the origin of species(物种起源)-第25节

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iations　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　for　life。　The　limbs　divided　into　great　branches；　and　these　into　lesser　and　lesser　branches；　were　themselves　once；　when　the　tree　was　small；　budding　twigs；　and　this　connexion　of　the　former　and　present　buds　by　ramifying　branches　may　well　represent　the　classification　of　all　extinct　and　living　species　in　groups　subordinate　to　groups。　Of　the　many　twigs　which　flourished　when　the　tree　was　a　mere　bush；　only　two　or　three；　now　grown　into　great　branches；　yet　survive　and　bear　all　the　other　branches；　so　with　the　species　which　lived　during　long…past　geological　periods；　very　few　now　have　living　and　modified　descendants。　From　the　first　growth　of　the　tree；　many　a　limb　and　branch　has　decayed　and　dropped　off；　and　these　lost　branches　of　various　sizes　may　represent　those　whole　orders；　families；　and　genera　which　have　now　no　living　representatives；　and　which　are　known　to　us　only　from　having　been　found　in　a　fossil　state。　As　we　here　and　there　see　a　thin　straggling　branch　springing　from　a　fork　low　down　in　a　tree；　and　which　by　some　chance　has　been　favoured　and　is　still　alive　on　its　summit；　so　we　occasionally　see　an　animal　like　the　Ornithorhynchus　or　Lepidosiren；　which　in　some　small　degree　connects　by　its　affinities　two　large　branches　of　life；　and　which　has　apparently　been　saved　from　fatal　competition　by　having　inhabited　a　protected　station。　As　buds　give　rise　by　growth　to　fresh　buds；　and　these；　if　vigorous；　branch　out　and　overtop　on　all　sides　many　a　feebler　branch；　so　by　generation　I　believe　it　has　been　with　the　great　Tree　of　Life；　which　fills　with　its　dead　and　broken　branches　the　crust　of　the　earth；　and　covers　the　surface　with　its　ever　branching　and　beautiful　ramifications。　　

Chapter　5　…　Laws　of　Variation

I　HAVE　hitherto　sometimes　spoken　as　if　the　variations　so　common　and　multiform　in　organic　beings　under　domestication；　and　in　a　lesser　degree　in　those　in　a　state　of　nature　had　been　due　to　chance。　This；　of　course；　is　a　wholly　incorrect　expression；　but　it　serves　to　acknowledge　plainly　our　ignorance　of　the　cause　of　each　particular　variation。　Some　authors　believe　it　to　be　as　much　the　function　of　the　reproductive　system　to　produce　individual　differences；　or　very　slight　deviations　of　structure；　as　to　make　the　child　like　its　parents。　But　the　much　greater　variability；　as　well　as　the　greater　frequency　of　monstrosities；　under　domestication　or　cultivation；　than　under　nature；　leads　me　to　believe　that　deviations　of　structure　are　in　some　way　due　to　the　nature　of　the　conditions　of　life；　to　which　the　parents　and　their　more　remote　ancestors　have　been　exposed　during　several　generations。　I　have　remarked　in　the　first　chapter　but　a　long　catalogue　of　facts　which　cannot　be　here　given　would　be　necessary　to　show　the　truth　of　the　remark　that　the　reproductive　system　is　eminently　susceptible　to　changes　in　the　conditions　of　life；　and　to　this　system　being　functionally　disturbed　in　the　parents；　I　chiefly　attribute　the　varying　or　plastic　condition　of　the　offspring。　The　male　and　female　sexual　elements　seem　to　be　affected　before　that　union　takes　place　which　is　to　form　a　new　being。　In　the　case　of　'sporting'　plants；　the　bud；　which　in　its　earliest　condition　does　not　apparently　differ　essentially　from　an　ovule；　is　alone　affected。　But　why；　because　the　reproductive　system　is　disturbed；　this　or　that　part　should　vary　more　or　less；　we　are　profoundly　ignorant。　Nevertheless；　we　can　here　and　there　dimly　catch　a　faint　ray　of　light；　and　we　may　feel　sure　that　there　must　be　some　cause　for　each　deviation　of　structure；　however　slight。　
How　much　direct　effect　difference　of　climate；　food；　&c。；　produces　on　any　being　is　extremely　doubtful。　My　impression　is；　that　the　effect　is　extremely　small　in　the　case　of　animals；　but　perhaps　rather　more　in　that　of　plants。　We　may；　at　least；　safely　conclude　that　such　influences　cannot　have　produced　the　many　striking　and　complex　co…adaptations　of　structure　between　one　organic　being　and　another；　which　we　see　everywhere　throughout　nature。　Some　little　influence　may　be　attributed　to　climate；　food；　&c。：　thus；　E。　Forbes　speaks　confidently　that　shells　at　their　southern　limit；　and　when　living　in　shallow　water；　are　more　brightly　coloured　than　those　of　the　same　species　further　north　or　from　greater　depths。　Gould　believes　that　birds　of　the　same　species　are　more　brightly　coloured　under　a　clear　atmosphere；　than　when　living　on　islands　or　near　the　coast。　So　with　insects；　Wollaston　is　convinced　that　residence　near　the　sea　affects　their　colours。　Moquin…Tandon　gives　a　list　of　plants　which　when　growing　near　the　sea…shore　have　their　leaves　in　some　degree　fleshy；　though　not　elsewhere　fleshy。　Several　other　such　cases　could　be　given。　
The　fact　of　varieties　of　one　species；　when　they　range　into　the　zone　of　habitation　of　other　species；　often　acquiring　in　a　very　slight　degree　some　of　the　characters　of　such　species；　accords　with　our　view　that　species　of　all　kinds　are　only　well…marked　and　permanent　varieties。　Thus　the　species　of　shells　which　are　confined　to　tropical　and　shallow　seas　are　generally　brighter…coloured　than　those　confined　to　cold　and　deeper　seas。　The　birds　which　are　confined　to　continen