the origin of species(物种起源)-第41部分

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


ork；　by　supposing　masons　first　to　pile　up　a　broad　ridge　of　cement；　and　then　to　begin　cutting　it　away　equally　on　both　sides　near　the　ground；　till　a　smooth；　very　thin　wall　is　left　in　the　middle；　the　masons　always　piling　up　the　cut…away　cement；　and　adding　fresh　cement；　on　the　summit　of　the　ridge。　We　shall　thus　have　a　thin　wall　steadily　growing　upward；　but　always　crowned　by　a　gigantic　coping。　From　all　the　cells；　both　those　just　commenced　and　those　completed；　being　thus　crowned　by　a　strong　coping　of　wax；　the　bees　can　cluster　and　crawl　over　the　comb　without　injuring　the　delicate　hexagonal　walls；　which　are　only　about　one　four…hundredth　of　an　inch　in　thickness；　the　plates　of　the　pyramidal　basis　being　about　twice　as　thick。　By　this　singular　manner　of　building；　strength　is　continually　given　to　the　comb；　with　the　utmost　ultimate　economy　of　wax。　
It　seems　at　first　to　add　to　the　difficulty　of　understanding　how　the　cells　are　made；　that　a　multitude　of　bees　all　work　together；　one　bee　after　working　a　short　time　at　one　cell　going　to　another；　so　that；　as　Huber　has　stated；　a　score　of　individuals　work　even　at　the　commencement　of　the　first　cell。　I　was　able　practically　to　show　this　fact；　by　covering　the　edges　of　the　hexagonal　walls　of　a　single　cell；　or　the　extreme　margin　of　the　circumferential　rim　of　a　growing　comb；　with　an　extremely　thin　layer　of　melted　vermilion　wax；　and　I　invariably　found　that　the　colour　was　most　delicately　diffused　by　the　bees　as　delicately　as　a　painter　could　have　done　with　his　brush　by　atoms　of　the　coloured　wax　having　been　taken　from　the　spot　on　which　it　had　been　placed；　and　worked　into　the　growing　edges　of　the　cells　all　round。　The　work　of　construction　seems　to　be　a　sort　of　balance　struck　between　many　bees；　all　instinctively　standing　at　the　same　relative　distance　from　each　other；　all　trying　to　sweep　equal　spheres；　and　then　building　up；　or　leaving　ungnawed；　the　planes　of　intersection　between　these　spheres。　It　was　really　curious　to　note　in　cases　of　difficulty；　as　when　two　pieces　of　comb　met　at　an　angle；　how　often　the　bees　would　entirely　pull　down　and　rebuild　in　different　ways　the　same　cell；　sometimes　recurring　to　a　shape　which　they　had　at　first　rejected。　
When　bees　have　a　place　on　which　they　can　stand　in　their　proper　positions　for　working；　for　instance；　on　a　slip　of　wood；　placed　directly　under　the　middle　of　a　comb　growing　downwards　so　that　the　comb　has　to　be　built　over　one　face　of　the　slip　in　this　case　the　bees　can　lay　the　foundations　of　one　wall　of　a　new　hexagon；　in　its　strictly　proper　place；　projecting　beyond　the　other　completed　cells。　It　suffices　that　the　bees　should　be　enabled　to　stand　at　their　proper　relative　distances　from　each　other　and　from　the　walls　of　the　last　completed　cells；　and　then；　by　striking　imaginary　spheres；　they　can　build　up　a　wall　intermediate　between　two　adjoining　spheres；　but；　as　far　as　I　have　seen；　they　never　gnaw　away　and　finish　off　the　angles　of　a　cell　till　a　large　part　both　of　that　cell　and　of　the　adjoining　cells　has　been　built。　This　capacity　in　bees　of　laying　down　under　certain　circumstances　a　rough　wall　in　its　proper　place　between　two　just…commenced　cells；　is　important；　as　it　bears　on　a　fact；　which　seems　at　first　quite　subversive　of　the　foregoing　theory；　namely；　that　the　cells　on　the　extreme　margin　of　wasp…combs　are　sometimes　strictly　hexagonal；　but　I　have　not　space　here　to　enter　on　this　subject。　Nor　does　there　seem　to　me　any　great　difficulty　in　a　single　insect　（as　in　the　case　of　a　queen…wasp）　making　hexagonal　cells；　if　she　work　alternately　on　the　inside　and　outside　of　two　or　three　cells　commenced　at　the　same　time；　always　standing　at　the　proper　relative　distance　from　the　parts　of　the　cells　just　begun；　sweeping　spheres　or　cylinders；　and　building　up　intermediate　planes。　It　is　even　conceivable　that　an　insect　might；　by　fixing　on　a　point　at　which　to　commence　a　cell；　and　then　moving　outside；　first　to　one　point；　and　then　to　five　other　points；　at　the　proper　relative　distances　from　the　central　point　and　from　each　other；　strike　the　planes　of　intersection；　and　so　make　an　isolated　hexagon：　but　I　am　not　aware　that　any　such　case　has　been　observed；　nor　would　any　good　be　derived　from　a　single　hexagon　being　built；　as　in　its　construction　more　materials　would　be　required　than　for　a　cylinder。　
As　natural　selection　acts　only　by　the　accumulation　of　slight　modifications　of　structure　or　instinct；　each　profitable　to　the　individual　under　its　conditions　of　life；　it　may　reasonably　be　asked；　how　a　long　and　graduated　succession　of　modified　architectural　instincts；　all　tending　towards　the　present　perfect　plan　of　construction；　could　have　profited　the　progenitors　of　the　hive…bee？　I　think　the　answer　is　not　difficult：　it　is　known　that　bees　are　often　hard　pressed　to　get　sufficient　nectar；　and　I　am　informed　by　Mr。　Tegetmeier　that　it　has　been　experimentally　found　that　no　less　than　from　twelve　to　fifteen　pounds　of　dry　sugar　are　consumed　by　a　hive　of　bees　for　the　secretion　of　each　pound　of　wax；　so　that　a　prodigious　quantity　of　fluid　nectar　must　be　collected　and　consumed　by　the　bees　in　a　hive　for　the　secretion　of　the　wax　necessary　for　the　construction　of　their　combs。　Moreover；　many　bees　have　to　remain　idle　for　many　days　during　the　process　of　secretion。　A　large　store　of　honey　is　indispensable　to　support　a　large　stock　of　bees　during　the　winter；　and　the　security　of　the　hive　is　known　mainly　to　depend　on　a　large　number　of　bees　being　supported。　Hence　the　saving　of　wax　by　largely　saving　honey　must　be　a　most　important　element　of　success　in　any　family　of　bees。　Of　course　the　success　of　any　species　of　bee　may　be　dependent　on　the　number　of　its　parasites　or　other　enemies；　or　on　quite　distinct　causes；　and　so　be　altogether　independent　of　the　quantity　of　honey　which　the　bees　could　collect。　But　let　us　suppose　that　this　latter　circumstance　determined；　as　it　probably　often　does　determine；　the　numbers　of　a　humble…bee　which　could　exist　in　a　country；　and　let　us　further　suppose　that　the　community　lived　throughout　the　winter；　and　consequently　required　a　store　of　honey：　there　can　in　this　case　be　no　doubt　that　it　would　be　an　advantage　to　our　humble…bee；　if　a　slight　modification　of　her　instinct　led　her　to　make　her　waxen　cells　near　together；　so　as　to　intersect　a　little；　for　a　wall　in　common　even　to　two　adjoining　cells；　would　save　some　little　wax。　Hence　it　would　continually　be　more　and　more　advantageous　to　our　humble…bee；　if　she　were　to　make　her　cells　more　and　more　regular；　nearer　together；　and　aggregated　into　a　mass；　like　the　cells　of　the　Melipona；　for　in　this　case　a　large　part　of　the　bounding　surface　of　each　cell　would　serve　to　bound　other　cells；　and　much　wax　would　be　saved。　Again；　from　the　same　cause；　it　would　be　advantageous　to　the　Melipona；　if　she　were　to　make　her　cells　closer　together；　and　more　regular　in　every　way　than　at　present；　for　then；　as　we　have　seen；　the　spherical　surfaces　would　wholly　disappear；　and　would　all　be　replaced　by　plane　surfaces；　and　the　Melipona　would　make　a　comb　as　perfect　as　that　of　the　hive…bee。　Beyond　this　stage　of　perfection　in　architecture；　natural　selection　could　not　lead；　for　the　comb　of　the　hive…bee；　as　far　as　we　can　see；　is　absolutely　perfect　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　bette