s　which　it　connects；　consequently　the　two　latter；　during　the　course　of　further　modification；　from　existing　in　greater　numbers；　will　have　a　great　advantage　over　the　less　numerous　intermediate　variety；　and　will　thus　generally　succeed　in　supplanting　and　exterminating　it。
  We　have　seen　in　this　chapter　how　cautious　we　should　be　in　concluding　that　the　most　different　habits　of　life　could　not　graduate　into　each　other；　that　a　bat；　for　instance；　could　not　have　been　formed　by　natural　selection　from　an　animal　which　at　first　could　only　glide　through　the　air。
  We　have　seen　that　a　species　may　under　new　conditions　of　life　change　its　habits；　or　have　diversified　habits；　with　some　habits　very　unlike　those　of　its　nearest　congeners。　Hence　we　can　understand　bearing　in　mind　that　each　organic　being　is　trying　to　live　wherever　it　can　live；　how　it　has　arisen　that　there　are　upland　geese　with　webbed　feet；　ground　woodpeckers；　diving　thrushes；　and　petrels　with　the　habits　of　auks。
  Although　the　belief　that　an　organ　so　perfect　as　the　eye　could　have　been　formed　by　natural　selection；　is　more　than　enough　to　stagger　any　one；　yet　in　the　case　of　any　organ；　if　we　know　of　a　long　series　of　gradations　in　complexity；　each　good　for　its　possessor；　then；　under　changing　conditions　of　life；　there　is　no　logical　impossibility　in　the　acquirement　of　any　conceivable　degree　of　perfection　through　natural　selection。　In　the　cases　in　which　we　know　of　no　intermediate　or　transitional　states；　we　should　be　very　cautious　in　concluding　that　none　could　have　existed；　for　the　homologies　of　many　organs　and　their　intermediate　states　show　that　wonderful　metamorphoses　in　function　are　at　least　possible。　For　instance；　a　swim…bladder　has　apparently　been　converted　into　an　air…breathing　lung。　The　same　organ　having　performed　simultaneously　very　different　functions；　and　then　having　been　specialised　for　one　function；　and　two　very　distinct　organs　having　performed　at　the　same　time　the　same　function；　the　one　having　been　perfected　whilst　aided　by　the　other；　must　often　have　largely　facilitated　transitions。
  We　are　far　too　ignorant；　in　almost　every　case；　to　be　enabled　to　assert　that　any　part　or　organ　is　so　unimportant　for　the　welfare　of　a　species；　that　modifications　in　its　structure　could　not　have　been　slowly　accumulated　by　means　of　natural　selection。　But　we　may　confidently　believe　that　many　modifications；　wholly　due　to　the　laws　of　growth；　and　at　first　in　no　way　advantageous　to　a　species；　have　been　subsequently　taken　advantage　of　by　the　still　further　modified　descendants　of　this　species。　We　may；　also；　believe　that　a　part　formerly　of　high　importance　has　often　been　retained　（as　the　tail　of　an　aquatic　animal　by　its　terrestrial　descendants）；　though　it　has　become　of　such　small　importance　that　it　could　not；　in　its　present　state；　have　been　acquired　by　natural　selection；　a　power　which　acts　solely　by　the　preservation　of　profitable　variations　in　the　struggle　for　life。
  Natural　selection　will　produce　nothing　in　one　species　for　the　exclusive　good　or　injury　of　another；　though　it　may　well　produce　parts；　organs；　and　excretions　highly　useful　or　even　indispensable；　or　highly　injurious　to　another　species；　but　in　all　cases　at　the　same　time　useful　to　the　owner。　Natural　selection　in　each　well…stocked　country；　must　act　chiefly　through　the　competition　of　the　inhabitants　one　with　another；　and　consequently　will　produce　perfection；　or　strength　in　the　battle　for　life；　only　according　to　the　standard　of　that　country。　Hence　the　inhabitants　of　one　country；　generally　the　smaller　one；　will　often　yield；　as　we　see　they　do　yield；　to　the　inhabitants　of　another　and　generally　larger　country。　For　in　the　larger　country　there　will　have　existed　more　individuals；　and　more　diversified　forms；　and　the　competition　will　have　been　severer；　and　thus　the　standard　of　perfection　will　have　been　rendered　higher。　Natural　selection　will　not　necessarily　produce　absolute　perfection；　nor；　as　far　as　we　can　judge　by　our　limited　faculties；　can　absolute　perfection　be　everywhere　found。
  On　the　theory　of　natural　selection　we　can　clearly　understand　the　full　meaning　of　that　old　canon　in　natural　history；　'Natura　non　facit　saltum。'　This　canon；　if　we　look　only　to　the　present　inhabitants　of　the　world；　is　not　strictly　correct；　but　if　we　include　all　those　of　past　times；　it　must　by　my　theory　be　strictly　true。
  It　is　generally　acknowledged　that　all　organic　beings　have　been　formed　on　two　great　laws　Unity　of　Type；　and　the　Conditions　of　Existence。　By　unity　of　type　is　meant　that　fundamental　agreement　in　structure；　which　we　see　in　organic　beings　of　the　same　class；　and　which　is　quite　independent　of　their　habits　of　life。　On　my　theory；　unity　of　type　is　explained　by　unity　of　descent。　The　expression　of　conditions　of　existence；　so　often　insisted　on　by　the　illustrious　Cuvier；　is　fully　embraced　by　the　principle　of　natural　selection。　For　natural　selection　acts　by　either　now　adapting　the　varying　parts　of　each　being　to　its　organic　and　inorganic　conditions　of　life；　or　by　having　adapted　them　during　long…past　periods　of　time：　the　adaptations　being　aided　in　some　cases　by　use　and　disuse；　being　slightly　affected　by　the　direct　action　of　the　external　conditions　of　life；　and　being　in　all　cases　subjected　to　the　several　laws　of　growth。　Hence；　in　fact；　the　law　of　the　Conditions　of　Existence　is　the　higher　law；　as　it　includes；　through　the　inheritance　of　former　adaptations；　that　of　Unity　of　Type。
  Chapter　7　…　Instinct
  THE　subject　of　instinct　might　have　been　worked　into　the　previous　chapters；　but　I　have　thought　that　it　would　be　more　convenient　to　treat　the　subject　separately；　especially　as　so　wonderful　an　instinct　as　that　of　the　hive…bee　making　its　cells　will　probably　have　occurred　to　many　readers；　as　a　difficulty　sufficient　to　overthrow　my　whole　theory。　I　must　premise；　that　I　have　nothing　to　do　with　the　origin　of　the　primary　mental　powers；　any　more　than　I　have　with　that　of　life　itself。　We　are　concerned　only　with　the　diversities　of　instinct　and　of　the　other　mental　qualities　of　animals　within　the　same　class。
  I　will　not　attempt　any　definition　of　instinct。　It　would　be　easy　to　show　that　several　distinct　mental　actions　are　commonly　embraced　by　this　term；　but　every　one　understands　what　is　meant；　when　it　is　said　that　instinct　impels　the　cuckoo　to　migrate　and　to　lay　her　eggs　in　other　birds'　nests。　An　action；　which　we　ourselves　should　require　experience　to　enable　us　to　perform；　when　performed　by　an　animal；　more　especially　by　a　very　young　one；　without　any　experience；　and　when　performed　by　many　individuals　in　the　same　way；　without　their　knowing　for　what　purpose　it　is　performed；　is　usually　said　to　be　instinctive。　But　I　could　show　that　none　of　these　characters　of　instinct　are　universal。　A　little　dose；　as　Pierre　Huber　expresses　it；　of　judgment　or　reason；　often　comes　into　play；　even　in　animals　very　low　in　the　scale　of　nature。
  Frederick　Cuvier　and　several　of　the　older　metaphysicians　have　compared　instinct　with　habit。　This　comparison　gives；　I　think；　a　remarkably　accurate　notion　of　the　frame　of　mind　under　which　an　instinctive　action　is　performed；　but　not　of　its　origin。　How　unconsciously　many　habitual　actions　are　performed；　indeed　not　rarely　in　direct　opposition　to　our　conscious　will！　yet　they　may　be　modified　by　the　will　or　reason。　Habits　easily　become　associated　with　other　habits；　and　with　certain　periods　of　time　and　states　of　the　body。　When　once　acquired；　they　often　remain　constant　throughout　life。　Several　other　points　of　resemblance　between　instincts　and　habits　could　be　pointed　out。　As　in　repeating　a　well…known　song；　so　in　instincts；　one　action　follows　another　by　a　sort　of　rhythm；　if　a　person　be　interrupted　in　a　song；　or　in　repeating　anything　by　rote；　he　is　generally　forced　to　go　back　to　recover　the　habitual　train　of　thought：　so　P。　Huber　found　it　was　with　a　caterpillar；　which　makes　a　very　complicated　hammock；　for　if　he　took　a　caterpillar　which　had　completed　its　hammock　up　to；　say；　the　sixth　stage　of　construction；　and　put　it　into　a　hammock　completed　up　only　to　the　third　stage；　the　caterpillar　simply　re…performed　the　fourth；　fifth；　and　sixth　stages　of　construction。　If；　however；　a　caterpillar　were　taken　out　of　a　hammock　made　up；　for　instance；　to　the　third　stage；　and　were　put　into　one　finished　up　to　the　sixth　stage；　so　that　much　of　its　work　was　already　done　for　it；　far　from　feeling　the　benefit　of　this；　it　was　much　embarrassed；　and；　in　order　to　complete　its　hammock；　seemed　forced　to　start　from　the　third　stage；　where　it　had　left　off；　and　thus　tried　to　complete　the　already　finished　work。
  If　we　suppose　any　habitual　action　to　become　inherited　and　I　think　it　can　be　shown　that　this　does　sometimes　happen　then　the　resemblance　between　what　originally　was　a　habit　and　an　instinct　becomes　so　close　as　not　to　be　distinguished。　If　Mozart；　instead　of　playing　the　pianoforte　at　three　years　old　with　wonderfully　little　practice；　had　played　a　tune　with　no　practice　at　all；　be　might　truly　be　said　to　have　done　so　instinctively。　But　it　would　be　the　most　serious　error　to　suppose　that　the　greater　number　of　instincts　have　been　acquired　by　habit　in　one　generation；　and　then　transmitted　by　inheritance　to　succeeding　generations。　It　can　be　clearly　shown　that　the　most　wonderful　instincts　with　which　we　are　acquainted；　namely；　those　of　the　hive…bee　and　of　many　ants；　c