darwin and modern science-第16节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


ection。

NATURAL　SELECTION　chooses　out　for　reproduction　the　individuals　that　are　best　equipped　for　the　struggle　for　existence；　and　it　does　so　at　every　stage　of　development；　it　thus　improves　the　species　in　all　its　stages　and　forms。　　SEXUAL　SELECTION　operates　only　on　individuals　that　are　already　capable　of　reproduction；　and　does　so　only　in　relation　to　the　attainment　of　reproduction。　　It　arises　from　the　rivalry　of　one　sex；　usually　the　male；　for　the　possession　of　the　other；　usually　the　female。　　Its　influence　can　therefore　only　DIRECTLY　affect　one　sex；　in　that　it　equips　it　better　for　attaining　possession　of　the　other。　　But　the　effect　may　extend　indirectly　to　the　female　sex；　and　thus　the　whole　species　may　be　modified；　without；　however；　becoming　any　more　capable　of　resistance　in　the　struggle　for　existence；　for　sexual　selection　only　gives　rise　to　adaptations　which　are　likely　to　give　their　possessor　the　victory　over　rivals　in　the　struggle　for　possession　of　the　female；　and　which　are　therefore　peculiar　to　the　wooing　sex：　　the　manifold　〃secondary　sexual　characters。〃　　The　diversity　of　these　characters　is　so　great　that　I　cannot　here　attempt　to　give　anything　approaching　a　complete　treatment　of　them；　but　I　should　like　to　give　a　sufficient　number　of　examples　to　make　the　principle　itself；　in　its　various　modes　of　expression；　quite　clear。

One　of　the　chief　preliminary　postulates　of　sexual　selection　is　the　unequal　number　of　individuals　in　the　two　sexes；　for　if　every　male　immediately　finds　his　mate　there　can　be　no　competition　for　the　possession　of　the　female。　　Darwin　has　shown　that；　for　the　most　part；　the　inequality　between　the　sexes　is　due　simply　to　the　fact　that　there　are　more　males　than　females；　and　therefore　the　males　must　take　some　pains　to　secure　a　mate。　　But　the　inequality　does　not　always　depend　on　the　numerical　preponderance　of　the　males；　it　is　often　due　to　polygamy；　for；　if　one　male　claims　several　females；　the　number　of　females　in　proportion　to　the　rest　of　the　males　will　be　reduced。　　Since　it　is　almost　always　the　males　that　are　the　wooers；　we　must　expect　to　find　the　occurrence　of　secondary　sexual　characters　chiefly　among　them；　and　to　find　it　especially　frequent　in　polygamous　species。　　And　this　is　actually　the　case。

If　we　were　to　try　to　guesswithout　knowing　the　factswhat　means　the　male　animals　make　use　of　to　overcome　their　rivals　in　the　struggle　for　the　possession　of　the　female；　we　might　name　many　kinds　of　means；　but　it　would　be　difficult　to　suggest　any　which　is　not　actually　employed　in　some　animal　group　or　other。　　I　begin　with　the　mere　difference　in　strength；　through　which　the　male　of　many　animals　is　so　sharply　distinguished　from　the　female；　as；　for　instance；　the　lion；　walrus；　〃sea…elephant；〃　and　others。　　Among　these　the　males　fight　violently　for　the　possession　of　the　female；　who　falls　to　the　victor　in　the　combat。　　In　this　simple　case　no　one　can　doubt　the　operation　of　selection；　and　there　is　just　as　little　room　for　doubt　as　to　the　selection…value　of　the　initial　stages　of　the　variation。　　Differences　in　bodily　strength　are　apparent　even　among　human　beings；　although　in　their　case　the　struggle　for　the　possession　of　the　female　is　no　longer　decided　by　bodily　strength　alone。

Combats　between　male　animals　are　often　violent　and　obstinate；　and　the　employment　of　the　natural　weapons　of　the　species　in　this　way　has　led　to　perfecting　of　these；　e。g。　the　tusks　of　the　boar；　the　antlers　of　the　stag；　and　the　enormous；　antler…like　jaws　of　the　stag…beetle。　　Here　again　it　is　impossible　to　doubt　that　variations　in　these　organs　presented　themselves；　and　that　these　were　considerable　enough　to　be　decisive　in　combat；　and　so　to　lead　to　the　improvement　of　the　weapon。

Among　many　animals；　however；　the　females　at　first　withdraw　from　the　males；　they　are　coy；　and　have　to　be　sought　out；　and　sometimes　held　by　force。　　This　tracking　and　grasping　of　the　females　by　the　males　has　given　rise　to　many　different　characters　in　the　latter；　as；　for　instance；　the　larger　eyes　of　the　male　bee；　and　especially　of　the　males　of　the　Ephemerids　（May…flies）；　some　species　of　which　show；　in　addition　to　the　usual　compound　eyes；　large；　so…called　turban…eyes；　so　that　the　whole　head　is　covered　with　seeing　surfaces。　In　these　species　the　females　are　very　greatly　in　the　minority　（1…　100）；　and　it　is　easy　to　understand　that　a　keen　competition　for　them　must　take　place；　and　that；　when　the　insects　of　both　sexes　are　floating　freely　in　the　air；　an　unusually　wide　range　of　vision　will　carry　with　it　a　decided　advantage。　　Here　again　the　actual　adaptations　are　in　accordance　with　the　preliminary　postulates　of　the　theory。　　We　do　not　know　the　stages　through　which　the　eye　has　passed　to　its　present　perfected　state；　but；　since　the　number　of　simple　eyes　（facets）　has　become　very　much　greater　in　the　male　than　in　the　female；　we　may　assume　that　their　increase　is　due　to　a　gradual　duplication　of　the　determinants　of　the　ommatidium　in　the　germ…plasm；　as　I　have　already　indicated　in　regard　to　sense…organs　in　general。　　In　this　case；　again；　the　selection…value　of　the　initial　stages　hardly　admits　of　doubt；　better　vision　DIRECTLY　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