the origin of species(物种起源)-第34节

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as　been　continuous　during　a　long　period。　Geology　would　lead　us　to　believe　that　almost　every　continent　has　been　broken　up　into　islands　even　during　the　later　tertiary　periods；　and　in　such　islands　distinct　species　might　have　been　separately　formed　without　the　possibility　of　intermediate　varieties　existing　in　the　intermediate　zones。　By　changes　in　the　form　of　the　land　and　of　climate；　marine　areas　now　continuous　must　often　have　existed　within　recent　times　in　a　far　less　continuous　and　uniform　condition　than　at　present。　But　I　will　pass　over　this　way　of　escaping　from　the　difficulty；　for　I　believe　that　many　perfectly　defined　species　have　been　formed　on　strictly　continuous　areas；　though　I　do　not　doubt　that　the　formerly　broken　condition　of　areas　now　continuous　has　played　an　important　part　in　the　formation　of　new　species；　more　especially　with　freely…crossing　and　wandering　animals。　
In　looking　at　species　as　they　are　now　distributed　over　a　wide　area；　we　generally　find　them　tolerably　numerous　over　a　large　territory；　then　becoming　somewhat　abruptly　rarer　and　rarer　on　the　confines；　and　finally　disappearing。　Hence　the　neutral　territory　between　two　representative　species　is　generally　narrow　in　comparison　with　the　territory　proper　to　each。　We　see　the　same　fact　in　ascending　mountains；　and　sometimes　it　is　quite　remarkable　how　abruptly；　as　Alph。　De　Candolle　has　observed；　a　common　alpine　species　disappears。　The　same　fact　has　been　noticed　by　Forbes　in　sounding　the　depths　of　the　sea　with　the　dredge。　To　those　who　look　at　climate　and　the　physical　conditions　of　life　as　the　all…important　elements　of　distribution；　these　facts　ought　to　cause　surprise；　as　climate　and　height　or　depth　graduate　away　insensibly。　But　when　we　bear　in　mind　that　almost　every　species；　even　in　its　metropolis；　would　increase　immensely　in　numbers；　were　it　not　for　other　competing　species；　that　nearly　all　either　prey　on　or　serve　as　prey　for　others；　in　short；　that　each　organic　being　is　either　directly　or　indirectly　related　in　the　most　important　manner　to　other　organic　beings；　we　must　see　that　the　range　of　the　inhabitants　of　any　country　by　no　means　exclusively　depends　on　insensibly　changing　physical　conditions；　but　in　large　part　on　the　presence　of　other　species；　on　which　it　depends；　or　by　which　it　is　destroyed；　or　with　which　it　comes　into　competition；　and　as　these　species　are　already　defined　objects　（however　they　may　have　become　so）；　not　blending　one　into　another　by　insensible　gradations；　the　range　of　any　one　species；　depending　as　it　does　on　the　range　of　others；　will　tend　to　be　sharply　defined。　Moreover；　each　species　on　the　confines　of　its　range；　where　it　exists　in　lessened　numbers；　will；　during　fluctuations　in　the　number　of　its　enemies　or　of　its　prey；　or　in　the　seasons；　be　extremely　liable　to　utter　extermination；　and　thus　its　geographical　range　will　come　to　be　still　more　sharply　defined。　
If　I　am　right　in　believing　that　allied　or　representative　species；　when　inhabiting　a　continuous　area；　are　generally　so　distributed　that　each　has　a　wide　range；　with　a　comparatively　narrow　neutral　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