darwin and modern science-第92节

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pecies　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　〃Origin〃。　　Thus　he　wrote　to　H。W。　Bates；　April　4；　1861：　　〃If　I　had　to　cut　up　myself　in　a　review　I　would　have　（worried？）　and　quizzed　sexual　selection；　therefore；　though　I　am　fully　convinced　that　it　is　largely　true；　you　may　imagine　how　pleased　I　am　at　what　you　say　on　your　belief。〃　　（〃More　Letters〃；　I。　page　183。）

The　existence　of　sound…producing　organs　in　the　males　of　insects　was；　Darwin　considered；　the　strongest　evidence　in　favour　of　the　operation　of　sexual　selection　in　this　group。　　（〃Life　and　Letters〃；　III。　pages　94；　138。）　　Such　a　conclusion　has　received　strong　support　in　recent　years　by　the　numerous　careful　observations　of　Dr　F。A。　Dixey　（〃Proc。　Ent。　Soc。　Lond。〃　1904；　page　lvi；　1905；　pages　xxxvii；　liv；　1906；　page　ii。）　and　Dr　G。B。　Longstaff　（〃Proc。　Ent。　Soc。　Lond。〃　1905；　page　xxxv；　〃Trans。　Ent。　Soc。　Lond。〃　1905；　page　136；　1908；　page　607。）　on　the　scents　of　male　butterflies。　　The　experience　of　these　naturalists　abundantly　confirms　and　extends　the　account　given　by　Fritz　Muller　（〃Jen。　Zeit。〃　Vol。　XI。　1877；　page　99；　〃Trans。　Ent。　Soc。　Lond。〃　1878；　page　211。）　of　the　scents　of　certain　Brazilian　butterflies。　　It　is　a　remarkable　fact　that　the　apparently　epigamic　scents　of　male　butterflies　should　be　pleasing　to　man　while　the　apparently　aposematic　scents　in　both　sexes　of　species　with　warning　colours　should　be　displeasing　to　him。　　But　the　former　is　far　more　surprising　than　the　latter。　　It　is　not　perhaps　astonishing　that　a　scent　which　is　ex　hypothesi　unpleasant　to　an　insect…　eating　Vertebrate　should　be　displeasing　to　the　human　sense；　but　it　is　certainly　wonderful　that　an　odour　which　is　ex　hypothesi　agreeable　to　a　female　butterfly　should　also　be　agreeable　to　man。

Entirely　new　light　upon　the　seasonal　appearance　of　epigamic　characters　is　shed　by　the　recent　researches　of　C。W。　Beebe　（〃The　American　Naturalist〃；　Vol。　XLII。　No。　493；　Jan。　1908；　page　34。）；　who　caused　the　scarlet　tanager　（Piranga　erythromelas）　and　the　bobolink　（Dolichonyx　oryzivorus）　to　retain　their　breeding　plumage　through　the　whole　year　by　means　of　fattening　food；　dim　illumination；　and　reduced　activity。　　Gradual　restoration　to　the　light　and　the　addition　of　meal…worms　to　the　diet　invariably　brought　back　the　spring　song；　even　in　the　middle　of　winter。　　A　sudden　alteration　of　temperature；　either　higher　or　lower；　caused　the　birds　nearly　to　stop　feeding；　and　one　tanager　lost　weight　rapidly　and　in　two　weeks　moulted　into　the　olive…green　winter　plumage。　　After　a　year；　and　at　the　beginning　of　the　normal　breeding　season；　〃individual　tanagers　and　bobolinks　were　gradually　brought　under　normal　conditions　and　activities；〃　and　in　every　case　moulted　from　nuptial　plumage　to　nuptial　plumage。　　〃The　dull　colours　of　the　winter　season　had　been　skipped。〃　　The　author　justly　claims　to　have　established　〃that　the　sequence　of　plumage　in　these　birds　is　not　in　any　way　predestined　through　inheritance。。。；　but　that　it　may　be　interrupted　by　certain　factors　in　the　environmental　complex。〃


XVI。　　GEOGRAPHICAL　DISTRIBUTION　OF　PLANTS。

By　SIR　WILLIAM　THISELTON…DYER；　K。C。M。G。；　C。I。E。　Sc。D。；　F。R。S。

The　publication　of　〃The　Origin　of　Species〃　placed　the　study　of　Botanical　Geography　on　an　entirely　new　basis。　　It　is　only　necessary　to　study　the　monumental　〃Geographie　Botanique　raisonnee〃　of　Alphonse　De　Candolle；　published　four　years　earlier　（1855）；　to　realise　how　profound　and　far…　reaching　was　the　change。　　After　a　masterly　and　exhaustive　discussion　of　all　available　data　De　Candolle　in　his　final　conclusions　could　only　arrive　at　a　deadlock。　　It　is　sufficient　to　quote　a　few　sentences：

〃L'opinion　de　Lamarck　est　aujourd'hui　abandonee　par　tous　les　naturalistes　qui　ont　etudie　sagement　les　modifications　possibles　des　etres　organises。。。

〃Et　si　l'on　s'ecarte　des　exagerations　de　Lamarck；　si　l'on　suppose　un　premier　type　de　chaque　genre；　de　chaque　famille　tout　au　moins；　on　se　trouve　encore　a　l'egard　de　l'origine　de　ces　types　en　presence　de　la　grande　question　de　la　creation。

〃Le　seul　parti　a　prendre　est　donc　d'envisager　les　etres　organises　comme　existant　depuis　certaines　epoques；　avec　leurs　qualites　particulieres。〃