the origin of species(物种起源)-第38章

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fect　in　economising　wax。　
Thus；　as　I　believe；　the　most　wonderful　of　all　known　instincts；　that　of　the　hive…bee；　can　be　explained　by　natural　selection　having　taken　advantage　of　numerous；　successive；　slight　modifications　of　simpler　instincts；　natural　selection　having　by　slow　degrees；　more　and　more　perfectly；　led　the　bees　to　sweep　equal　spheres　at　a　given　distance　from　each　other　in　a　double　layer；　and　to　build　up　and　excavate　the　wax　along　the　planes　of　intersection。　The　bees；　of　course；　no　more　knowing　that　they　swept　their　spheres　at　one　particular　distance　from　each　other；　than　they　know　what　are　the　several　angles　of　the　hexagonal　prisms　and　of　the　basal　rhombic　plates。　The　motive　power　of　the　process　of　natural　selection　having　been　economy　of　wax；　that　individual　swarm　which　wasted　least　honey　in　the　secretion　of　wax；　having　succeeded　best；　and　having　transmitted　by　inheritance　its　newly　acquired　economical　instinct　to　new　swarms；　which　in　their　turn　will　have　had　the　best　chance　of　succeeding　in　the　struggle　for　existence。　
No　doubt　many　instincts　of　very　difficult　explanation　could　be　opposed　to　the　theory　of　natural　selection；　cases；　in　which　we　cannot　see　how　an　instinct　could　possibly　have　originated；　cases；　in　which　no　intermediate　gradations　are　known　to　exist；　cases　of　instinct　of　apparently　such　trifling　importance；　that　they　could　hardly　have　been　acted　on　by　natural　selection；　cases　of　instincts　almost　identically　the　same　in　animals　so　remote　in　the　scale　of　nature；　that　we　cannot　account　for　their　similarity　by　inheritance　from　a　common　parent；　and　must　therefore　believe　that　they　have　been　acquired　by　independent　acts　of　natural　selection。　I　will　not　here　enter　on　these　several　cases；　but　will　confine　myself　to　one　special　difficulty；　which　at　first　appeared　to　me　insuperable；　and　actually　fatal　to　my　whole　theory。　I　allude　to　the　neuters　or　sterile　females　in　insect…communities：　for　these　neuters　often　differ　widely　in　instinct　and　in　structure　from　both　the　males　and　fertile　females；　and　yet；　from　being　sterile；　they　cannot　propagate　their　kind。　
The　subject　well　deserves　to　be　discussed　at　great　length；　but　I　will　here　take　only　a　single　case；　that　of　working　or　sterile　ants。　How　the　workers　have　been　rendered　sterile　is　a　difficulty；　but　not　much　greater　than　that　of　any　other　striking　modification　of　structure；　for　it　can　be　shown　that　some　insects　and　other　articulate　animals　in　a　state　of　nature　occasionally　become　sterile；　and　if　such　insects　had　been　social；　and　it　had　been　profitable　to　the　community　that　a　number　should　have　been　annually　born　capable　of　work；　but　incapable　of　procreation；　I　can　see　no　very　great　difficulty　in　this　being　effected　by　natural　selection。　But　I　must　pass　over　this　preliminary　difficulty。　The　great　difficulty　lies　in　the　working　ants　differing　widely　from　both　the　males　and　the　fertile　females　in　structure；　as　in　the　shape　of　the　thorax　and　in　being　destitute　of　wings　and　sometimes　of　eyes；　and　in　instinct。　As　far　as　instinct　alone　is　concerned；　the　prodigious　difference　in　this　respect　between　the　workers　and　the　perfect　females；　would　have　been　far　better　exemplified　by　the　hive…bee。　If　a　working　ant　or　other　neuter　insect　had　been　an　animal　in　the　ordinary　state；　I　should　have　unhesitatingly　assumed　that　all　its　characters　had　been　slowly　acquired　through　natural　selection；　namely；　by　an　individual　having　been　born　with　some　slight　profitable　modification　of　structure；　this　being　inherited　by　its　offspring；　which　again　varied　and　were　again　selected；　and　so　onwards。　But　with　the　working　ant　we　have　an　insect　differing　greatly　from　its　parents；　yet　absolutely　sterile；　so　that　it　could　never　have　transmitted　successively　acquired　modifications　of　structure　or　instinct　to　its　progeny。　It　may　well　be　asked　how　is　it　possible　to　reconcile　this　case　with　the　theory　of　natural　selection？　
First；　let　it　be　remembered　that　we　have　innumerable　instances；　both　in　our　domestic　productions　and　in　those　in　a　state　of　nature；　of　all　sorts　of　differences　of　structure　which　have　become　correlated　to　certain　ages；　and　to　either　sex。　We　have　differences　correlated　not　only　to　one　sex；　but　to　that　short　period　alone　when　the　reproductive　system　is　active；　as　in　the　nuptial　plumage　of　many　birds；　and　in　the　hooked　jaws　of　the　male　salmon。　We　have　even　slight　differences　in　the　horns　of　different　breeds　of　cattle　in　relation　to　an　artificially　imperfect　state　of　the　male　sex；　for　oxen　of　certain　breeds　have　longer　horns　than　in　other　breeds；　in　comparison　with　the　horns　of　the　bulls　or　cows　of　these　same　breeds。　Hence　I　can　see　no　real　difficulty　in　any　character　having　become　correlated　with　the　sterile　condition　of　certain　members　of　insect…communities：　the　difficulty　lies　in　understanding　how　such　correlated　modifications　of　structure　could　have　been　slowly　accumulated　by　natural　selection。　
This　difficulty；　though　appearing　insuperable；　is　lessened；　or；　as　I　believe；　disappears；　when　it　is　remembered　that　selection　may　be　applied　to　the　family；　as　well　as　to　the　individual；　and　may　thus　gain　the　desired　end。　Thus；　a　well…flavoured　vegetable　is　cooked；　and　the　individual　is　destroyed；　but　the　horticulturist　sows　seeds　of　the　same　stock；　and　confidently　expects　to　get　nearly　the　same　variety；　breeders　of　cattle　wish　the　flesh　and　fat　to　be　well　marbled　together；　the　animal　has　been　slaughtered；　but　the　breeder　goes　with　confidence　to　the　same　family。　I　have　such　faith　in　the　powers　of　selection；　that　I　do　not　doubt　that　a　breed　of　cattle；　always　yielding　oxen　with　extraordinarily　long　horns；　could　be　slowly　formed　by　carefully　watching　which　individual　bulls　and　cows；　when　matched；　produced　oxen　with　the　longest　horns；　and　yet　no　one　ox　could　ever　have　propagated　its　kind。　Thus　I　believe　it　has　been　with　social　insects：　a　slight　modification　of　structure；　or　instinct；　correlated　with　the　sterile　condition　of　certain　members　of　the　community；　has　been　advantageous　to　the　community：　consequently　the　fertile　males　and　females　of　the　same　community　flourished；　and　transmitted　to　their　fertile　offspring　a　tendency　to　produce　sterile　members　having　the　same　modification。　And　I　believe　that　this　process　has　been　repeated；　until　that　prodigious　amount　of　difference　between　the　fertile　and　sterile　females　of　the　same　species　has　been　produced；　which　we　see　in　many　social　insects。　
But　we　have　not　as　yet　touched　on　the　climax　of　the　difficulty；　namely；　the　fact　that　the　neuters　of　several　ants　differ；　not　only　from　the　fertile　females　and　males；　but　from　each　other；　sometimes　to　an　almost　incredible　degree；　and　are　thus　divided　into　two　or　even　three　castes。　The　castes；　moreover；　do　not　generally　graduate　into　each　other；　but　are　perfectly　well　defined；　being　as　distinct　from　each　other；　as　are　any　two　species　of　the　same　genus；　or　rather　as　any　two　genera　of　the　same　family。　Thus　in　Eciton；　there　are　working　and　soldier　neuters；　with　jaws　and　instincts　extraordinarily　different：　in　Cryptocerus；　the　workers　of　one　caste　alone　carry　a　wonderful　sort　of　shield　on　their　heads；　the　use　of　which　is　quite　unknown：　in　the　Mexican　Myrmecocystus；　the　workers　of　one　caste　never　leave　the　nest；　they　are　fed　by　the　workers　of　another　caste；　and　they　have　an　enormously　developed　abdomen　which　secretes　a　sort　of　honey；　supplying　the　place　of　that　excreted　by　the　aphides；　or　the　domestic　cattle　as　they　may　be　called；　which　our　European　ants　guard　or　imprison。　
It　will　indeed　be　thought　that　I　have　an　overweening　confidence　in　the　principle　of　natural　selection；　when　I　do　not　admit　that　such　wonderful　and　well…established　facts　at　once　annihilate　my　theory。　In　the　simpler　case　of　neuter　insects　all　of　one　caste　or　of　the　same　kind；　which　have　been　rendered　by　natural　selection；　as　I　believe　to　be　quite　possible；　different　from　the　fertile　males　and　females；　in　this　case；　we　may　safely　conclude　from　the　analogy　of　ordinary　variations；　that　each　successive；　slight；　profitable　modification　did　not　probably　at　first　appear　in　all　the　individual　neuters　in　the　same　nest；　but　in　a　few　alone；　and　that　by　the　long…continued　selection　of　the　fertile　parents　which　produced　most　neuters　with　the　profitable　modification；　all　the　neuters　ultimately　came　to　have　the　desired　character。　On　this　view　we　ought　occasionally　to　find　neuter…insects　of　the　same　species；　in　the　same　nest；　presenting　gradations　of　structure；　and　this　we　do　find；　even　often；　considering　how　few　neuter…insects　out　of　Europe　have　been　carefully　examined。　Mr　F。　Smith　has　shown　how　surprisingly　the　neuters　of　several　British　ants　differ　from　each　other　in　size　and　sometimes　in　colour；　and　that　the　extreme　forms　can　sometimes　be　perfectly　linked　together　by　individuals　taken　out　of　the　same　nest：　I　have　myself　compared　perfect　gradations　of　this　kind。　It　often　happens　that　the　larger　or　the　smaller　sized　workers　are　the　most　numerous；　or　that　both　large　and　small　are　numerous；　with　those　of　an　intermediate　size　scanty　in　numbers。　Formica　flava　has　larger　and　smaller　workers；　with　some　of　intermediate　size；　and；　in　this　species；　as　Mr　F。　Smith　has　observed；　the　larger　workers　have　simple　eyes　（ocelli）；　which　though　small　can　be　plainly　distinguished；　whereas　the　smaller　workers　have　their　ocelli　rudimentary。　Having　carefully　dissected　several　specimens　of　these　workers；　I　can　affirm　that　the　eyes　are　far　more　rudimentary　in　the　smaller　workers　than　can　be　accounted　for　merely　by　their　proportionally　lesser　size；　and　I　fully　believe；　though　I　dare　not　assert　so　positively；　that　the　workers　of　intermediate　size　have　their　ocelli　in　an　exactly　intermediate　condition。　So　that　we　here　have　two　bodies　of　sterile　workers　in　the　same　nest；　differing　not　only　in　size；　but　in　their　organs　of　vision；　yet　connected　by　some　few　members　in　an　intermediate　condition。　I　may　digress　by　adding；　that　if　the　smaller　workers　had　been　the　most　useful　to　the　community；　and　those　males　and　females　had　been　continually　selected；　which　produced　more　and　more　of　the　smaller　workers；　until　all　the　workers　had　come　to　be　in　this　condition；　we　should　then　have　had　a　species　of　ant　with　neuters　very　nearly　in　the　same　condition　with　those　of　Myrmica。　For　the　workers　of　Myrmica　have　not　even　rudiments　of　ocelli；　though　the　male　and　female　ants　of　this　genus　have　well…developed　ocelli。　
I　may　give　one　other　case：　so　confidently　did　I　expect　to　find　gradations　in　important　points　of　structure　between　the　different　castes　of　neuters　in　the　same　species；　that　I　gladly　availed　myself　of　Mr　F。　Smith's　offer　of　numerous　specimens　from　the　same　nest　of　the　driver　ant　（Anomma）　of　West　Africa。　The　reader　will　perhaps　best　appreciate　the　amount　of　difference　in　these　workers；　by　my　giving　not　the　actual　measurements；　but　a　strictly　accurate　illustration：　the　difference　was　the　same　as　if　we　were　to　see　a　set　of　workmen　building　a　house　of　whom　many　were　five　feet　four　inches　high；　and　many　sixteen　feet　high；　but　we　must　suppose　that　the　larger　workmen　had　heads　four　instead　of　three　times　as　big　as　those　of　the　smaller　men；　and　jaws　nearly　five　times　as　big。　The　jaws；　moreover；　of　the　working　ants　of　the　several　sizes　differed　wonderfully　in　shape；　and　in　the　form　and　number　of　the　teeth。　But　the　important　fact　fo