the origin of species(物种起源)-第16章

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If　several　varieties　of　the　cabbage；　radish；　onion；　and　of　some　other　plants；　be　allowed　to　seed　near　each　other；　a　large　majority；　as　I　have　found；　of　the　seedlings　thus　raised　will　turn　out　mongrels：　for　instance；　I　raised　233　seedling　cabbages　from　some　plants　of　different　varieties　growing　near　each　other；　and　of　these　only　78　were　true　to　their　kind；　and　some　even　of　these　were　not　perfectly　true。　Yet　the　pistil　of　each　cabbage…flower　is　surrounded　not　only　by　its　own　six　stamens；　but　by　those　of　the　many　other　flowers　on　the　same　plant。　How；　then；　comes　it　that　such　a　vast　number　of　the　seedlings　are　mongrelised？　I　suspect　that　it　must　arise　from　the　pollen　of　a　distinct　variety　having　a　prepotent　effect　over　a　flower's　own　pollen；　and　that　this　is　part　of　the　general　law　of　good　being　derived　from　the　intercrossing　of　distinct　individuals　of　the　same　species。　When　distinct　species　are　crossed　the　case　is　directly　the　reverse；　for　a　plant's　own　pollen　is　always　prepotent　over　foreign　pollen；　but　to　this　subject　we　shall　return　in　a　future　chapter。　
In　the　case　of　a　gigantic　tree　covered　with　innumerable　flowers；　it　may　be　objected　that　pollen　could　seldom　be　carried　from　tree　to　tree；　and　at　most　only　from　flower　to　flower　on　the　same　tree；　and　that　flowers　on　the　same　tree　can　be　considered　as　distinct　individuals　only　in　a　limited　sense。　I　believe　this　objection　to　be　valid；　but　that　nature　has　largely　provided　against　it　by　giving　to　trees　a　strong　tendency　to　bear　flowers　with　separated　sexes。　When　the　sexes　are　separated；　although　the　male　and　female　flowers　may　be　produced　on　the　same　tree；　we　can　see　that　pollen　must　be　regularly　carried　from　flower　to　flower；　and　this　will　give　a　better　chance　of　pollen　being　occasionally　carried　from　tree　to　tree。　That　trees　belonging　to　all　Orders　have　their　sexes　more　often　separated　than　other　plants；　I　find　to　be　the　case　in　this　country；　and　at　my　request　Dr　Hooker　tabulated　the　trees　of　New　Zealand；　and　Dr　Asa　Gray　those　of　the　United　States；　and　the　result　was　as　I　anticipated。　On　the　other　hand；　Dr　Hooker　has　recently　informed　me　that　he　finds　that　the　rule　does　not　hold　in　Australia；　and　I　have　made　these　few　remarks　on　the　sexes　of　trees　simply　to　call　attention　to　the　subject。　
Turning　for　a　very　brief　space　to　animals：　on　the　land　there　are　some　hermaphrodites；　as　land…mollusca　and　earth…worms；　but　these　all　pair。　As　yet　I　have　not　found　a　single　case　of　a　terrestrial　animal　which　fertilises　itself。　We　can　understand　this　remarkable　fact；　which　offers　so　strong　a　contrast　with　terrestrial　plants；　on　the　view　of　an　occasional　cross　being　indispensable；　by　considering　the　medium　in　which　terrestrial　animals　live；　and　the　nature　of　the　fertilising　element；　for　we　know　of　no　means；　analogous　to　the　action　of　insects　and　of　the　wind　in　the　case　of　plants；　by　which　an　occasional　cross　could　be　effected　with　terrestrial　animals　without　the　concurrence　of　two　individuals。　Of　aquatic　animals；　there　are　many　self…fertilising　hermaphrodites；　but　here　currents　in　the　water　offer　an　obvious　means　for　an　occasional　cross。　And；　as　in　the　case　of　flowers；　I　have　as　yet　failed；　after　consultation　with　one　of　the　highest　authorities；　namely；　Professor　Huxley；　to　discover　a　single　case　of　an　hermaphrodite　animal　with　the　organs　of　reproduction　so　perfectly　enclosed　within　the　body；　that　access　from　without　and　the　occasional　influence　of　a　distinct　individual　can　be　shown　to　be　physically　impossible。　Cirripedes　long　appeared　to　me　to　present　a　case　of　very　great　difficulty　under　this　point　of　view；　but　I　have　been　enabled；　by　a　fortunate　chance；　elsewhere　to　prove　that　two　individuals；　though　both　are　self…fertilising　hermaphrodites；　do　sometimes　cross。　
It　must　have　struck　most　naturalists　as　a　strange　anomaly　that；　in　the　case　of　both　animals　and　plants；　species　of　the　same　family　and　even　of　the　same　genus；　though　agreeing　closely　with　each　other　in　almost　their　whole　organisation；　yet　are　not　rarely；　some　of　them　hermaphrodites；　and　some　of　them　unisexual。　But　if；　in　fact；　all　hermaphrodites　do　occasionally　intercross　with　other　individuals；　the　difference　between　hermaphrodites　and　unisexual　species；　as　far　as　function　is　concerned；　becomes　very　small。　
From　these　several　considerations　and　from　the　many　special　facts　which　I　have　collected；　but　which　I　am　not　here　able　to　give；　I　am　strongly　inclined　to　suspect　that；　both　in　the　vegetable　and　animal　kingdoms；　an　occasional　intercross　with　a　distinct　individual　is　a　law　of　nature。　I　am　well　aware　that　there　are；　on　this　view；　many　cases　of　difficulty；　some　of　which　I　am　trying　to　investigate。　Finally　then；　we　may　conclude　that　in　many　organic　beings；　a　cross　between　two　individuals　is　an　obvious　necessity　for　each　birth；　in　many　others　it　occurs　perhaps　only　at　long　intervals；　but　in　none；　as　I　suspect；　can　self…fertilisation　go　on　for　perpetuity。　
Circumstances　favourable　to　Natural　Selection
This　is　an　extremely　intricate　subject。　A　large　amount　of　inheritable　and　diversified　variability　is　favourable；　but　I　believe　mere　individual　differences　suffice　for　the　work。　A　large　number　of　individuals；　by　giving　a　better　chance　for　the　appearance　within　any　given　period　of　profitable　variations；　will　compensate　for　a　lesser　amount　of　variability　in　each　individual；　and　is；　I　believe；　an　extremely　important　element　of　success。　Though　nature　grants　vast　periods　of　time　for　the　work　of　natural　selection；　she　does　not　grant　an　indefinite　period；　for　as　all　organic　beings　are　striving；　it　may　be　said；　to　seize　on　each　place　in　the　economy　of　nature；　if　any　one　species　does　not　become　modified　and　improved　in　a　corresponding　degree　with　its　competitors；　it　will　soon　be　exterminated。　
In　man's　methodical　selection；　a　breeder　selects　for　some　definite　object；　and　free　intercrossing　will　wholly　stop　his　work。　But　when　many　men；　without　intending　to　alter　the　breed；　have　a　nearly　common　standard　of　perfection；　and　all　try　to　get　and　breed　from　the　best　animals；　much　improvement　and　modification　surely　but　slowly　follow　from　this　unconscious　process　of　selection；　notwithstanding　a　large　amount　of　crossing　with　inferior　animals。　Thus　it　will　be　in　nature；　for　within　a　confined　area；　with　some　place　in　its　polity　not　so　perfectly　occupied　as　might　be；　natural　selection　will　always　tend　to　preserve　all　the　individuals　varying　in　the　right　direction；　though　in　different　degrees；　so　as　better　to　fill　up　the　unoccupied　place。　But　if　the　area　be　large；　its　several　districts　will　almost　certainly　present　different　conditions　of　life；　and　then　if　natural　selection　be　modifying　and　improving　a　species　in　the　several　districts；　there　will　be　intercrossing　with　the　other　individuals　of　the　same　species　on　the　confines　of　each。　And　in　this　case　the　effects　of　intercrossing　can　hardly　be　counterbalanced　by　natural　selection　always　tending　to　modify　all　the　individuals　in　each　district　in　exactly　the　same　manner　to　the　conditions　of　each；　for　in　a　continuous　area；　the　conditions　will　generally　graduate　away　insensibly　from　one　district　to　another。　The　intercrossing　will　most　affect　those　animals　which　unite　for　each　birth；　which　wander　much；　and　which　do　not　breed　at　a　very　quick　rate。　Hence　in　animals　of　this　nature；　for　instance　in　birds；　varieties　will　generally　be　confined　to　separated　countries；　and　this　I　believe　to　be　the　case。　In　hermaphrodite　organisms　which　cross　only　occasionally；　and　likewise　in　animals　which　unite　for　each　birth；　but　which　wander　little　and　which　can　increase　at　a　very　rapid　rate；　a　new　and　improved　variety　might　be　quickly　formed　on　any　one　spot；　and　might　there　maintain　itself　in　a　body；　so　that　whatever　intercrossing　took　place　would　be　chiefly　between　the　individuals　of　the　same　new　variety。　A　local　variety　when　once　thus　formed　might　subsequently　slowly　spread　to　other　districts。　On　the　above　principle；　nurserymen　always　prefer　getting　seed　from　a　large　body　of　plants　of　the　same　variety；　as　the　chance　of　intercrossing　with　other　varieties　is　thus　lessened。　
Even　in　the　case　of　slow…breeding　animals；　which　unite　for　each　birth；　we　must　not　overrate　the　effects　of　intercrosses　in　retarding　natural　selection；　for　I　can　bring　a　considerable　catalogue　of　facts；　showing　that　within　the　same　area；　varieties　of　the　same　animal　can　long　remain　distinct；　from　haunting　different　stations；　from　breeding　at　slightly　different　seasons；　or　from　varieties　of　the　same　kind　preferring　to　pair　together。　
Intercrossing　plays　a　very　important　part　in　nature　in　keeping　the　individuals　of　the　same　species；　or　of　the　same　variety；　true　and　uniform　in　character。　It　will　obviously　thus　act　far　more　efficiently　with　those　animals　which　unite　for　each　birth；　but　I　have　already　attempted　to　show　that　we　have　reason　to　believe　that　occasional　intercrosses　take　place　with　all　animals　and　with　all　plants。　Even　if　these　take　place　only　at　long　intervals；　I　am　convinced　that　the　young　thus　produced　will　gain　so　much　in　vigour　and　fertility　over　the　offspring　from　long…continued　self…fertilisation；　that　they　will　have　a　better　chance　of　surviving　and　propagating　their　kind；　and　thus；　in　the　long　run；　the　influence　of　intercrosses；　even　at　rare　intervals；　will　be　great。　If　there　exist　organic　beings　which　never　intercross；　uniformity　of　character　can　be　retained　amongst　them；　as　long　as　their　conditions　of　life　remain　the　same；　only　through　the　principle　of　inheritance；　and　through　natural　selection　destroying　any　which　depart　from　the　proper　type；　but　if　their　conditions　of　life　change　and　they　undergo　modification；　uniformity　of　character　can　be　given　to　their　modified　offspring；　solely　by　natural　selection　preserving　the　same　favourable　variations。　
Isolation；　also；　is　an　important　element　in　the　process　of　natural　selection。　In　a　confined　or　isolated　area；　if　not　very　large；　the　organic　and　inorganic　conditions　of　life　will　generally　be　in　a　great　degree　uniform；　so　that　natural　selection　will　tend　to　modify　all　the　individuals　of　a　varying　species　throughout　the　area　in　the　same　manner　in　relation　to　the　same　conditions。　Intercrosses；　also；　with　the　individuals　of　the　same　species；　which　otherwise　would　have　inhabited　the　surrounding　and　differently　circumstanced　districts；　will　be　prevented。　But　isolation　probably　acts　more　efficiently　in　checking　the　immigration　of　better　adapted　organisms；　after　any　physical　change；　such　as　of　climate　or　elevation　of　the　land；　&c。；　and　thus　new　places　in　the　natural　economy　of　the　country　are　left　open　for　the　old　inhabitants　to　struggle　for；　and　become　adapted　to；　through　modifications　in　their　structure　and　constitution。　Lastly；　isolation；　by　checking　immigration　and　consequently　competition；　will　give　time　for　any　new　variety　to　be　slowly　improved；　and　this　may　sometimes　be　of　importance　in　the　production　of　new　species。　If；　however；　an　isolated　area　be　very　small；　either　from　being　surrounded　by　barriers；　or　from　having　very　peculiar　physical　conditions；　the　total　number　of　the　individuals　supported　on　it　will　necessarily　be　very　small；　and　fewness　of　individuals　will　greatly　retard　the　production　of　new　species　through　natural　selection；　by　decreasing　the　chance　of　the　appearance　of　favourable　variations。　
If　we　turn　to　nat