the origin of species(物种起源)-第42节

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er　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　nest　and　sit　on　her　own　eggs；　those　first　laid　would　have　to　be　left　for　some　time　unincubated；　or　there　would　be　eggs　and　young　birds　of　different　ages　in　the　same　nest。　If　this　were　the　case；　the　process　of　laying　and　hatching　might　be　inconveniently　long；　more　especially　as　she　has　to　migrate　at　a　very　early　period；　and　the　first　hatched　young　would　probably　have　to　be　fed　by　the　male　alone。　But　the　American　cuckoo　is　in　this　predicament；　for　she　makes　her　own　nest　and　has　eggs　and　young　successively　hatched；　all　at　the　same　time。　It　has　been　asserted　that　the　American　cuckoo　occasionally　lays　her　eggs　in　other　birds'　nests；　but　I　hear　on　the　high　authority　of　Dr。　Brewer；　that　this　is　a　mistake。　Nevertheless；　I　could　give　several　instances　of　various　birds　which　have　been　known　occasionally　to　lay　their　eggs　in　other　birds'　nests。　Now　let　us　suppose　that　the　ancient　progenitor　of　our　European　cuckoo　had　the　habits　of　the　American　cuckoo；　but　that　occasionally　she　laid　an　egg　in　another　bird's　nest。　If　the　old　bird　profited　by　this　occasional　habit；　or　if　the　young　were　made　more　vigorous　by　advantage　having　been　taken　of　the　mistaken　maternal　instinct　of　another　bird；　than　by　their　own　mother's　care；　encumbered　as　she　can　hardly　fail　to　be　by　having　eggs　and　young　of　different　ages　at　the　same　time；　then　the　old　birds　or　the　fostered　young　would　gain　an　advantage。　And　analogy　would　lead　me　to　believe；　that　the　young　thus　reared　would　be　apt　to　follow　by　inheritance　the　occasional　and　aberrant　habit　of　their　mother；　and　in　their　turn　would　be　apt　to　lay　their　eggs　in　other　birds'　nests；　and　thus　be　successful　in　rearing　their　young。　By　a　continued　process　of　this　nature；　I　believe　that　the　strange　instinct　of　our　cuckoo　could　be；　and　has　been；　generated。　I　may　add　that；　according　to　Dr。　Gray　and　to　some　other　observers；　the　European　cuckoo　has　not　utterly　lost　all　maternal　love　and　care　for　her　own　offspring。　
The　occasional　habit　of　birds　laying　their　eggs　in　other　birds'　nests；　either　of　the　same　or　of　a　distinct　species；　is　not　very　uncommon　with　the　Gallinaceae；　and　this　perhaps　explains　the　origin　of　a　singular　instinct　in　the　allied　group　of　ostriches。　For　several　hen　ostriches；　at　least　in　the　case　of　the　American　species；　unite　and　lay　first　a　few　eggs　in　one　nest　and　then　in　another；　and　these　are　hatched　by　the　males。　This　instinct　may　probably　be　accounted　for　by　the　fact　of　the　hens　laying　a　large　number　of　eggs；　but；　as　in　the　case　of　the　cuckoo；　at　intervals　of　two　or　three　days。　This　instinct；　however；　of　the　American　ostrich　has　not　as　yet　been　perfected；　for　a　surprising　number　of　eggs　lie　strewed　over　the　plains；　so　that　in　one　day's　hunting　I　picked　up　no　less　than　twenty　lost　and　wasted　eggs。　
Many　bees　are　parasitic；　and　always　lay　their　eggs　in　the　nests　of　bees　of　other　kinds。　This　case　is　more　remarkable　than　that　of　the　cuckoo；　for　these　bees　have　not　only　their　instincts　but　their　structure　modified　in　accordance　with　their　parasitic　habits；　for　they　do　not　possess　the　pollen…collecting　apparatus　which　would　be　necessary　if　they　had　to　store　food　for　their　own　young。　Some　species；　likewise；　of　Sphegidae　（wasp…like　insects）　are　parasitic　on　other　species；　and　M。　Fabre　has　lately　shown　good　reason　for　believing　that　although　the　Tachytes　nigra　generally　makes　its　own　burrow　and　stores　it　with　paralysed　prey　for　its　own　larvae　to　feed　on；　yet　that　when　this　insect　finds　a　burrow　already　made　and　stored　by　another　sphex；　it　takes　advantage　of　the　prize；　and　becomes　for　the　occasion　parasitic。　In　this　case；　as　with　the　supposed　case　of　the　cuckoo；　I　can　see　no　difficulty　in　natural　selection　making　an　occasional　habit　permanent；　if　of　advantage　to　the　species；　and　if　the　insect　whose　nest　and　stored　food　are　thus　feloniously　appropriated；　be　not　thus　exterminated。　
Slave…making　instinct。　This　remarkable　instinct　was　first　discovered　in　the　Formica　（Polyerges）　rufescens　by　Pierre　Huber；　a　better　observer　even　than　his　celebrated　father。　This　ant　is　absolutely　dependent　on　its　slaves；　without　their