the origin of species(物种起源)-第34章

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！


emoved　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　either　parent：　for　example；　Le　Roy　describes　a　dog；　whose　great…grandfather　was　a　wolf；　and　this　dog　showed　a　trace　of　its　wild　parentage　only　in　one　way；　by　not　coming　in　a　straight　line　to　his　master　when　called。　
Domestic　instincts　are　sometimes　spoken　of　as　actions　which　have　become　inherited　solely　from　long…continued　and　compulsory　habit；　but　this；　I　think；　is　not　true。　No　one　would　ever　have　thought　of　teaching；　or　probably　could　have　taught；　the　tumbler…pigeon　to　tumble；　an　action　which；　as　I　have　witnessed；　is　performed　by　young　birds；　that　have　never　seen　a　pigeon　tumble。　We　may　believe　that　some　one　pigeon　showed　a　slight　tendency　to　this　strange　habit；　and　that　the　long…continued　selection　of　the　best　individuals　in　successive　generations　made　tumblers　what　they　now　are；　and　near　Glasgow　there　are　house…tumblers；　as　I　hear　from　Mr　Brent；　which　cannot　fly　eighteen　inches　high　without　going　head　over　heels。　It　may　be　doubted　whether　any　one　would　have　thought　of　training　a　dog　to　point；　had　not　some　one　dog　naturally　shown　a　tendency　in　this　line；　and　this　is　known　occasionally　to　happen；　as　I　once　saw　in　a　pure　terrier。　When　the　first　tendency　was　once　displayed；　methodical　selection　and　the　inherited　effects　of　compulsory　training　in　each　successive　generation　would　soon　complete　the　work；　and　unconscious　selection　is　still　at　work；　as　each　man　tries　to　procure；　without　intending　to　improve　the　breed；　dogs　which　will　stand　and　hunt　best。　On　the　other　hand；　habit　alone　in　some　cases　has　sufficed；　no　animal　is　more　difficult　to　tame　than　the　young　of　the　wild　rabbit；　scarcely　any　animal　is　tamer　than　the　young　of　the　tame　rabbit；　but　I　do　not　suppose　that　domestic　rabbits　have　ever　been　selected　for　tameness；　and　I　presume　that　we　must　attribute　the　whole　of　the　inherited　change　from　extreme　wildness　to　extreme　tameness；　simply　to　habit　and　long…continued　close　confinement。　
Natural　instincts　are　lost　under　domestication：　a　remarkable　instance　of　this　is　seen　in　those　breeds　of　fowls　which　very　rarely　or　never　become　'broody；'　that　is；　never　wish　to　sit　on　their　eggs。　Familiarity　alone　prevents　our　seeing　how　universally　and　largely　the　minds　of　our　domestic　animals　have　been　modified　by　domestication。　It　is　scarcely　possible　to　doubt　that　the　love　of　man　has　become　instinctive　in　the　dog。　All　wolves；　foxes；　jackals；　and　species　of　the　cat　genus；　when　kept　tame；　are　most　eager　to　attack　poultry；　sheep；　and　pigs；　and　this　tendency　has　been　found　incurable　in　dogs　which　have　been　brought　home　as　puppies　from　countries；　such　as　Tierra　del　Fuego　and　Australia；　where　the　savages　do　not　keep　these　domestic　animals。　How　rarely；　on　the　other　hand；　do　our　civilised　dogs；　even　when　quite　young；　require　to　be　taught　not　to　attack　poultry；　sheep；　and　pigs！　No　doubt　they　occasionally　do　make　an　attack；　and　are　then　beaten；　and　if　not　cured；　they　are　destroyed；　so　that　habit；　with　some　degree　of　selection；　has　probably　concurred　in　civilising　by　inheritance　our　dogs。　On　the　other　hand；　young　chickens　have　lost；　wholly　by　habit；　that　fear　of　the　dog　and　cat　which　no　doubt　was　originally　instinctive　in　them；　in　the　same　way　as　it　is　so　plainly　instinctive　in　young　pheasants；　though　reared　under　a　hen。　It　is　not　that　chickens　have　lost　all　fear；　but　fear　only　of　dogs　and　cats；　for　if　the　hen　gives　the　danger…chuckle；　they　will　run　（more　especially　young　turkeys）　from　under　her；　and　conceal　themselves　in　the　surrounding　grass　or　thickets；　and　this　is　evidently　done　for　the　instinctive　purpose　of　allowing；　as　we　see　in　wild　ground…birds；　their　mother　to　fly　away。　But　this　instinct　retained　by　our　chickens　has　become　useless　under　domestication；　for　the　mother…hen　has　almost　lost　by　disuse　the　power　of　flight。　
Hence；　we　may　conclude；　that　domestic　instincts　have　been　acquired　and　natural　instincts　have　been　lost　partly　by　habit；　and　partly　by　man　selecting　and　accumulating　during　successive　generations；　peculiar　mental　habits　and　actions；　which　at　first　appeared　from　what　we　must　in　our　ignorance　call　an　accident。　In　some　cases　compulsory　habit　alone　has　sufficed　to　produce　such　inherited　mental　changes；　in　other　cases　compulsory　habit　has　done　nothing；　and　all　has　been　the　result　of　selection；　pursued　both　methodically　and　unconsciously；　but　in　most　cases；　probably；　habit　and　selection　have　acted　together。　
We　shall；　perhaps；　best　understand　how　instincts　in　a　state　of　nature　have　become　modified　by　selection；　by　considering　a　few　cases。　I　will　select　only　three；　out　of　the　several　which　I　shall　have　to　discuss　in　my　future　work；　namely；　the　instinct　which　leads　the　cuckoo　to　lay　her　eggs　in　other　birds'　nests；　the　slave…making　instinct　of　certain　ants；　and　the　comb…making　power　of　the　hive…bee：　these　two　latter　instincts　have　generally；　and　most　justly；　been　ranked　by　naturalists　as　the　most　wonderful　of　all　known　instincts。　
It　is　now　commonly　admitted　that　the　more　immediate　and　final　cause　of　the　cuckoo's　instinct　is；　that　she　lays　her　eggs；　not　daily；　but　at　intervals　of　two　or　three　days；　so　that；　if　she　were　to　make　her　own　n