darwin and modern science-第15部分

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ECTLY　secures　reproduction。

In　many　cases　THE　ORGAN　OF　SMELL　shows　a　similar　improvement。　　Many　lower　Crustaceans　（Daphnidae）　have　better　developed　organs　of　smell　in　the　male　sex。　　The　difference　is　often　slight　and　amounts　only　to　one　or　two　olfactory　filaments；　but　certain　species　show　a　difference　of　nearly　a　hundred　of　these　filaments　（Leptodora）。　　The　same　thing　occurs　among　insects。

We　must　briefly　consider　the　clasping　or　grasping　organs　which　have　developed　in　the　males　among　many　lower　Crustaceans；　but　here　natural　selection　plays　its　part　along　with　sexual　selection；　for　the　union　of　the　sexes　is　an　indispensable　condition　for　the　maintenance　of　the　species；　and　as　Darwin　himself　pointed　out；　in　many　cases　the　two　forms　of　selection　merge　into　each　other。　　This　fact　has　always　seemed　to　me　to　be　a　proof　of　natural　selection；　for；　in　regard　to　sexual　selection；　it　is　quite　obvious　that　the　victory　of　the　best…equipped　could　have　brought　about　the　improvement　only　of　the　organs　concerned；　the　factors　in　the　struggle；　such　as　the　eye　and　the　olfactory　organ。

We　come　now　to　the　EXCITANTS；　that　is；　to　the　group　of　sexual　characters　whose　origin　through　processes　of　selection　has　been　most　frequently　called　in　question。　　We　may　cite　the　LOVE…CALLS　produced　by　many　male　insects；　such　as　crickets　and　cicadas。　　These　could　only　have　arisen　in　animal　groups　in　which　the　female　did　not　rapidly　flee　from　the　male；　but　was　inclined　to　accept　his　wooing　from　the　first。　　Thus；　notes　like　the　chirping　of　the　male　cricket　serve　to　entice　the　females。　　At　first　they　were　merely　the　signal　which　showed　the　presence　of　a　male　in　the　neighbourhood；　and　the　female　was　gradually　enticed　nearer　and　nearer　by　the　continued　chirping。　　The　male　that　could　make　himself　heard　to　the　greatest　distance　would　obtain　the　largest　following；　and　would　transmit　the　beginnings；　and；　later；　the　improvement　of　his　voice　to　the　greatest　number　of　descendants。　　But　sexual　excitement　in　the　female　became　associated　with　the　hearing　of　the　love…call；　and　then　the　sound…producing　organ　of　the　male　began　to　improve；　until　it　attained　to　the　emission　of　the　long…drawn…out　soft　notes　of　the　mole…cricket　or　the　maenad…like　cry　of　the　cicadas。　　I　cannot　here　follow　the　process　of　development　in　detail；　but　will　call　attention　to　the　fact　that　the　original　purpose　of　the　voice；　the　announcing　of　the　male's　presence；　became　subsidiary；　and　the　exciting　of　the　female　became　the　chief　goal　to　be　aimed　at。　　The　loudest　singers　awakened　the　strongest　excitement；　and　the　improvement　resulted　as　a　matter　of　course。　　I　conceive　of　the　origin　of　bird…song　in　a　somewhat　similar　manner；　first　as　a　means　of　enticing；　then　of　exciting　the　female。

One　more　kind　of　secondary　sexual　character　must　here　be　mentioned：　　the　odour　which　emanates　from　so　many　animals　at　the　breeding　season。　　It　is　possible　that　this　odour　also　served　at　first　merely　to　give　notice　of　the　presence　of　individuals　of　the　other　sex；　but　it　soon　became　an　excitant；　and　as　the　individuals　which　caused　the　greatest　degree　of　excitement　were　preferred；　it　reached　as　high　a　pitch　of　perfection　as　was　possible　to　it。　　I　shall　confine　myself　here　to　the　comparatively　recently　discovered　fragrance　of　butterflies。　　Since　Fritz　Muller　found　out　that　certain　Brazilian　butterflies　gave　off　fragrance　〃like　a　flower；〃　we　have　become　acquainted　with　many　such　cases；　and　we　now　know　that　in　all　lands；　not　only　many　diurnal　Lepidoptera　but　nocturnal　ones　also　give　off　a　delicate　odour；　which　is　agreeable　even　to　man。　　The　ethereal　oil　to　which　this　fragrance　is　due　is　secreted　by　the　skin…cells；　usually　of　the　wing；　as　I　showed　soon　after　the　discovery　of　the　SCENT…SCALES。　　This　is　the　case　in　the　males；　the　females　have　no　SPECIAL　scent…scales　recognisable　as　such　by　their　form；　but　they　must；　nevertheless；　give　off　an　extremely　delicate　fragrance；　although　our　imperfect　organ　of　smell　cannot　perceive　it；　for　the　males　become　aware　of　the　presence　of　a　female；　even　at　night；　from　a　long　distance　off；　and　gather　round　her。　　We　may　therefore　conclude；　that　both　sexes　have　long　given　forth　a　very　delicate　perfume；　which　announced　their　presence　to　others　of　the　same　species；　and　that　in　many　species　（NOT　IN　ALL）　these　small　beginnings　became；　in　the　males；　particularly　strong　scent…scales　of　characteristic　form　（lute；　brush；　or　lyre…shaped）。　　At　first　these　scales　were　scattered　over　the　surface　of　the　wing；　but　gradually　they　concentrated　themselves；　and　formed　broad；　velvety　bands；　or　strong；　prominent　brushes；　and　they　attained　their　highest　pitch　of　evolution　when　they　became　enclosed　within　pits　or　folds　of　the　skin；　which　could　be　opened　to　let　the　delicious　fragrance　stream　forth　suddenly　towards　the　female。　　Thus　in　this　case　also　we　see　that　characters；　the　original　use　of　which　was　to　bring　the　sexes　together；　and　so　to　maintain　the　species；　have　been　evolved　in　the　males　into　means　for　exciting　the　female。　　And　we　can　hardly　doubt；　that　the　females　are　most　readily　enticed　to　yield　to　the　butterfly　that　sends　out　the　strongest　fragrance；that　is　to　say；　that　excites　them　to　the　highest　degree。　　It　is　a　pity　that　our　organs　of　smell　are　not　fine　enough　to　examine　the　fragrance　of　male　Lepidoptera　in　general；　and　to　compare　it　with　other　perfumes　which　attract　these　insects。　　（See　Poulton；　〃Essays　on　Evolution〃；　1908；　pages　316；　317。）　　As　far　as　we　can　perceive　them　they　resemble　the　fragrance　of　flowers；　but　there　are　Lepidoptera　whose　scent　suggests　musk。　　A　smell　of　musk　is　also　given　off　by　several　plants：　　it　is　a　sexual　excitant　in　the　musk…deer；　the　musk…sheep；　and　the　crocodile。

As　far　as　we　know；　then；　it　is　perfumes　similar　to　those　of　flowers　that　the　male　Lepidoptera　give　off　in　order　to　entice　their　mates；　and　this　is　a　further　indication　that　animals；　like　plants；　can　to　a　large　extent　meet　the　claims　made　upon　them　by　life；　and　produce　the　adaptations　which　are　most　purposive；a　further　proof；　too；　of　my　proposition　that　the　useful　variations；　so　to　speak；　are　ALWAYS　THERE。　　The　flowers　developed　the　perfumes　which　entice　their　visitors；　and　the　male　Lepidoptera　developed　the　perfumes　which　entice　and　excite　their　mates。

There　are　many　pretty　little　problems　to　be　solved　in　this　connection；　for　there　are　insects；　such　as　some　flies；　that　are　attracted　by　smells　which　are　unpleasant　to　us；　like　those　from　decaying　flesh　and　carrion。　　But　there　are　also　certain　flowers；　some　orchids　for　instance；　which　give　forth　no　very　agreeable　odour；　but　one　which　is　to　us　repulsive　and　disgusting；　and　we　should　therefore　expect　that　the　males　of　such　insects　would　give　off　a　smell　unpleasant　to　us；　but　there　is　no　case　known　to　me　in　which　this　has　been　demonstrated。

In　cases　such　as　we　have　discussed；　it　is　obvious　that　there　is　no　possible　explanation　except　through　selection。　　This　brings　us　to　the　last　kind　of　secondary　sexual　characters；　and　the　one　in　regard　to　which　doubt　has　been　most　frequently　expressed；decorative　colours　and　decorative　forms；　the　brilliant　plumage　of　the　male　pheasant；　the　humming…birds；　and　the　bird　of　Paradise；　as　well　as　the　bright　colours　of　many　species　of　butterfly；　from　the　beautiful　blue　of　our　little　Lycaenidae　to　the　magnificent　azure　of　the　large　Morphinae　of　Brazil。　　In　a　great　many　cases；　though　not　by　any　means　in　all；　the　male　butterflies　are　〃more　beautiful〃　than　the　females；　and　in　the　Tropics　in　particular　they　shine　and　glow　in　the　most　superb　colours。　　I　really　see　no　reason　why　we　should　doubt　the　power　of　sexual　selection；　and　I　myself　stand　wholly　on　Darwin's　side。　　Even　though　we　certainly　cannot　assume　that　the　females　exercise　a　conscious　choice　of　the　〃handsomest〃　mate；　and　deliberate　like　the　judges　in　a　court　of　justice　over　the　perfections　of　their　wooers；　we　have　no　reason　to　doubt　that　distinctive　forms　（decorative　feathers）　and　colours　have　a　particularly　exciting　effect　upon　the　female；　just　as　certain　odours　have　among　animals　of　so　many　different　groups；　including　the　butterflies。　　The　doubts　which　existed　for　a　considerable　time；　as　a　result　of　fallacious　experiments；　as　to　whether　the　colours　of　flowers　really　had　any　influence　in　attracting　butterflies　have　now　been　set　at　rest　through　a　series　of　more　careful　investigations；　we　now　know　that　the　colours　of　flowers　are　there　on　account　of　the　butterflies；　as　Sprengel　first　showed；　and　that　the　blossoms　of　Phanerogams　are　selected　in　relation　to　them；　as　Darwin　pointed　out。

Certainly　it　is　not　possible　to　bring　forward　any　convincing　proof　of　the　origin　of　decorative　colours　through　sexual　selection；　but　there　are　many　weighty　arguments　in　favour　of　it；　and　these　form　a　body　of　presumptive　evidence　so　strong　that　it　almost　amounts　to　certainty。

In　the　first　place；　there　is　the　analogy　with　other　secondary　sexual　characters。　If　the　song　of　birds　and　the　chirping　of　the　cricket　have　been　evolved　through　sexual　selection；　if　the　penetrating　odours　of　male　animals；the　crocodile；　the　musk…deer；　the　beaver；　the　carnivores；　and；　finally；　the　flower…like　fragrances　of　the　butterflies　have　been　evolved　to　their　present　pitch　in　this　way；　why　should　decorative　colours　have　arisen　in　some　other　way？　　Why　should　the　eye　be　less　sensitive　to　SPECIFICALLY　MALE　colours　and　other　VISIBLE　signs　ENTICING　TO　THE　FEMALE；　than　the　olfactory　sense　to　specifically　male　odours；　or　the　sense　of　hearing　to　specifically　male　sounds？　　Moreover；　the　decorative　feathers　of　birds　are　almost　always　spread　out　and　displayed　before　the　female　during　courtship。　　I　have　elsewhere　（〃The　Evolution　Theory〃；　London；　1904；　I。　page　219。）　pointed　out　that　decorative　colouring　and　sweet…scentedness　may　replace　one　another　in　Lepidoptera　as　well　as　in　flowers；　for　just　as　some　modestly　coloured　flowers　（mignonette　and　violet）　have　often　a　strong　perfume；　while　strikingly　coloured　ones　are　sometimes　quite　devoid　of　fragrance；　so　we　find　that　the　most　beautiful　and　gaily…coloured　of　our　native　Lepidoptera；　the　species　of　Vanessa；　have　no　scent…scales；　while　these　are　often　markedly　developed　in　grey　nocturnal　Lepidoptera。　　Both　attractions　may；　however；　be　combined　in　butterflies；　just　as　in　flowers。　　Of　course；　we　cannot　explain　why　bot