　swallowtails〃　（Pharmacophagus）　by　the　females　of　Papilio　swallowtails　was　probably　begun　in　this　way。

（4）　　It　is　probably　incorrect　to　assume　with　Haase　that　mimicry　always　arose　in　the　female　and　was　later　acquired　by　the　male。　　Both　sexes　of　the　third　section　of　swallowtails　（Cosmodesmus）　mimic　Pharmacophagus　in　America；　far　more　perfectly　than　do　the　females　of　Papilio。　　But　this　is　not　due　to　Cosmodesmus　presenting　us　with　a　later　stage　of　history　begun　in　Papilio；　for　in　Africa　Cosmodesmus　is　still　mimetic　（of　Danainae）　in　both　sexes　although　the　resemblances　attained　are　imperfect；　while　many　African　species　of　Papilio　have　non…mimetic　males　with　beautifully　mimetic　females。　The　explanation　is　probab