darwin and modern science-第103章

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inks；　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　probably　to　be　sought　in　the　fact　that　the　females　of　Papilio　are　more　variable　and　more　often　tend　to　become　dimorphic　than　those　of　Cosmodesmus；　while　the　latter　group　has　more　often　happened　to　possess　a　sufficient　foundation　for　the　origin　of　the　resemblance　in　patterns　which；　from　the　start；　were　common　to　male　and　female。

（5）　　In　very　variable　species　with　sexes　alike；　mimicry　can　be　rapidly　evolved　in　both　sexes　out　of　very　small　beginnings。　　Thus　the　reddish　marks　which　are　common　in　many　individuals　of　Limenitis　arthemis　were　almost　certainly　the　starting…point　for　the　evolution　of　the　beautifully　mimetic　L。　archippus。　　Nevertheless　in　such　cases；　although　there　is　no　reason　to　suspect　any　greater　variability；　the　female　is　commonly　a　somewhat　better　mimic　than　the　male　and　often　a　very　much　better　mimic。　　Wallace's　principle　seems　here　to　supply　the　obvious　interpretation。

（6）　　When　the　difference　between　the　patterns　of　the　model　and　presumed　ancestor　of　the　mimic　is　very　great；　the　female　is　often　alone　mimetic；　when　the　difference　is　comparatively　small；　both　sexes　are　commonly　mimetic。　　The　Nymphaline　genus　Hypolimnas　is　a　good　example。　　In　Hypolimnas　itself　the　females　mimic　Danainae　with　patterns　very　different　from　those　preserved　by　the　non…mimetic　males：　　in　the　sub…genus　Euralia；　both　sexes　resemble　the　black　and　white　Ethiopian　Danaines　with　patterns　not　very　dissimilar　from　that　which　we　infer　to　have　existed　in　the　non…mimetic　ancestor。

（7）　　Although　a　melanic　form　or　other　large　variation　may　be　of　the　utmost　importance　in　facilitating　the　start　of　a　mimetic　likeness；　it　is　impossible　to　explain　the　evolution　of　any　detailed　resemblance　in　this　manner。　　And　even　the　large　colour　variation　itself　may　well　be　the　expression　of　a　minute　and　〃continuous〃　change　in　the　chemical　and　physical　constitution　of　pigments。

SEXUAL　SELECTION　（EPIGAMIC　CHARACTERS）。

We　do　not　know　the　date　at　which　the　idea　of　Sexual　Selection　arose　in　Darwin's　mind；　but　it　was　probably　not　many　years　after　the　sudden　flash　of　insight　which；　in　October　1838；　gave　to　him　the　theory　of　Natural　Selection。　　An　excellent　account　of　Sexual　Selection　occupies　the　concluding　paragraph　of　Part　I。　of　Darwin's　Section　of　the　Joint　Essay　on　Natural　Selection；　read　July　1st；　1858；　before　the　Linnean　Society。　　（〃Journ。　Proc。　Linn。　Soc。〃　Vol。　III。　1859；　page　50。）　　The　principles　are　so　clearly　and　sufficiently　stated　in　these　brief　sentences　that　it　is　appropriate　to　quote　the　whole：　　〃Besides　this　natural　means　of　selection；　by　which　those　individuals　are　preserved；　whether　in　their　egg；　or　larval；　or　mature　state；　which　are　best　adapted　to　the　place　they　fill　in　nature；　there　is　a　second　agency　at　work　in　most　unisexual　animals；　tending　to　produce　the　same　effect；　namely；　the　struggle　of　the　males　for　the　females。　　These　struggles　are　generally　decided　by　the　law　of　battle；　but　in　the　case　of　birds；　apparently；　by　the　charms　of　their　song；　by　their　beauty　or　their　power　of　courtship；　as　in　the　dancing　rock…thrush　of　Guiana。　　The　most　vigorous　and　healthy　males；　implying　perfect　adaptation；　must　generally　gain　the　victory　in　their　contests。　　This　kind　of　selection；　however；　is　less　rigorous　than　the　other；　it　does　not　require　the　death　of　the　less　successful；　but　gives　to　them　fewer　descendants。　　The　struggle　falls；　moreover；　at　a　time　of　year　when　food　is　generally　abundant；　and　perhaps　the　effect　chiefly　produced　would　be　the　modification　of　the　secondary　sexual　characters；　which　are　not　related　to　the　power　of　obtaining　food；　or　to　defence　from　enemies；　but　to　fighting　with　or　rivalling　other　males。　　The　result　of　this　struggle　amongst　the　males　may　be　compared　in　some　respects　to　that　produced　by　those　agriculturists　who　pay　less　attention　to　the　careful　selection　of　all　their　young　animals；　and　more　to　the　occasional　use　of　a　choice　mate。〃

A　full　exposition　of　Sexual　Selection　appeared　in　the　〃The　Descent　of　Man〃　in　1871；　and　in　the　greatly　augmented　second　edition；　in　1874。　　It　has　been　remarked　that　the　two　subjects；　〃The　Descent　of　Man　and　Selection　in　Relation　to　Sex〃；　seem　to　fuse　somewhat　imperfectly　into　the　single　work　of　which　they　form　the　title。　　The　reason　for　their　association　is　clearly　shown　in　a　letter　to　Wallace；　dated　May　28；　1864：　　〃。。。I　suspect　that　a　sort　of　sexual　selection　has　been　the　most　powerful　means　of　changing　the　races　of　man。〃　　（〃More　Letters〃；　II。　page　33。）

Darwin；　as　we　know　from　his　Autobiography　（〃Life　and　Letters〃；　I。　page　94。）；　was　always　greatly　interested　in　this　hypothesis；　and　it　has　been　shown　in　the　preceding　pages　that　he　was　inclined　to　look　favourably　upon　it　as　an　interpretation　of　many　appearances　usually　explained　by　Natural　Selection。　　Hence　Sexual　Selection；　incidentally　discussed　in　other　sections　of　the　present　essay；　need　not　be　considered　at　any　length；　in　the　section　specially　allotted　to　it。

Although　so　interested　in　the　subject　and　notwithstanding　his　conviction　that　the　hypothesis　was　sound；　Darwin　was　quite　aware　that　it　was　probably　the　most　vulnerable　part　of　the