the origin of species(物种起源)-第19节

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；　that　their　fertility　is　greatly　diminished　if　these　visits　be　prevented。　Now；　it　is　scarcely　possible　that　bees　should　fly　from　flower　to　flower；　and　not　carry　pollen　from　one　to　the　other；　to　the　great　good；　as　I　believe；　of　the　plant。　Bees　will　act　like　a　camel…hair　pencil；　and　it　is　quite　sufficient　just　to　touch　the　anthers　of　one　flower　and　then　the　stigma　of　another　with　the　same　brush　to　ensure　fertilisation；　but　it　must　not　be　supposed　that　bees　would　thus　produce　a　multitude　of　hybrids　between　distinct　species；　for　if　you　bring　on　the　same　brush　a　plant's　own　pollen　and　pollen　from　another　species；　the　former　will　have　such　a　prepotent　effect；　that　it　will　invariably　and　completely　destroy；　as　has　been　shown　by　G？rtner；　any　influence　from　the　foreign　pollen。　
When　the　stamens　of　a　flower　suddenly　spring　towards　the　pistil；　or　slowly　move　one　after　the　other　towards　it；　the　contrivance　seems　adapted　solely　to　ensure　self…fertilisation；　and　no　doubt　it　is　useful　for　this　end：　but；　the　agency　of　insects　is　often　required　to　cause　the　stamens　to　spring　forward；　as　K？lreuter　has　shown　to　be　the　case　with　the　barberry；　and　curiously　in　this　very　genus；　which　seems　to　have　a　special　contrivance　for　self…fertilisation；　it　is　well　known　that　if　very　closely…allied　forms　or　varieties　are　planted　near　each　other；　it　is　hardly　possible　to　raise　pure　seedlings；　so　largely　do　they　naturally　cross。　In　many　other　cases；　far　from　there　being　any　aids　for　self…fertilisation；　there　are　special　contrivances；　as　I　could　show　from　the　writings　of　C。　C。　Sprengel　and　from　my　own　observations；　which　effectually　prevent　the　stigma　receiving　pollen　from　its　own　flower：　for　instance；　in　Lobelia　fulgens；　there　is　a　really　beautiful　and　elaborate　contrivance　by　which　every　one　of　the　infinitely　numerous　pollen…granules　are　swept　out　of　the　conjoined　anthers　of　each　flower；　before　the　stigma　of　that　individual　flower　is　ready　to　receive　them；　and　as　this　flower　is　never　visited；　at　least　in　my　garden；　by　insects；　it　never　sets　a　seed；　though　by　placing　pollen　from　one　flower　on　the　stigma　of　another；　I　raised　plenty　of　seedlings；　and　whilst　another　species　of　Lobelia　growing　close　by；　which　is　visited　by　bees；　seeds　freely。　In　very　many　other　cases；　though　there　be　no　special　mechanical　contrivance　to　prevent　the　stigma　of　a　flower　receiving　its　own　pollen；　yet；　as　C。　C。　Sprengel　has　shown；　and　as　I　can　confirm；　either　the　anthers　burst　before　the　stigma　is　ready　for　fertilisation；　or　the　stigma　is　ready　before　the　pollen　of　that　flower　is　ready；　so　that　these　plants　have　in　fact　separated　sexes；　and　must　habitually　be　crossed。　How　strange　are　these　facts！　How　strange　that　the　pollen　and　stigmatic　surface　of　the　same　flower；　though　placed　so　close　together；　as　if　for　the　very　purpose　of　self…fertilisation；　should　in　so　many　cases　be　mutually　useless　to　each　other！　How　simply　are　these　facts　explained　on　the　view　of　an　occasional　cross　with　a　distinct　individual　being　advantageous　or　indispensable！　
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　confine