the origin of species(物种起源)-第43章

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uld　be　able　to　range　into　new　pastures　and　thus　gain　a　great　advantage。　It　is　not　that　the　larger　quadrupeds　are　actually　destroyed　（except　in　some　rare　cases）　by　the　flies；　but　they　are　incessantly　harassed　and　their　strength　reduced；　so　that　they　are　more　subject　to　disease；　or　not　so　well　enabled　in　a　coming　dearth　to　search　for　food；　or　to　escape　from　beasts　of　prey。　
Organs　now　of　trifling　importance　have　probably　in　some　cases　been　of　high　importance　to　an　early　progenitor；　and；　after　having　been　slowly　perfected　at　a　former　period；　have　been　transmitted　in　nearly　the　same　state；　although　now　become　of　very　slight　use；　and　any　actually　injurious　deviations　in　their　structure　will　always　have　been　checked　by　natural　selection。　Seeing　how　important　an　organ　of　locomotion　the　tail　is　in　most　aquatic　animals；　its　general　presence　and　use　for　many　purposes　in　so　many　land　animals；　which　in　their　lungs　or　modified　swim…bladders　betray　their　aquatic　origin；　may　perhaps　be　thus　accounted　for。　A　well…developed　tail　having　been　formed　in　an　aquatic　animal；　it　might　subsequently　come　to　be　worked　in　for　all　sorts　of　purposes；　as　a　fly…flapper；　an　organ　of　prehension；　or　as　an　aid　in　turning；　as　with　the　dog；　though　the　aid　must　be　slight；　for　the　hare；　with　hardly　any　tail；　can　double　quickly　enough。　
In　the　second　place；　we　may　sometimes　attribute　importance　to　characters　which　are　really　of　very　little　importance；　and　which　have　originated　from　quite　secondary　causes；　independently　of　natural　selection。　We　should　remember　that　climate；　food；　&c。；　probably　have　some　little　direct　influence　on　the　organisation；　that　characters　reappear　from　the　law　of　reversion；；　that　correlation　of　growth　will　have　had　a　most　important　influence　in　modifying　various　structures；　and　finally；　that　sexual　selection　will　often　have　largely　modified　the　external　characters　of　animals　having　a　will；　to　give　one　male　an　advantage　in　fighting　with　another　or　in　charming　the　females。　Moreover　when　a　modification　of　structure　has　primarily　arisen　from　the　above　or　other　unknown　causes；　it　may　at　first　have　been　of　no　advantage　to　the　species；　but　may　subsequently　have　been　taken　advantage　of　by　the　descendants　of　the　species　under　new　conditions　of　life　and　with　newly　acquired　habits。　
To　give　a　few　instances　to　illustrate　these　latter　remarks。　If　green　woodpeckers　alone　had　existed；　and　we　did　not　know　that　there　were　many　black　and　pied　kinds；　I　dare　say　that　we　should　have　thought　that　the　green　colour　was　a　beautiful　adaptation　to　hide　this　tree…frequenting　bird　from　its　enemies；　and　consequently　that　it　was　a　character　of　importance　and　might　have　been　acquired　through　natural　selection；　as　it　is；　I　have　no　doubt　that　the　colour　is　due　to　some　quite　distinct　cause；　probably　to　sexual　selection。　A　trailing　bamboo　in　the　Malay　Archipelago　climbs　the　loftiest　trees　by　the　aid　of　exquisitely　constructed　hooks　clustered　around　the　ends　of　the　branches；　and　this　contrivance；　no　doubt；　is　of　the　highest　service　to　the　plant；　but　as　we　see　nearly　similar　hooks　on　many　trees　which　are　not　climbers　the　hooks　on　the　bamboo　may　have　arisen　from　unknown　laws　of　growth；　and　have　been　subsequently　taken　advantage　of　by　the　plant　undergoing　further　modification　and　becoming　a　climber。　The　naked　skin　on　the　head　of　a　vulture　is　generally　looked　at　as　a　direct　adaptation　for　wallowing　in　putridity；　and　so　it　may　be；　or　it　may　possibly　be　due　to　the　direct　action　of　putrid　matter；　but　we　should　be　very　cautious　in　drawing　any　such　inference；　when　we　see　that　the　skin　on　the　head　of　the　clean…feeding　male　turkey　is　likewise　naked。　The　sutures　in　the　skulls　of　young　mammals　have　been　advanced　as　a　beautiful　adaptation　for　aiding　parturition；　and　no　doubt　they　facilitate；　or　may　be　indispensable　for　this　act；　but　as　sutures　occur　in　the　skulls　of　young　birds　and　reptiles；　which　have　only　to　escape　from　a　broken　egg；　we　may　infer　that　this　structure　has　arisen　from　the　laws　of　growth；　and　has　been　taken　advantage　of　in　the　parturition　of　the　higher　animals。　
We　are　profoundly　ignorant　of　the　causes　producing　slight　and　unimportant　variations；　and　we　are　immediately　made　conscious　of　this　by　reflecting　on　the　differences　in　the　breeds　of　our　domesticated　animals　in　different　countries；　more　especially　in　the　less　civilized　countries　where　there　has　been　but　little　artificial　selection。　Careful　observers　are　convinced　that　a　damp　climate　affects　the　growth　of　the　hair；　and　that　with　the　hair　the　horns　are　correlated。　Mountain　breeds　always　differ　from　lowland　breeds；　and　a　mountainous　country　would　probably　affect　the　hind　limbs　from　exercising　them　more；　and　possibly　even　the　form　of　the　pelvis；　and　then　by　the　law　of　homologous　variation；　the　front　limbs　and　even　the　head　would　probably　be　affected。　The　shape；　also；　of　the　pelvis　might　affect　by　pressure　the　shape　of　the　head　of　the　young　in　the　womb。　The　laborious　breathing　necessary　in　high　regions　would；　we　have　some　reason　to　believe；　increase　the　size　of　the　chest；　and　again　correlation　would　come　into　play。　Animals　kept　by　savages　in　different　countries　often　have　to　struggle　for　their　own　subsistence；　and　would　be　exposed　to　a　certain　extent　to　natural　selection；　and　individuals　with　slightly　different　constitutions　would　succeed　best　under　different　climates；　and　there　is　reason　to　believe　that　constitution　and　colour　are　correlated。　A　good　observer；　also；　states　that　in　cattle　susceptibility　to　the　attacks　of　flies　is　correlated　with　colour；　as　is　the　liability　to　be　poisoned　by　certain　plants；　so　that　colour　would　be　thus　subjected　to　the　action　of　natural　selection。　But　we　are　far　too　ignorant　to　speculate　on　the　relative　importance　of　the　several　known　and　unknown　laws　of　variation；　and　I　have　here　alluded　to　them　only　to　show　that；　if　we　are　unable　to　account　for　the　characteristic　differences　of　our　domestic　breeds；　which　nevertheless　we　generally　admit　to　have　arisen　through　ordinary　generation；　we　ought　not　to　lay　too　much　stress　on　our　ignorance　of　the　precise　cause　of　the　slight　analogous　differences　between　species。　I　might　have　adduced　for　this　same　purpose　the　differences　between　the　races　of　man；　which　are　so　strongly　marked；　I　may　add　that　some　little　light　can　apparently　be　thrown　on　the　origin　of　these　differences；　chiefly　through　sexual　selection　of　a　particular　kind；　but　without　here　entering　on　copious　details　my　reasoning　would　appear　frivolous。　
The　foregoing　remarks　lead　me　to　say　a　few　words　on　the　protest　lately　made　by　some　naturalists；　against　the　utilitarian　doctrine　that　every　detail　of　structure　has　been　produced　for　the　good　of　its　possessor。　They　believe　that　very　many　structures　have　been　created　for　beauty　in　the　eyes　of　man；　or　for　mere　variety。　This　doctrine；　if　true；　would　be　absolutely　fatal　to　my　theory。　Yet　I　fully　admit　that　many　structures　are　of　no　direct　use　to　their　possessors。　Physical　conditions　probably　have　had　some　little　effect　on　structure；　quite　independently　of　any　good　thus　gained。　Correlation　of　growth　has　no　doubt　played　a　most　important　part；　and　a　useful　modification　of　one　part　will　often　have　entailed　on　other　parts　diversified　changes　of　no　direct　use。　So　again　characters　which　formerly　were　useful；　or　which　formerly　had　arisen　from　correlation　of　growth；　or　from　other　unknown　cause；　may　reappear　from　the　law　of　reversion；　though　now　of　no　direct　use。　The　effects　of　sexual　selection；　when　displayed　in　beauty　to　charm　the　females；　can　be　called　useful　only　in　rather　a　forced　sense。　But　by　far　the　most　important　consideration　is　that　the　chief　part　of　the　organisation　of　every　being　is　simply　due　to　inheritance；　and　consequently；　though　each　being　assuredly　is　well　fitted　for　its　place　in　nature；　many　structures　now　have　no　direct　relation　to　the　habits　of　life　of　each　species。　Thus；　we　can　hardly　believe　that　the　webbed　feet　of　the　upland　goose　or　of　the　frigate…bird　are　of　special　use　to　these　birds；　we　cannot　believe　that　the　same　bones　in　the　arm　of　the　monkey；　in　the　fore　leg　of　the　horse；　in　the　wing　of　the　bat；　and　in　the　flipper　of　the　seal；　are　of　special　use　to　these　animals。　We　may　safely　attribute　these　structures　to　inheritance。　But　to　the　progenit