te　unknown：　in　the　Mexican　Myrmecocystus；　the　workers　of　one　caste　never　leave　the　nest；　they　are　fed　by　the　workers　of　another　caste；　and　they　have　an　enormously　developed　abdomen　which　secretes　a　sort　of　honey；　supplying　the　place　of　that　excreted　by　the　aphides；　or　the　domestic　cattle　as　they　may　be　called；　which　our　European　ants　guard　or　imprison。
  It　will　indeed　be　thought　that　I　have　an　overweening　confidence　in　the　principle　of　natural　selection；　when　I　do　not　admit　that　such　wonderful　and　well…established　facts　at　once　annihilate　my　theory。　In　the　simpler　case　of　neuter　insects　all　of　one　caste　or　of　the　same　kind；　which　have　been　rendered　by　natural　selection；　as　I　believe　to　be　quite　possible；　different　from　the　fertile　males　and　females；　in　this　case；　we　may　safely　conclude　from　the　analogy　of　ordinary　variations；　that　each　successive；　slight；　profitable　modification　did　not　probably　at　first　appear　in　all　the　individual　neuters　in　the　same　nest；　but　in　a　few　alone；　and　that　by　the　long…continued　selection　of　the　fertile　parents　which　produced　most　neuters　with　the　profitable　modification；　all　the　neuters　ultimately　came　to　have　the　desired　character。　On　this　view　we　ought　occasionally　to　find　neuter…insects　of　the　same　species；　in　the　same　nest；　presenting　gradations　of　structure；　and　this　we　do　find；　even　often；　considering　how　few　neuter…insects　out　of　Europe　have　been　carefully　examined。　Mr　F。　Smith　has　shown　how　surprisingly　the　neuters　of　several　British　ants　differ　from　each　other　in　size　and　sometimes　in　colour；　and　that　the　extreme　forms　can　sometimes　be　perfectly　linked　together　by　individuals　taken　out　of　the　same　nest：　I　have　myself　compared　perfect　gradations　of　this　kind。　It　often　happens　that　the　larger　or　the　smaller　sized　workers　are　the　most　numerous；　or　that　both　large　and　small　are　numerous；　with　those　of　an　intermediate　size　scanty　in　numbers。　Formica　flava　has　larger　and　smaller　workers；　with　some　of　intermediate　size；　and；　in　this　species；　as　Mr　F。　Smith　has　observed；　the　larger　workers　have　simple　eyes　（ocelli）；　which　though　small　can　be　plainly　distinguished；　whereas　the　smaller　workers　have　their　ocelli　rudimentary。　Having　carefully　dissected　several　specimens　of　these　workers；　I　can　affirm　that　the　eyes　are　far　more　rudimentary　in　the　smaller　workers　than　can　be　accounted　for　merely　by　their　proportionally　lesser　size；　and　I　fully　believe；　though　I　dare　not　assert　so　positively；　that　the　workers　of　intermediate　size　have　their　ocelli　in　an　exactly　intermediate　condition。　So　that　we　here　have　two　bodies　of　sterile　workers　in　the　same　nest；　differing　not　only　in　size；　but　in　their　organs　of　vision；　yet　connected　by　some　few　members　in　an　intermediate　condition。　I　may　digress　by　adding；　that　if　the　smaller　workers　had　been　the　most　useful　to　the　community；　and　those　males　and　females　had　been　continually　selected；　which　produced　more　and　more　of　the　smaller　workers；　until　all　the　workers　had　come　to　be　in　this　condition；　we　should　then　have　had　a　species　of　ant　with　neuters　very　nearly　in　the　same　condition　with　those　of　Myrmica。　For　the　workers　of　Myrmica　have　not　even　rudiments　of　ocelli；　though　the　male　and　female　ants　of　this　genus　have　well…developed　ocelli。
  I　may　give　one　other　case：　so　confidently　did　I　expect　to　find　gradations　in　important　points　of　structure　between　the　different　castes　of　neuters　in　the　same　species；　that　I　gladly　availed　myself　of　Mr　F。　Smith's　offer　of　numerous　specimens　from　the　same　nest　of　the　driver　ant　（Anomma）　of　West　Africa。　The　reader　will　perhaps　best　appreciate　the　amount　of　difference　in　these　workers；　by　my　giving　not　the　actual　measurements；　but　a　strictly　accurate　illustration：　the　difference　was　the　same　as　if　we　were　to　see　a　set　of　workmen　building　a　house　of　whom　many　were　five　feet　four　inches　high；　and　many　sixteen　feet　high；　but　we　must　suppose　that　the　larger　workmen　had　heads　four　instead　of　three　times　as　big　as　those　of　the　smaller　men；　and　jaws　nearly　five　times　as　big。　The　jaws；　moreover；　of　the　working　ants　of　the　several　sizes　differed　wonderfully　in　shape；　and　in　the　form　and　number　of　the　teeth。　But　the　important　fact　for　us　is；　that　though　the　workers　can　be　grouped　into　castes　of　different　sizes；　yet　they　graduate　insensibly　into　each　other；　as　does　the　widely…different　structure　of　their　jaws。　I　speak　confidently　on　this　latter　point；　as　Mr　Lubbock　made　drawings　for　me　with　the　camera　lucida　of　the　jaws　which　I　had　dissected　from　the　workers　of　the　several　sizes。
  With　these　facts　before　me；　I　believe　that　natural　selection；　by　acting　on　the　fertile　parents；　could　form　a　species　which　should　regularly　produce　neuters；　either　all　of　large　size　with　one　form　of　jaw；　or　all　of　small　size　with　jaws　having　a　widely　different　structure；　or　lastly；　and　this　is　our　climax　of　difficulty；　one　set　of　workers　of　one　size　and　structure；　and　simultaneously　another　set　of　workers　of　a　different　size　and　structure；　a　graduated　series　having　been　first　formed；　as　in　the　case　of　the　driver　ant；　and　then　the　extreme　forms；　from　being　the　most　useful　to　the　community；　having　been　produced　in　greater　and　greater　numbers　through　the　natural　selection　of　the　parents　which　generated　them；　until　none　with　an　intermediate　structure　were　produced。
  Thus；　as　I　believe；　the　wonderful　fact　of　two　distinctly　defined　castes　of　sterile　workers　existing　in　the　same　nest；　both　widely　different　from　each　other　and　from　their　parents；　has　originated。　We　can　see　how　useful　their　production　may　have　been　to　a　social　community　of　insects；　on　the　same　principle　that　the　division　of　labour　is　useful　to　civilised　man。　As　ants　work　by　inherited　instincts　and　by　inherited　tools　or　weapons；　and　not　by　acquired　knowledge　and　manufactured　instruments；　a　perfect　division　of　labour　could　be　effected　with　them　only　by　the　workers　being　sterile；　for　had　they　been　fertile；　they　would　have　intercrossed；　and　their　instincts　and　structure　would　have　become　blended。　And　nature　has；　as　I　believe；　effected　this　admirable　division　of　labour　in　the　communities　of　ants；　by　the　means　of　natural　selection。　But　I　am　bound　to　confess；　that；　with　all　my　faith　in　this　principle；　I　should　never　have　anticipated　that　natural　selection　could　have　been　efficient　in　so　high　a　degree；　had　not　the　case　of　these　neuter　insects　convinced　me　of　the　fact。　I　have；　therefore；　discussed　this　case；　at　some　little　but　wholly　insufficient　length；　in　order　to　show　the　power　of　natural　selection；　and　likewise　because　this　is　by　far　the　most　serious　special　difficulty；　which　my　theory　has　encountered。　The　case；　also；　is　very　interesting；　as　it　proves　that　with　animals；　as　with　plants；　any　amount　of　modification　in　structure　can　be　effected　by　the　accumulation　of　numerous；　slight；　and　as　we　must　call　them　accidental；　variations；　which　are　in　any　manner　profitable；　without　exercise　or　habit　having　come　into　play。　For　no　amount　of　exercise；　or　habit；　or　volition；　in　the　utterly　sterile　members　of　a　community　could　possibly　have　affected　the　structure　or　instincts　of　the　fertile　members；　which　alone　leave　descendants。　I　am　surprised　that　no　one　has　advanced　this　demonstrative　case　of　neuter　insects；　against　the　well…known　doctrine　of　Lamarck。
  Summary。　I　have　endeavoured　briefly　in　this　chapter　to　show　that　the　mental　qualities　of　our　domestic　animals　vary；　and　that　the　variations　are　inherited。　Still　more　briefly　I　have　attempted　to　show　that　instincts　vary　slightly　in　a　state　of　nature。　No　one　will　dispute　that　instincts　are　of　the　highest　importance　to　each　animal。　Therefore　I　can　see　no　difficulty；　under　changing　conditions　of　life；　in　natural　selection　accumulating　slight　modifications　of　instinct　to　any　extent；　in　any　useful　direction。　In　some　cases　habit　or　use　and　disuse　have　probably　come　into　play。　I　do　not　pretend　that　the　facts　given　in　this　chapter　strengthen　in　any　great　degree　my　theory；　but　none　of　the　cases　of　difficulty；　to　the　best　of　my　judgment；　annihilate　it。　On　the　other　hand；　the　fact　that　instincts　are　not　always　absolutely　perfect　and　are　liable　to　mistakes；　that　no　instinct　has　been　produced　for　the　exclusive　good　of　other　animals；　but　that　each　animal　takes　advantage　of　the　instincts　of　others；　that　the　canon　in　natural　history；　of　'natura　non　facit　saltum'　is　applicable　to　instincts　as　well　as　to　corporeal　structure；　and　is　plainly　explicable　on　the　foregoing　views；　but　is　otherwise　inexplicable；　all　tend　to　corroborate　the　theory　of　natural　selection。
  This　theory　is；　also；　strengthened　by　some　few　other　facts　in　regard　to　instincts；　as　by　that　common　case　of　closely　allied；　but　certainly　distinct；　species；　when　inhabiting　distant　parts　of　the　world　and　living　under　considerably　different　conditions　of　life；　yet　often　retaining　nearly　the　same　instincts。　For　instance；　we　can　understand　on　the　principle　of　inheritance；　how　it　is　that　the　thrush　of　South　America　lines　its　nest　with　mud；　in　the　same　peculiar　manner　as　does　our　British　thrush：　how　it　is　that　the　male　wrens　（Troglodytes）　of　North　America；　build　'cock…nests；'　to　roost　in；　like　the　males　of　our　distinct　Kitty…wrens；　a　habit　wholly　unlike　that　of　any　other　known　bird。　Finally；　it　may　not　be　a　logical　deduction；　but　to　my　imagination　it