the origin of species(物种起源)-第35章

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s　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　aid；　the　species　would　certainly　become　extinct　in　a　single　year。　The　males　and　fertile　females　do　no　work。　The　workers　or　sterile　females；　though　most　energetic　and　courageous　in　capturing　slaves；　do　no　other　work。　They　are　incapable　of　making　their　own　nests；　or　of　feeding　their　own　larvae。　When　the　old　nest　is　found　inconvenient；　and　they　have　to　migrate；　it　is　the　slaves　which　determine　the　migration；　and　actually　carry　their　masters　in　their　jaws。　So　utterly　helpless　are　the　masters；　that　when　Huber　shut　up　thirty　of　them　without　a　slave；　but　with　plenty　of　the　food　which　they　like　best；　and　with　their　larvae　and　pupae　to　stimulate　them　to　work；　they　did　nothing；　they　could　not　even　feed　themselves；　and　many　perished　of　hunger。　Huber　then　introduced　a　single　slave　（F。　fusca）；　and　she　instantly　set　to　work；　fed　and　saved　the　survivors；　made　some　cells　and　tended　the　larvae；　and　put　all　to　rights。　What　can　be　more　extraordinary　than　these　well…ascertained　facts？　If　we　had　not　known　of　any　other　slave…making　ant；　it　would　have　been　hopeless　to　have　speculated　how　so　wonderful　an　instinct　could　have　been　perfected。　
Formica　sanguinea　was　likewise　first　discovered　by　P。　Huber　to　be　a　slave…making　ant。　This　species　is　found　in　the　southern　parts　of　England；　and　its　habits　have　been　attended　to　by　Mr。　F。　Smith；　of　the　British　Museum；　to　whom　I　am　much　indebted　for　information　on　this　and　other　subjects。　Although　fully　trusting　to　the　statements　of　Huber　and　Mr。　Smith；　I　tried　to　approach　the　subject　in　a　sceptical　frame　of　mind；　as　any　one　may　well　be　excused　for　doubting　the　truth　of　so　extraordinary　and　odious　an　instinct　as　that　of　making　slaves。　Hence　I　will　give　the　observations　which　I　have　made　myself　made；　in　some　little　detail。　I　opened　fourteen　nests　of　F。　sanguinea；　and　found　a　few　slaves　in　all。　Males　and　fertile　females　of　the　slave…species　are　found　only　in　their　own　proper　communities；　and　have　never　been　observed　in　the　nests　of　F。　sanguinea。　The　slaves　are　black　and　not　above　half　the　size　of　their　red　masters；　so　that　the　contrast　in　their　appearance　is　very　great。　When　the　nest　is　slightly　disturbed；　the　slaves　occasionally　come　out；　and　like　their　masters　are　much　agitated　and　defend　their　nest：　when　the　nest　is　much　disturbed　and　the　larvae　and　pupae　are　exposed；　the　slaves　work　energetically　with　their　masters　in　carrying　them　away　to　a　place　of　safety。　Hence；　it　is　clear；　that　the　slaves　feel　quite　at　home。　During　the　months　of　June　and　July；　on　three　successive　years；　I　have　watched　for　many　hours　several　nests　in　Surrey　and　Sussex；　and　never　saw　a　slave　either　leave　or　enter　a　nest。　As；　during　these　months；　the　slaves　are　very　few　in　number；　I　thought　that　they　might　behave　differently　when　more　numerous；　but　Mr。　Smith　informs　me　that　he　has　watched　the　nests　at　various　hours　during　May；　June　and　August；　both　in　Surrey　and　Hampshire；　and　has　never　seen　the　slaves；　though　present　in　large　numbers　in　August；　either　leave　or　enter　the　nest。　Hence　he　considers　them　as　strictly　household　slaves。　The　masters；　on　the　other　hand；　may　be　constantly　seen　bringing　in　materials　for　the　nest；　and　food　of　all　kinds。　During　the　present　year；　however；　in　the　month　of　July；　I　came　across　a　community　with　an　unusually　large　stock　of　slaves；　and　I　observed　a　few　slaves　mingled　with　their　masters　leaving　the　nest；　and　marching　along　the　same　road　to　a　tall　Scotch…fir…tree；　twenty…five　yards　distant；　which　they　ascended　together；　probably　in　search　of　aphides　or　cocci。　According　to　Huber；　who　had　ample　opportunities　for　observation；　in　Switzerland　the　slaves　habitually　work　with　their　masters　in　making　the　nest；　and　they　alone　open　and　close　the　doors　in　the　morning　and　evening；　and；　as　Huber　expressly　states；　their　principal　office　is　to　search　for　aphides。　This　difference　in　the　usual　habits　of　the　masters　and　slaves　in　the　two　countries；　probably　depends　merely　on　the　slaves　being　captured　in　greater　numbers　in　Switzerland　than　in　England。　
One　day　I　fortunately　chanced　to　witness　a　migration　from　one　nest　to　another；　and　it　was　a　most　interesting　spectacle　to　behold　the　masters　carefully　carrying；　as　Huber　has　described；　their　slaves　in　their　jaws。　Another　day　my　attention　was　struck　by　about　a　score　of　the　slave…makers　haunting　the　same　spot；　and　evidently　not　in　search　of　food；　they　approached　and　were　vigorously　repulsed　by　an　independent　community　of　the　slave　species　（F。　fusca）；　sometimes　as　many　as　three　of　these　ants　clinging　to　the　legs　of　the　slave…making　F。　sanguinea。　The　latter　ruthlessly　killed　their　small　opponents；　and　carried　their　dead　bodies　as　food　to　their　nest；　twenty…nine　yards　distant；　but　they　were　prevented　from　getting　any　pupae　to　rear　as　slaves。　I　then　dug　up　a　small　parcel　of　the　pupae　of　F。　fusca　from　another　nest；　and　put　them　down　on　a　bare　spot　near　the　place　of　combat；　they　were　eagerly　seized；　and　carried　off　by　the　tyrants；　who　perhaps　fancied　that；　after　all；　they　had　been　victorious　in　their　late　combat。　
At　the　same　time　I　laid　on　the　same　place　a　small　parcel　of　the　pupae　of　another　species；　F。　flava；　with　a　few　of　these　little　yellow　ants　still　clinging　to　the　fragments　of　the　nest。　This　species　is　sometimes；　though　rarely；　made　into　slaves；　as　has　been　described　by　Mr　Smith。　Although　so　small　a　species；　it　is　very　courageous；　and　I　have　seen　it　ferociously　attack　other　ants。　In　one　instance　I　found　to　my　surprise　an　independent　community　of　F。　flava　under　a　stone　beneath　a　nest　of　the　slave…making　F。　sanguinea；　and　when　I　had　accidentally　disturbed　both　nests；　the　little　ants　attacked　their　big　neighbours　with　surprising　courage。　Now　I　was　curious　to　ascertain　whether　F。　sanguinea　could　distinguish　the　pupae　of　F。　fusca；　which　they　habitually　make　into　slaves；　from　those　of　the　little　and　furious　F。　flava；　which　they　rarely　capture；　and　it　was　evident　that　they　did　at　once　distinguish　them：　for　we　have　seen　that　they　eagerly　and　instantly　seized　the　pupae　of　F。　fusca；　whereas　they　were　much　terrified　when　they　came　across　the　pupae；　or　even　the　earth　from　the　nest　of　F。　flava；　and　quickly　ran　away；　but　in　about　a　quarter　of　an　hour；　shortly　after　all　the　little　yellow　ants　had　crawled　away；　they　took　heart　and　carried　off　the　pupae。　
One　evening　I　visited　another　community　of　F。　sanguinea；　and　found　a　number　of　these　ants　entering　their　nest；　carrying　the　dead　bodies　of　F。　fusca　（showing　that　it　was　not　a　migration）　and　numerous　pupae。　I　traced　the　returning　file　burthened　with　booty；　for　about　forty　yards；　to　a　very　thick　clump　of　heath。　whence　I　saw　the　last　individual　of　F。　sanguinea　emerge；　carrying　a　pupa；　but　I　was　not　able　to　find　the　desolated　nest　in　the　thick　heath。　The　nest；　however；　must　have　been　close　at　hand；　for　two　or　three　individuals　of　F。　fusca　were　rushing　about　in　the　greatest　agitation；　and　one　was　perched　motionless　with　its　own　pupa　in　its　mouth　on　the　top　of　a　spray　of　heath　over　its　ravaged　home。　
Such　are　the　facts；　though　they　did　not　need　confirmation　by　me；　in　regard　to　the　wonderful　instinct　of　making　slaves。　Let　it　be　observed　what　a　contrast　the　i