darwin and modern science-第17节

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　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　both　means　of　attraction　should　exist　in　one　genus；　and　only　one　of　them　in　another；　since　we　do　not　know　the　minutest　details　of　the　conditions　of　life　of　the　genera　concerned。　　But　from　the　sporadic　distribution　of　scent…scales　in　Lepidoptera；　and　from　their　occurrence　or　absence　in　nearly　related　species；　we　may　conclude　that　fragrance　is　a　relatively　MODERN　acquirement；　　more　recent　than　brilliant　colouring。

One　thing　in　particular　that　stamps　decorative　colouring　as　a　product　of　selection　is　ITS　GRADUAL　INTENSIFICATION　by　the　addition　of　new　spots；　which　we　can　quite　well　observe；　because　in　many　cases　the　colours　have　been　first　acquired　by　the　males；　and　later　transmitted　to　the　females　by　inheritance。　　The　scent…scales　are　never　thus　transmitted；　probably　for　the　same　reason　that　the　decorative　colours　of　many　birds　are　often　not　transmitted　to　the　females：　　because　with　these　they　would　be　exposed　to　too　great　elimination　by　enemies。　　Wallace　was　the　first　to　point　out　that　in　species　with　concealed　nests　the　beautiful　feathers　of　the　male　occurred　in　the　female　also；　as　in　the　parrots；　for　instance；　but　this　is　not　the　case　in　species　which　brood　on　an　exposed　nest。　　In　the　parrots　one　can　often　observe　that　the　general　brilliant　colouring　of　the　male　is　found　in　the　female；　but　that　certain　spots　of　colour　are　absent；　and　these　have　probably　been　acquired　comparatively　recently　by　the　male　and　have　not　yet　been　transmitted　to　the　female。

Isolation　of　the　group　of　individuals　which　is　in　process　of　varying　is　undoubtedly　of　great　value　in　sexual　selection；　for　even　a　solitary　conspicuous　variation　will　become　dominant　much　sooner　in　a　small　isolated　colony；　than　among　a　large　number　of　members　of　a　species。

Anyone　who　agrees　with　me　in　deriving　variations　from　germinal　selection　will　regard　that　process　as　an　essential　aid　towards　explaining　the　selection　of　distinctive　courtship…characters；　such　as　coloured　spots；　decorative　feathers；　horny　outgrowths　in　birds　and　reptiles；　combs；　feather…tufts；　and　the　like；　since　the　beginnings　of　these　would　be　presented　with　relative　frequency　in　the　struggle　between　the　determinants　within　the　germ…plasm。　　The　process　of　transmission　of　decorative　feathers　to　the　female　results；　as　Darwin　pointed　out　and　illustrated　by　interesting　examples；　in　the　COLOUR…TRANSFORMATION　OF　A　WHOLE　SPECIES；　and　this　process；　as　the　phyletically　older　colouring　of　young　birds　shows；　must；　in　the　course　of　thousands　of　years；　have　repeated　itself　several　times　in　a　line　of　descent。

If　we　survey　the　wealth　of　phenomena　presented　to　us　by　secondary　sexual　characters；　we　can　hardly　fail　to　be　convinced　of　the　truth　of　the　principle　of　sexual　selection。　　And　certainly　no　one　who　has　accepted　natural　selection　should　reject　sexual　selection；　for；　not　only　do　the　two　processes　rest　upon　the　same　basis；　but　they　merge　into　one　another；　so　that　it　is　often　impossible　to　say　how　much　of　a　particular　character　depends　on　one　and　how　much　on　the　other　form　of　selection。

（b）　　NATURAL　SELE