the origin of species(物种起源)-第41节

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　wonderful　instincts　with　which　we　are　acquainted；　namely；　those　of　the　hive…bee　and　of　many　ants；　could　not　possibly　have　been　thus　acquired。　
It　will　be　universally　admitted　that　instincts　are　as　important　as　corporeal　structure　for　the　welfare　of　each　species；　under　its　present　conditions　of　life。　Under　changed　conditions　of　life；　it　is　at　least　possible　that　slight　modifications　of　instinct　might　be　profitable　to　a　species；　and　if　it　can　be　shown　that　instincts　do　vary　ever　so　little；　then　I　can　see　no　difficulty　in　natural　selection　preserving　and　continually　accumulating　variations　of　instinct　to　any　extent　that　may　be　profitable。　It　is　thus；　as　I　believe；　that　all　the　most　complex　and　wonderful　instincts　have　originated。　As　modifications　of　corporeal　structure　arise　from；　and　are　increased　by；　use　or　habit；　and　are　diminished　or　lost　by　disuse；　so　I　do　not　doubt　it　has　been　with　instincts。　But　I　believe　that　the　effects　of　habit　are　of　quite　subordinate　importance　to　the　effects　of　the　natural　selection　of　what　may　be　called　accidental　variations　of　instincts；　that　is　of　variations　produced　by　the　same　unknown　causes　which　produce　slight　deviations　of　bodily　structure。　
No　complex　instinct　can　possibly　be　produced　through　natural　selection；　except　by　the　slow　and　gradual　accumulation　of　numerous；　slight；　yet　profitable；　variations。　Hence；　as　in　the　case　of　corporeal　structures；　we　ought　to　find　in　nature；　not　the　actual　transitional　gradations　by　which　each　complex　instinct　has　been　acquired　for　these　could　be　found　only　in　the　lineal　ancestors　of　each　species　but　we　ought　to　find　in　the　collateral　lines　of　descent　some　evidence　of　such　gradations；　or　we　ought　at　least　to　be　able　to　show　that　gradations　of　some　kind　are　possible；　and　this　we　certainly　can　do。　I　have　been　surprised　to　find；　making　allowance　for　the　instincts　of　animals　having　been　but　little　observed　except　in　Europe　and　North　America；　and　for　no　instinct　being　known　amongst　extinct　species；　how　very　generally　gradations；　leading　to　the　most　complex　instincts；　can　be　discovered。　The　canon　of　'Natura　non　facit　saltum'　applies　with　almost　equal　force　to　instincts　as　to　bodily　organs。　Changes　of　instinct　may　sometimes　be　facilitated　by　the　same　species　having　different　instincts　at　different　periods　of　life；　or　at　different　seasons　of　the　year；　or　when　placed　under　different　circumstances；　&c。；　in　which　case　either　one　or　the　other　instinct　might　be　preserved　by　natural　selection。　And　such　instances　of　diversity　of　instinct　in　the　same　species　can　be　shown　to　occur　in　nature。　
Again　as　in　the　case　of　corporeal　structure；　and　conformably　with　my　theory；　the　instinct　of　each　species　is　good　for　itself；　but　has　never；　as　far　as　we　can　judge；　been　produced　for　the　exclusive　good　of　others。　One　of　the　strongest　instances　of　an　animal　apparently　performing　an　action　for　the　sole　good　of　another；　with　which　I　am　acquainted；　is　that　of　aphides　voluntarily　yielding　their　sweet　excretion　to　ants：　that　they　do　so　voluntarily；　the　following　facts　show。　I　removed　all　the　ants　from　a　group　of　about　a　dozen　aphides　on　a　dock…plant；　and　prevented　their　attendance　during　several　hours。　After　this　interval；　I　felt　sure　that　the　aphides　would　want　to　excrete。　I　watched　them　for　some　time　through　a　lens；　but　not　one　excreted；　I　then　tickled　and　stroked　them　with　a　hair　in　the　same　manner；　as　well　as　I　could；　as　the　ants　do　with　their　antennae；　but　not　one　excreted。　Afterwards　I　allowed　an　ant　to　visit　them；　and　it　immediately　seemed；　by　its　eager　way　of　running　about；　to　be　well　aware　what　a　rich　flock　it　had　discovered；　it　then　began　to　play　with　its　antennae　on　the　abdomen　first　of　one　aphis　and　then　of　another；　and　each　aphis；　as　soon　as　it　felt　the　antennae；　immediately　lifted　up　its　abdomen　and　excreted　a　limpid　drop　of　sweet　juice；　which　was　eagerly　devoured　by　the　ant。　Even　the　quite　young　aphides　behaved　in　this　manner；　showing　that　the　action　was　instinctive；　and　not　the　result　of　experience。　But　as　the　excretion　is　extremely　viscid；　it　is　probably　a　convenience　to　the　aphides　to　have　it　removed；　and　therefore　probably　the　aphides　do　not　instinctively　excrete　for　the　sole　good　of　the　ants。　Although　I　do　not　believe　that　any　animal　in　the　world　performs　an　action　for　the　exclusive　good　of　another　of　a　distinct　species；　yet　each　species　tries　to　take　advantage　of　the　instincts　of　others；　as　each　takes　advantage　of　the　weaker　bodily　structure　of　others。　So　again；　in　some　few　cases；　certain　instincts　cannot　be　considered　as　absolutely　perfect；　but　as　details　on　this　and　other　such　points　are　not　indispensable；　they　may　be　here　passed　over。　
As　some　degree　of　variation　in　instincts　under　a　state　of　nature；　and　the　inheritance　of　such　variations；　are　indispensable　for　the　action　of　natural　selection；　as　many　instances　as　possible　ought　to　have　been　here　given；　but　want　of　space　prevents　me。　I　can　only　assert；　that　instincts　certainly　do　vary　for　instance；　the　migratory　instinct；　both　in　extent　and　direction；　and　in　its　total　loss。　So　it　is　with　the　nests　of　birds；　which　vary　partly　in　dependence　on　the　situations　chosen；　and　on　the　nature　and　temperature　of　the　country　inhabited；　but　often　from　causes　wholly　unknown　to　us：　Audubon　has　given　several　remarkable　cases　of　differences　in　nests　of　the　same　species　in　the　northern　and　southern　United　States。　Fear　of　any　particular　enemy　is　certainly　an　instinctive　quality；　as　may　be　seen　in　nestling　birds；　though　it　is　strengthened　by　experience；　and　by　the　sight　of　fear　of　the　same　enemy　in　other　animals。　But　fear　of　man　is　slowly　acquired；　as　I　have　elsewhere　shown；　by　various　animals　inhabiting　desert　islands；　and　we　may　see　an　instance　of　this；　even　in　England；　in　the　greater　wildness　of　all　our　large　birds　than　of　our　small　birds；　for　the　large　birds　have　been　most　persecuted　by　man。　We　may　safely　attribute　the　greater　wildness　of　our　large　birds　to　this　cause；　for　in　uninhabited　islands　large　birds　are　not　more　fearful　than　small；　and　the　magpie；　so　wary　in　England；　is　tame　in　Norway；　as　is　the　hooded　crow　in　Egypt。　
That　the　general　disposition　of　individuals　of　the　same　species；　born　in　a　state　of　nature；　is　extremely　diversified；　can　be　shown　by　a　multitude　of　facts。　Several　cases　also；　could　be　given；　of　occasional　and　strange　habits　in　certain　species；　which　might；　if　advantageous　to　the　species；　give　rise；　through　natural　selection；　to　quite　new　instincts。　But　I　am　well　aware　that　these　general　statements；　without　facts　given　in　detail；　can　produce　but　a　feeble　effect　on　the　reader's　mind。　I　can　only　repeat　my　assurance；　that　I　do　not　speak　without　good　evidence。　
The　possibility；　or　even　probability；　of　inherited　variations　of　instinct　in　a　state　of　nature　will　be　strengthened　by　briefly　considering　a　few　cases　under　domestication。　We　shall　thus　also　be　enabled　to　see　the　respective　parts　which　habit　and　the　selection　of　so…called　accidental　variations　have　played　in　modifying　the　mental　qualities　of　our　domestic　animals。　A　number　of　curious　and　authentic　instances　could　be　given　of　the　inheritance　of　all　shades　of　disposition　and　tastes；　and　likewise　of　the　oddest　tricks；　associated　with　certain　frames　of　mind　or　periods　of　time。　But　let　us　look　to　the　familiar　case　of　the　several　breeds　of　dogs：　it　cannot　be　doubted　that　young　pointers　（I　have　myself　seen　a　striking　instance）　will　sometimes　point　and　even　back　other　dogs　the　very　first　time　that　they　are　taken　out；　retrieving　is　certainly　in　some　degree　inherited　by　retrievers；　and　a　tendency　to　run　round；　instead　of　at；　a　flock　of　sheep；　by　shepherd…dogs。　I　cannot　see　that　these　actions；　performed　without　experience　by　the　young；　and　in　nearly　the　same　manner　by　each　individual；　performed　with　eager　delight　by　each　breed；　and　without　the　end　being　known；　for　the　young　pointer　can　no　more　know　that　he　points　to　aid　his　master；　than　the　white　butterfly　knows　why　she　lays　her　eggs　on　the　leaf　of　the　cabbage；　I　cannot　see　that　these　actions　differ　essentially　from　true　instincts。　If　we　were　to　see　one　kind　of　wolf；　when　young　and　without　any　training；　as　soon　as　it　scented　its　prey；　stand　motionless　like　a　statue；　and　then　slowly　crawl　forward　with　a　peculiar　gait；　and　another　kind　of　wolf　rushing　round；　instead　of　at；　a　herd　of　deer；　and　driving　them　to　a　distant　point；　we　should　assuredly　call　these　actions　instinctive。　Domestic　instincts；　as　they　may　be　called；　are　certify　far　less　fixed　or　invariable　than　natural　instincts；　but　they　have　been　acted　on　by　far　less　rigorous　selection；　and　have　been　transmitted　for　an　incomparably　shorter　period；　under　less　fixed　conditions　of　life。　
How　strongly　these　domestic　instincts；　habits；　and　dispositions　are　inherited；　and　how　curiously　they　become　mingled；　is　well　shown　when　different　breeds　of　dogs　are　crossed。　Thus　it　is　known　that　a　cross　with　a　bull…dog　has　affected　for　many　generations　the　courage　and　obstinacy　of　greyhounds；　and　a　cross　with　a　greyhound　has　given　to　a　whole　family　of　shepherd…dogs　a　tendency　to　hunt　hares。　These　domestic　instincts；　when　thus　tested　by　crossing；　resemble　natural　instincts；　which　in　a　like　manner　become　curiously　blended　together；　and　for　a　long　period　exhibit　traces　of　the　instincts　of　eithe