the origin of species(物种起源)-第20章

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es；　and　may　constitute　a　sub…genus　or　even　a　distinct　genus。　The　six　descendants　from　（I）　will　form　two　sub…genera　or　even　genera。　But　as　the　original　species　（I）　differed　largely　from　（A）；　standing　nearly　at　the　extreme　points　of　the　original　genus；　the　six　descendants　from　（I）　will；　owing　to　inheritance；　differ　considerably　from　the　eight　descendants　from　（A）；　the　two　groups；　moreover；　are　supposed　to　have　gone　on　diverging　in　different　directions。　The　intermediate　species；　also　（and　this　is　a　very　important　consideration）；　which　connected　the　original　species　（A）　and　（I）；　have　all　become；　excepting　（F）；　extinct；　and　have　left　no　descendants。　Hence　the　six　new　species　descended　from　（I）；　and　the　eight　descended　from　（A）；　will　have　to　be　ranked　as　very　distinct　genera；　or　even　as　distinct　sub…families。　
Thus　it　is；　as　I　believe；　that　two　or　more　genera　are　produced　by　descent；　with　modification；　from　two　or　more　species　of　the　same　genus。　And　the　two　or　more　parent…species　are　supposed　to　have　descended　from　some　one　species　of　an　earlier　genus。　In　our　diagram；　this　is　indicated　by　the　broken　lines；　beneath　the　capital　letters；　converging　in　sub…branches　downwards　towards　a　single　point；　this　point　representing　a　single　species；　the　supposed　single　parent　of　our　several　new　sub…genera　and　genera。　
It　is　worth　while　to　reflect　for　a　moment　on　the　character　of　the　new　species　F14；　which　is　supposed　not　to　have　diverged　much　in　character；　but　to　have　retained　the　form　of　（F）；　either　unaltered　or　altered　only　in　a　slight　degree。　In　this　case；　its　affinities　to　the　other　fourteen　new　species　will　be　of　a　curious　and　circuitous　nature。　Having　descended　from　a　form　which　stood　between　the　two　parent…species　（A）　and　（I）；　now　supposed　to　be　extinct　and　unknown；　it　will　be　in　some　degree　intermediate　in　character　between　the　two　groups　descended　from　these　species。　But　as　these　two　groups　have　gone　on　diverging　in　character　from　the　type　of　their　parents；　the　new　species　（F14）　will　not　be　directly　intermediate　between　them；　but　rather　between　types　of　the　two　groups；　and　every　naturalist　will　be　able　to　bring　some　such　case　before　his　mind。　
In　the　diagram；　each　horizontal　line　has　hitherto　been　supposed　to　represent　a　thousand　generations；　but　each　may　represent　a　million　or　hundred　million　generations；　and　likewise　a　section　of　the　successive　strata　of　the　earth's　crust　including　extinct　remains。　We　shall；　when　we　come　to　our　chapter　on　Geology；　have　to　refer　again　to　this　subject；　and　I　think　we　shall　then　see　that　the　diagram　throws　light　on　the　affinities　of　extinct　beings；　which；　though　generally　belonging　to　the　same　orders；　or　families；　or　genera；　with　those　now　living；　yet　are　often；　in　some　degree；　intermediate　in　character　between　existing　groups；　and　we　can　understand　this　fact；　for　the　extinct　species　lived　at　very　ancient　epochs　when　the　branching　lines　of　descent　had　diverged　less。　
I　see　no　reason　to　limit　the　process　of　modification；　as　now　explained；　to　the　formation　of　genera　alone。　If；　in　our　diagram；　we　suppose　the　amount　of　change　represented　by　each　successive　group　of　diverging　dotted　lines　to　be　very　great；　the　forms　marked　a214　to　p14；　those　marked　b14　and　f14；　and　those　marked　o14　to　m14；　will　form　three　very　distinct　genera。　We　shall　also　have　two　very　distinct　genera　descended　from　（I）　and　as　these　latter　two　genera；　both　from　continued　divergence　of　character　and　from　inheritance　from　a　different　parent；　will　differ　widely　from　the　three　genera　descended　from　（A）；　the　two　little　groups　of　genera　will　form　two　distinct　families；　or　even　orders；　according　to　the　amount　of　divergent　modification　supposed　to　be　represented　in　the　diagram。　And　the　two　new　families；　or　orders；　will　have　descended　from　two　species　of　the　original　genus；　and　these　two　species　are　supposed　to　have　descended　from　one　species　of　a　still　more　ancient　and　unknown　genus。　
We　have　seen　that　in　each　country　it　is　the　species　of　the　larger　genera　which　oftenest　present　varieties　or　incipient　species。　This；　indeed；　might　have　been　expected；　for　as　natural　selection　acts　through　one　form　having　some　advantage　over　other　forms　in　the　struggle　for　existence；　it　will　chiefly　act　on　those　which　already　have　some　advantage；　and　the　largeness　of　any　group　shows　that　its　species　have　inherited　from　a　common　ancestor　some　advantage　in　common。　Hence；　the　struggle　for　the　production　of　new　and　modified　descendants；　will　mainly　lie　between　the　larger　groups；　which　are　all　trying　to　increase　in　number。　One　large　group　will　slowly　conquer　another　large　group；　reduce　its　numbers；　and　thus　lessen　its　chance　of　further　variation　and　improvement。　Within　the　same　large　group；　the　later　and　more　highly　perfected　sub…groups；　from　branching　out　and　seizing　on　many　new　places　in　the　polity　of　Nature；　will　constantly　tend　to　supplant　and　destroy　the　earlier　and　less　improved　sub…groups。　Small　and　broken　groups　and　sub…groups　will　finally　tend　to　disappear。　Looking　to　the　future；　we　can　predict　that　the　groups　of　organic　beings　which　are　now　large　and　triumphant；　and　which　are　least　broken　up；　that　is；　which　as　yet　have　suffered　least　extinction；　will　for　a　long　period　continue　to　increase。　But　which　groups　will　ultimately　prevail；　no　man　can　predict；　for　we　well　know　that　many　groups；　formerly　most　extensively　developed；　have　now　become　extinct。　Looking　still　more　remotely　to　the　future；　we　may　predict　that；　owing　to　the　continued　and　steady　increase　of　the　larger　groups；　a　multitude　of　smaller　groups　will　become　utterly　extinct；　and　leave　no　modified　descendants；　and　consequently　that　of　the　species　living　at　any　one　period；　extremely　few　will　transmit　descendants　to　a　remote　futurity。　I　shall　have　to　return　to　this　subject　in　the　chapter　on　Classification；　but　I　may　add　that　on　this　view　of　extremely　few　of　the　more　ancient　species　having　transmitted　descendants；　and　on　the　view　of　all　the　descendants　of　the　same　species　making　a　class；　we　can　understand　how　it　is　that　there　exist　but　very　few　classes　in　each　main　division　of　the　animal　and　vegetable　kingdoms。　Although　extremely　few　of　the　most　ancient　species　may　now　have　living　and　modified　descendants；　yet　at　the　most　remote　geological　period；　the　earth　may　have　been　as　well　peopled　with　many　species　of　many　genera；　families；　orders；　and　classes；　as　at　the　present　day。　
Summary　of　Chapter
If　during　the　long　course　of　ages　and　under　varying　conditions　of　life；　organic　beings　vary　at　all　in　the　several　parts　of　their　organisation；　and　I　think　this　cannot　be　disputed；　if　there　be；　owing　to　the　high　geometrical　powers　of　increase　of　each　species；　at　some　age；　season；　or　year；　a　severe　struggle　for　life；　and　this　certainly　cannot　be　disputed；　then；　considering　the　infinite　complexity　of　the　relations　of　all　organic　beings　to　each　other　and　to　their　conditions　of　existence；　causing　an　infinite　diversity　in　structure；　constitution；　and　habits；　to　be　advantageous　to　them；　I　think　it　would　be　a　most　extraordinary　fact　if　no　variation　ever　had　occurred　useful　to　each　being's　own　welfare；　in　the　same　way　as　so　many　variations　have　occurred　useful　to　man。　But　if　variations　useful　to　any　organic　being　do　occur；　assuredly　individuals　thus　characterised　will　have　the　best　chance　of　being　preserved　in　the　struggle　for　life；　and　from　the　strong　principle　of　inheritance　they　will　tend　to　produce　offspring　similarly　characterised。　This　principle　of　preservation；　I　have　called；　for　the　sake　of　brevity；　Natural　Selection。　Natural　selection；　on　the　principle　of　qualities　being　inherited　at　corresponding　ages；　can　modify　the　egg；　seed；　or　young；　as　easily　as　the　adult。　Amongst　many　animals；　sexual　selection　will　give　its　aid　to　ordinary　selection；　by　assuring　to　the　most　vigorous　and　best　adapted　males　the　greatest　number　of　offspring。　Sexual　selection　will　also　give　characters　useful　to　the　males　alone；　in　their　struggles　with　other　males。　
Whether　natural　selection　has　really　thus　acted　in　nature；　in　modifying　and　adapting　the　various　forms　of　life　to　their　several　conditions　and　stations；　must　be　judged　of　by　the　general　tenour　and　balance　of　evidence　given　in　the　following　chapters。　But　we　already　see　how　it　entails　extinction；　and　how　largely　extinction　has　acted　in　the　world's　history；　geology　plainly　declares。　Natural　selection；　also；　leads　to　divergence　of　character；　for　more　living　beings　can　be　supported　on　the　same　area　the　more　they　diverge　in　structure；　habits；　and　constitution；　of　which　we　see　proof　by　looking　at　the　inhabitants　of　any　small　spot　or　at　naturalised　productions。　Therefore　during　the　modification　of　the　descendants　of　any　one　species；　and　during　the　incessant　struggle　of　all　species　to　increase　in　numbers；　the　more　diversified　these　descendants　become；　the　better　will　be　their　chance　of　succeeding　in　the　battle　of　life。　Thus　the　small　differences　distinguishing　varieties　of　the　same　species；　will　steadily　tend　to　increase　till　they　come　to　equal　the　greater　differences　between　species　of　the　same　genus；　or　even　of　distinct　genera。　
We　have　seen　that　it　is　the　common；　the　widely…diffused；　and　widely…ranging　species；　belonging　to　the　larger　genera；　which　vary　most；　and　these　will　tend　to　transmit　to　their　modified　offspring　that　superiority　which　now　makes　them　dominant　in　their　own　countries。　Natural　selection；　as　has　just　been　remarked；　leads　to　divergence　of　character　and　to　much　extinction　of　the　less　improved　and　intermediate　forms　of　life。　On　these　principles；　I　believe；　the　nature　of　the　affinities　of　all　organic　beings　may　be　explained。　It　is　a　truly　wonderful　fact　the　wonder　of　which　we　are　apt　to　overlook　from　familiarity　that　all　animals　and　all　plants　throughout　all　time　and　space　should　be　related　to　each　other　in　group　subordinate　to　group；　in　the　manner　which　we　everywhere　behold　namely；　varieties　of　the　same　species　most　closely　related　together；　species　of　the　same　genus　less　closely　and　unequally　related　together；　forming　sections　and　sub…genera；　species　of　distinct　genera　much　less　closely　related；　and　genera　related　in　different　degrees；　forming　sub…families；　families；　orders；　sub…classes；　and　classes。　The　several　subordinate　groups　in　any　class　cannot　be　ranked　in　a　single　file；　but　seem　rather　to　be　clustered　round　points；　and　these　round　other　points；　and　so　on　in　almost　endless　cycles。　On　the　view　that　each　species　has　been　independently　created；　I　can　see　no　explanation　of　this　great　fact　in　the　classification　of　all　organic　beings；　but；　to　the　best　of　my　judgment；　it　is　explained　through　inheritance　and　the　complex　action　of　natural　selection；　entailing　extinction　and　divergence　of　character；　as　we　have　seen　illustrated　in　the　diagram。　
The　affinities　of　all　the　beings　of　the　same　class　have　sometimes　been　represented　by　a　great　tree。　I　believe　this　simile　largely　speaks　the　truth。　The　green　and　budding　twigs　may　represent　existing　species；　and　those　produced　during　each　former　year　may　represent　the　long　succession　of　extinct　species。　At　each　period　of　growth　all　the　growing　twigs　have　tried　to　branch　out　on　all　sides；　and　to　overtop　and　kill　the　surrounding　twigs　and　branches；　in　the　same　manner　as　species　and　groups　of　species　have　tried　to　overmaster　other　species　in　the　great　battle　fo