Lastly；　looking　not　to　any　one　time；　but　to　all　time；　if　my　theory　be　true；　numberless　intermediate　varieties；　linking　most　closely　all　the　species　of　the　same　group　together；　must　assuredly　have　existed；　but　the　very　process　of　natural　selection　constantly　tends；　as　has　been　so　often　remarked；　to　exterminate　the　parent　forms　and　the　intermediate　links。　Consequently　evidence　of　their　former　existence　could　be　found　only　amongst　fossil　remains；　which　are　preserved；　as　we　shall　in　a　future　chapter　attempt　to　show；　in　an　extremely　imperfect　and　intermittent　record。
  On　the　origin　and　transitions　of　organic　beings　with　peculiar　habits　and　structure。　It　has　been　asked　by　the　opponents　of　such　views　as　I　hold；　how；　for　instance；　a　land　carnivorous　animal　could　have　been　converted　into　one　with　aquatic　habits；　for　how　could　the　animal　in　its　transitional　state　have　subsisted？　It　would　be　easy　to　show　that　within　the　same　group　carnivorous　animals　exist　having　every　intermediate　grade　between　truly　aquatic　and　strictly　terrestrial　habits；　and　as　each　exists　by　a　struggle　for　life；　it　is　clear　that　each　is　well　adapted　in　its　habits　to　its　place　in　nature。　Look　at　the　Mustela　vison　of　North　America；　which　has　webbed　feet　and　which　resembles　an　otter　in　its　fur；　short　legs；　and　form　of　tail；　during　summer　this　animal　dives　for　and　preys　on　fish；　but　during　the　long　winter　it　leaves　the　frozen　waters；　and　preys　like　other　polecats　on　mice　and　land　animals。　If　a　different　case　had　been　taken；　and　it　had　been　asked　how　an　insectivorous　quadruped　could　possibly　have　been　converted　into　a　flying　bat；　the　question　would　have　been　far　more　difficult；　and　I　could　have　given　no　answer。　Yet　I　think　such　difficulties　have　very　little　weight。
  Here；　as　on　other　occasions；　I　lie　under　a　heavy　disadvantage；　for　out　of　the　many　striking　cases　which　I　have　collected；　I　can　give　only　one　or　two　instances　of　transitional　habits　and　structures　in　closely　allied　species　of　the　same　genus；　and　of　diversified　habits；　either　constant　or　occasional；　in　the　same　species。　And　it　seems　to　me　that　nothing　less　than　a　long　list　of　such　cases　is　sufficient　to　lessen　the　difficulty　in　any　particular　case　like　that　of　the　bat。
  Look　at　the　family　of　squirrels；　here　we　have　the　finest　gradation　from　animals　with　their　tails　only　slightly　flattened；　and　from　others；　as　Sir　J。　Richardson　has　remarked；　with　the　posterior　part　of　their　bodies　rather　wide　and　with　the　skin　on　their　flanks　rather　full；　to　the　so…called　flying　squirrels；　and　flying　squirrels　have　their　limbs　and　even　the　base　of　the　tail　united　by　a　broad　expanse　of　skin；　which　serves　as　a　parachute　and　allows　them　to　glide　through　the　air　to　an　astonishing　distance　from　tree　to　tree。　We　cannot　doubt　that　each　structure　is　of　use　to　each　kind　of　squirrel　in　its　own　country；　by　enabling　it　to　escape　birds　or　beasts　of　prey；　or　to　collect　food　more　quickly；　or；　as　there　is　reason　to　believe；　by　lessening　the　danger　from　occasional　falls。　But　it　does　not　follow　from　this　fact　that　the　structure　of　each　squirrel　is　the　best　that　it　is　possible　to　conceive　under　all　natural　conditions。　Let　the　climate　and　vegetation　change；　let　other　competing　rodents　or　new　beasts　of　prey　immigrate；　or　old　ones　become　modified；　and　all　analogy　would　lead　us　to　believe　that　some　at　least　of　the　squirrels　would　decrease　in　numbers　or　become　exterminated；　unless　they　also　became　modified　and　improved　in　structure　in　a　corresponding　manner。　Therefore；　I　can　see　no　difficulty；　more　especially　under　changing　conditions　of　life；　in　the　continued　preservation　of　individuals　with　fuller　and　fuller　flank…membranes；　each　modification　being　useful；　each　being　propagated；　until　by　the　accumulated　effects　of　this　process　of　natural　selection；　a　perfect　so…called　flying　squirrel　was　produced。
  Now　look　at　the　Galeopithecus　or　flying　lemur；　which　formerly　was　falsely　ranked　amongst　bats。　It　has　an　extremely　wide　flank…membrane；　stretching　from　the　corners　of　the　jaw　to　the　tail；　and　including　the　limbs　and　the　elongated　fingers：　the　flank　membrane　is；　also；　furnished　with　an　extensor　muscle。　Although　no　graduated　links　of　structure；　fitted　for　gliding　through　the　air；　now　connect　the　Galeopithecus　with　the　other　Lemuridae；　yet　I　can　see　no　difficulty　in　supposing　that　such　links　formerly　existed；　and　that　each　had　been　formed　by　the　same　steps　as　in　the　case　of　the　less　perfectly　gliding　squirrels；　and　that　each　grade　of　structure　had　been　useful　to　its　possessor。　Nor　can　I　see　any　insuperable　difficulty　in　further　believing　it　possible　that　the　membrane…connected　fingers　and　fore…arm　of　the　Galeopithecus　might　be　greatly　lengthened　by　natural　selection；　and　this；　as　far　as　the　organs　of　flight　are　concerned；　would　convert　it　into　a　bat。　In　bats　which　have　the　wing…membrane　extended　from　the　top　of　the　shoulder　to　the　tail；　including　the　hind…legs；　we　perhaps　see　traces　of　an　apparatus　originally　constructed　for　gliding　through　the　air　rather　than　for　flight。
  If　about　a　dozen　genera　of　birds　had　become　extinct　or　were　unknown；　who　would　have　ventured　to　have　surmised　that　birds　might　have　existed　which　used　their　wings　solely　as　flappers；　like　the　logger…headed　duck　（Micropterus　of　Eyton）；　as　fins　in　the　water　and　front　legs　on　the　land；　like　the　penguin；　as　sails；　like　the　ostrich；　and　functionally　for　no　purpose；　like　the　Apteryx。　Yet　the　structure　of　each　of　these　birds　is　good　for　it；　under　the　conditions　of　life　to　which　it　is　exposed；　for　each　has　to　live　by　a　struggle；　but　it　is　not　necessarily　the　best　possible　under　all　possible　conditions。　It　must　not　be　inferred　from　these　remarks　that　any　of　the　grades　of　wing…structure　here　alluded　to；　which　perhaps　may　all　have　resulted　from　disuse；　indicate　the　natural　steps　by　which　birds　have　acquired　their　perfect　power　of　flight；　but　they　serve；　at　least；　to　show　what　diversified　means　of　transition　are　possible。
  Seeing　that　a　few　members　of　such　water…breathing　classes　as　the　Crustacea　and　Mollusca　are　adapted　to　live　on　the　land；　and　seeing　that　we　have　flying　birds　and　mammals；　flying　insects　of　the　most　diversified　types；　and　formerly　had　flying　reptiles；　it　is　conceivable　that　flying…fish；　which　now　glide　far　through　the　air；　slightly　rising　and　turning　by　the　aid　of　their　fluttering　fins；　might　have　been　modified　into　perfectly　winged　animals。　If　early　transitional　state　they　had　been　inhabitants　of　the　open　ocean；　and　had　used　their　incipient　organs　of　flight　exclusively；　as　far　as　we　know；　to　escape　being　devoured　by　other　fish？
  When　we　see　any　structure　highly　perfected　for　any　particular　habit；　as　the　wings　of　a　bird　for　flight；　we　should　bear　in　mind　that　animals　displaying　early　transitional　grades　of　the　structure　will　seldom　continue　to　exist　to　the　present　day；　for　they　will　have　been　supplanted　by　the　very　process　of　perfection　through　natural　selection。　Furthermore；　we　may　conclude　that　transitional　grades　between　structures　fitted　for　very　different　habits　of　life　will　rarely　have　been　developed　at　an　early　period　in　great　numbers　and　under　many　subordinate　forms。　Thus；　to　return　to　our　imaginary　illustration　of　the　flying…fish；　it　does　not　seem　probable　that　fishes　capable　of　true　flight　would　have　been　developed　under　many　subordinate　forms；　for　taking　prey　of　many　kinds　in　many　ways；　on　the　land　and　in　the　water；　until　their　organs　of　flight　had　come　to　a　high　stage　of　perfection；　so　as　to　have　given　them　a　decided　advantage　over　other　animals　in　the　battle　for　life。　Hence　the　chance　of　discovering　species　with　transitional　grades　of　structure　in　a　fossil　condition　will　always　be　less；　from　their　having　existed　in　lesser　numbers；　than　in　the　case　of　species　with　fully　developed　structures。
  I　will　now　give　two　or　three　instances　of　diversified　and　of　changed　habits　in　the　individuals　of　the　same　species。　When　either　case　occurs；　it　would　be　easy　for　natural　selection　to　fit　the　animal；　by　some　modification　of　its　structure；　for　its　changed　habits；　or　exclusively　for　one　of　its　several　different　habits。　But　it　is　difficult　to　tell；　and　immaterial　for　us；　whether　habits　generally　change　first　and　structure　afterwards；　or　whether　slight　modifications　of　structure　lead　to　changed　habits；　both　probably　often　change　almost　simultaneously。　Of　cases　of　changed　habits　it　will　suffice　merely　to　allude　to　that　of　the　many　British　insects　which　now　feed　on　exotic　plants；　or　exclusively　on　artificial　substances。　Of　diversified　habits　innumerable　instances　could　be　given：　I　have　often　watched　a　tyrant　flycatcher　（Saurophagus　sulphuratus）　in　South　America；　hovering　over　one　spot　and　then　proceeding　to　another；　like　a　kestrel；　and　at　other　times　standing　stationary　on　the　margin　of　water；　and　then　dashing　like　a　kingfisher　at　a　fish。　In　our　own　country　the　larger　titmouse　（Parus　major）　may　be　seen　climbing　branches；　almost　like　a　creeper；　it　often；　like　a　shrike；　kills　small　birds　by　blows　on　the　head；　and　I　have　many　times　seen　and　heard　it　hammering　the　seeds　of　the　yew　on　a　branch；　and　thus　breaking　them　like　a　nuthatch。　In　North　America　the　black　bear　was　seen　by　Hearne　swimming　for　hours　with　widely　open　mouth；　thus　catching；　like　a　whale；　insects　in　the　water。　Even　in　so　extreme　a　ca