the origin of species(物种起源)-第31部分

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territory　between　them；　in　which　they　become　rather　suddenly　rarer　and　rarer；　then；　as　varieties　do　not　essentially　differ　from　species；　the　same　rule　will　probably　apply　to　both；　and　if　we　in　imagination　adapt　a　varying　species　to　a　very　large　area；　we　shall　have　to　adapt　two　varieties　to　two　large　areas；　and　a　third　variety　to　a　narrow　intermediate　zone。　The　intermediate　variety；　consequently；　will　exist　in　lesser　numbers　from　inhabiting　a　narrow　and　lesser　area；　and　practically；　as　far　as　I　can　make　out；　this　rule　holds　good　with　varieties　in　a　state　of　nature。　I　have　met　with　striking　instances　of　the　rule　in　the　case　of　varieties　intermediate　between　well…marked　varieties　in　the　genus　Balanus。　And　it　would　appear　from　information　given　me　by　Mr　Watson；　Dr　Asa　Gray；　and　Mr　Wollaston；　that　generally　when　varieties　intermediate　between　two　other　forms　occur；　they　are　much　rarer　numerically　than　the　forms　which　they　connect。　Now；　if　we　may　trust　these　facts　and　inferences；　and　therefore　conclude　that　varieties　linking　two　other　varieties　together　have　generally　existed　in　lesser　numbers　than　the　forms　which　they　connect；　then；　I　think；　we　can　understand　why　intermediate　varieties　should　not　endure　for　very　long　periods；　why　as　a　general　rule　they　should　be　exterminated　and　disappear；　sooner　than　the　forms　which　they　originally　linked　together。　
For　any　form　existing　in　lesser　numbers　would；　as　already　remarked；　run　a　greater　chance　of　being　exterminated　than　one　existing　in　large　numbers；　and　in　this　particular　case　the　intermediate　form　would　be　eminently　liable　to　the　inroads　of　closely　allied　forms　existing　on　both　sides　of　it。　But　a　far　more　important　consideration；　as　I　believe；　is　that；　during　the　process　of　further　modification；　by　which　two　varieties　are　supposed　on　my　theory　to　be　converted　and　perfected　into　two　distinct　species；　the　two　which　exist　in　larger　numbers　from　inhabiting　larger　areas；　will　have　a　great　advantage　over　the　intermediate　variety；　which　exists　in　smaller　numbers　in　a　narrow　and　intermediate　zone。　For　forms　existing　in　larger　numbers　will　always　have　a　better　chance；　within　any　given　period；　of　presenting　further　favourable　variations　for　natural　selection　to　seize　on；　than　will　the　rarer　forms　which　exist　in　lesser　numbers。　Hence；　the　more　common　forms；　in　the　race　for　life；　will　tend　to　beat　and　supplant　the　less　common　forms；　for　these　will　be　more　slowly　modified　and　improved。　It　is　the　same　principle　which；　as　I　believe；　accounts　for　the　common　species　in　each　country；　as　shown　in　the　second　chapter；　presenting　on　an　average　a　greater　number　of　well…marked　varieties　than　do　the　rarer　species。　I　may　illustrate　what　I　mean　by　supposing　three　varieties　of　sheep　to　be　kept；　one　adapted　to　an　extensive　mountainous　region；　a　second　to　a　comparatively　narrow；　hilly　tract；　and　a　third　to　wide　plains　at　the　base；　and　that　the　inhabitants　are　all　trying　with　equal　steadiness　and　skill　to　improve　their　stocks　by　selection；　the　chances　in　this　case　will　be　strongly　in　favour　of　the　great　holders　on　the　mountains　or　on　the　plains　improving　their　breeds　more　quickly　than　the　small　holders　on　the　intermediate　narrow；　hilly　tract；　and　consequently　the　improved　mountain　or　plain　breed　will　soon　take　the　place　of　the　less　improved　hill　breed；　and　thus　the　two　breeds；　which　originally　existed　in　greater　numbers；　will　come　into　close　contact　with　each　other；　without　the　interposition　of　the　supplanted；　intermediate　hill…variety。　
To　sum　up；　I　believe　that　species　come　to　be　tolerably　well…defined　objects；　and　do　not　at　any　one　period　present　an　inextricable　chaos　of　varying　and　intermediate　links：　firstly；　because　new　varieties　are　very　slowly　formed；　for　variation　is　a　very　slow　process；　and　natural　selection　can　do　nothing　until　favourable　variations　chance　to　occur；　and　until　a　place　in　the　natural　polity　of　the　country　can　be　better　filled　by　some　modification　of　some　one　or　more　of　its　inhabitants。　And　such　new　places　will　depend　on　slow　changes　of　climate；　or　on　the　occasional　immigration　of　new　inhabitants；　and；　probably；　in　a　still　more　important　degree；　on　some　of　the　old　inhabitants　becoming　slowly　modified；　with　the　new　forms　thus　produced　and　the　old　ones　acting　and　reacting　on　each　other。　So　that；　in　any　one　region　and　at　any　one　time；　we　ought　only　to　see　a　few　species　presenting　slight　modifications　of　structure　in　some　degree　permanent；　and　this　assuredly　we　do　see。　
Secondly；　areas　now　continuous　must　often　have　existed　within　the　recent　period　in　isolated　portions；　in　which　many　forms；　more　especially　amongst　the　classes　which　unite　for　each　birth　and　wander　much；　may　have　separately　been　rendered　sufficiently　distinct　to　rank　as　representative　species。　In　this　case；　intermediate　varieties　between　the　several　representative　species　and　their　common　parent；　must　formerly　have　existed　in　each　broken　portion　of　the　land；　but　these　links　will　have　been　supplanted　and　exterminated　during　the　process　of　natural　selection；　so　that　they　will　no　longer　exist　in　a　living　state。　
Thirdly；　when　two　or　more　varieties　have　been　formed　in　different　portions　of　a　strictly　continuous　area；　intermediate　varieties　will；　it　is　probable；　at　first　have　been　formed　in　the　intermediate　zones；　but　they　will　generally　have　had　a　short　duration。　For　these　intermediate　varieties　will；　from　reasons　already　assigned　（namely　from　what　we　know　of　the　actual　distribution　of　closely　allied　or　representative　species；　and　likewise　of　acknowledged　varieties）；　exist　in　the　intermediate　zones　in　lesser　numbers　than　the　varieties　which　they　tend　to　connect。　From　this　cause　alone　the　intermediate　varieties　will　be　liable　to　accidental　extermination；　and　during　the　process　of　further　modification　through　natural　selection；　they　will　almost　certainly　be　beaten　and　supplanted　by　the　forms　which　they　connect；　for　these　from　existing　in　greater　numbers　will；　in　the　aggregate；　present　more　variation；　and　thus　be　further　improved　through　natural　selection　and　gain　further　advantages。　
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　th