ave　its　nutriment　wasted　in　building　up　an　useless　structure。　I　can　thus　only　understand　a　fact　with　which　I　was　much　struck　when　examining　cirripedes；　and　of　which　many　other　instances　could　be　given：　namely；　that　when　a　cirripede　is　parasitic　within　another　and　is　thus　protected；　it　loses　more　or　less　completely　its　own　shell　or　carapace。　This　is　the　case　with　the　male　Ibla；　and　in　a　truly　extraordinary　manner　with　the　Proteolepas：　for　the　carapace　in　all　other　cirripedes　consists　of　the　three　highly…important　anterior　segments　of　the　head　enormously　developed；　and　furnished　with　great　nerves　and　muscles；　but　in　the　parasitic　and　protected　Proteolepas；　the　whole　anterior　part　of　the　head　is　reduced　to　the　merest　rudiment　attached　to　the　bases　of　the　prehensile　antennae。　Now　the　saving　of　a　large　and　complex　structure；　when　rendered　superfluous　by　the　parasitic　habits　of　the　Proteolepas；　though　effected　by　slow　steps；　would　be　a　decided　advantage　to　each　successive　individual　of　the　species；　for　in　the　struggle　for　life　to　which　every　animal　is　exposed；　each　individual　Proteolepas　would　have　a　better　chance　of　supporting　itself；　by　less　nutriment　being　wasted　in　developing　a　structure　now　become　useless。
  Thus；　as　I　believe；　natural　selection　will　always　succeed　in　the　long　run　in　reducing　and　saving　every　part　of　the　organisation；　as　soon　as　it　is　rendered　superfluous；　without　by　any　means　causing　some　other　part　to　be　largely　developed　in　a　corresponding　degree。　And；　conversely；　that　natural　selection　may　perfectly　well　succeed　in　largely　developing　any　organ；　without　requiring　as　a　necessary　compensation　the　reduction　of　some　adjoining　part。
  It　seems　to　be　a　rule；　as　remarked　by　Is。　Geoffroy　St　Hilaire；　both　in　varieties　and　in　species；　that　when　any　part　or　organ　is　repeated　many　times　in　the　structure　of　the　same　individual　（as　the　vertebrae　in　snakes；　and　the　stamens　in　polyandrous　flowers）　the　number　is　variable；　whereas　the　number　of　the　same　part　or　organ；　when　it　occurs　in　lesser　numbers；　is　constant。　The　same　author　and　some　botanists　have　further　remarked　that　multiple　parts　are　also　very　liable　to　variation　in　structure。　Inasmuch　as　this　'vegetative　repetition；'　to　use　Prof。　Owen's　expression；　seems　to　be　a　sign　of　low　organisation；　the　foregoing　remark　seems　connected　with　the　very　general　opinion　of　naturalists；　that　beings　low　in　the　scale　of　nature　are　more　variable　than　those　which　are　higher。　I　presume　that　lowness　in　this　case　means　that　the　several　parts　of　the　organisation　have　been　but　little　specialised　for　particular　functions；　and　as　long　as　the　same　part　has　to　perform　diversified　work；　we　can　perhaps　see　why　it　should　remain　variable；　that　is；　why　natural　selection　should　have　preserved　or　rejected　each　little　deviation　of　form　less　carefully　than　when　the　part　has　to　serve　for　one　special　purpose　alone。　In　the　same　way　that　a　knife　which　has　to　cut　all　sorts　of　things　may　be　of　almost　any　shape；　whilst　a　tool　for　some　particular　object　had　better　be　of　some　particular　shape。　Natural　selection；　it　should　never　be　forgotten；　can　act　on　each　part　of　each　being；　solely　through　and　for　its　advantage。
  Rudimentary　parts；　it　has　been　stated　by　some　authors；　and　I　believe　with　truth；　are　apt　to　be　highly　variable。　We　shall　have　to　recur　to　the　general　subject　of　rudimentary　and　aborted　organs；　and　I　will　here　only　add　that　their　variability　seems　to　be　owing　to　their　uselessness；　and　therefore　to　natural　selection　having　no　power　to　check　deviations　in　their　structure。　Thus　rudimentary　parts　are　left　to　the　free　play　of　the　various　laws　of　growth；　to　the　effects　of　long…continued　disuse；　and　to　the　tendency　to　reversion。
  A　part　developed　in　any　species　in　an　extraordinary　degree　or　manner；　in　comparison　with　the　same　part　in　allied　species；　tends　to　be　highly　variable。
  Several　years　ago　I　was　much　struck　with　a　remark；　nearly　to　the　above　effect；　published　by　Mr　Waterhouse。　I　infer　also　from　an　observation　made　by　Professor　Owen；　with　respect　to　the　length　of　the　arms　of　the　ourang…outang；　that　he　has　come　to　a　nearly　similar　conclusion。　It　is　hopeless　to　attempt　to　convince　any　one　of　the　truth　of　this　proposition　without　giving　the　long　array　of　facts　which　I　have　collected；　and　which　cannot　possibly　be　here　introduced。　I　can　only　state　my　conviction　that　it　is　a　rule　of　high　generality。　I　am　aware　of　several　causes　of　error；　but　I　hope　that　I　have　made　due　allowance　for　them。　It　should　be　understood　that　the　rule　by　no　means　applies　to　any　part；　however　unusually　developed；　unless　it　be　unusually　developed　in　comparison　with　the　same　part　in　closely　allied　species。　Thus；　the　bat's　wing　is　a　most　abnormal　structure　in　the　class　mammalia；　but　the　rule　would　not　here　apply；　because　there　is　a　whole　group　of　bats　having　wings；　it　would　apply　only　if　some　one　species　of　bat　had　its　wings　developed　in　some　remarkable　manner　in　comparison　with　the　other　species　of　the　same　genus。　The　rule　applies　very　strongly　in　the　case　of　secondary　sexual　characters；　when　displayed　in　any　unusual　manner。　The　term；　secondary　sexual　characters；　used　by　Hunter；　applies　to　characters　which　are　attached　to　one　sex；　but　are　not　directly　connected　with　the　act　of　reproduction。　The　rule　applies　to　males　and　females；　but　as　females　more　rarely　offer　remarkable　secondary　sexual　characters；　it　applies　more　rarely　to　them。　The　rule　being　so　plainly　applicable　in　the　case　of　secondary　sexual　characters；　may　be　due　to　the　great　variability　of　these　characters；　whether　or　not　displayed　in　any　unusual　manner　of　which　fact　I　think　there　can　be　little　doubt。　But　that　our　rule　is　not　confined　to　secondary　sexual　characters　is　clearly　shown　in　the　case　of　hermaphrodite　cirripedes；　and　I　may　here　add；　that　I　particularly　attended　to　Mr。　Waterhouse's　remark；　whilst　investigating　this　Order；　and　I　am　fully　convinced　that　the　rule　almost　invariably　holds　good　with　cirripedes。　I　shall；　in　my　future　work；　give　a　list　of　the　more　remarkable　cases；　I　will　here　only　briefly　give　one；　as　it　illustrates　the　rule　in　its　largest　application。　The　opercular　valves　of　sessile　cirripedes　（rock　barnacles）　are；　in　every　sense　of　the　word；　very　important　structures；　and　they　differ　extremely　little　even　in　different　genera；　but　in　the　several　species　of　one　genus；　Pyrgoma；　these　valves　present　a　marvellous　amount　of　diversification：　the　homologous　valves　in　the　different　species　being　sometimes　wholly　unlike　in　shape；　and　the　amount　of　variation　in　the　individuals　of　several　of　the　species　is　so　great；　that　it　is　no　exaggeration　to　state　that　the　varieties　differ　more　from　each　other　in　the　characters　of　these　important　valves　than　do　other　species　of　distinct　genera。
  As　birds　within　the　same　country　vary　in　a　remarkably　small　degree；　I　have　particularly　attended　to　them；　and　the　rule　seems　to　me　certainly　to　hold　good　in　this　class。　I　cannot　make　out　that　it　applies　to　plants；　and　this　would　seriously　have　shaken　my　belief　in　its　truth；　had　not　the　great　variability　in　plants　made　it　particularly　difficult　to　compare　their　relative　degrees　of　variability。
  When　we　see　any　part　or　organ　developed　in　a　remarkable　degree　or　manner　in　any　species；　the　fair　presumption　is　that　it　is　of　high　importance　to　that　species；　nevertheless　the　part　in　this　case　is　eminently　liable　to　variation。　Why　should　this　be　so？　On　the　view　that　each　species　has　been　independently　created；　with　all　its　parts　as　we　now　see　them；　I　can　see　no　explanation。　But　on　the　view　that　groups　of　species　have　descended　from　other　species；　and　have　been　modified　through　natural　selection；　I　think　we　can　obtain　some　light。　In　our　domestic　animals；　if　any　part；　or　the　whole　animal；　be　neglected　and　no　selection　be　applied；　that　part　（for　instance；　the　comb　in　the　Dorking　fowl）　or　the　whole　breed　will　cease　to　have　a　nearly　uniform　character。　The　breed　will　then　be　said　to　have　degenerated。　In　rudimentary　organs；　and　in　those　which　have　been　but　little　specialized　for　any　particular　purpose；　and　perhaps　in　polymorphic　groups；　we　see　a　nearly　parallel　natural　case；　for　in　such　cases　natural　selection　either　has　not　or　cannot　come　into　full　play；　and　thus　the　organisation　is　left　in　a　fluctuating　condition。　But　what　here　more　especially　concerns　us　is；　that　in　our　domestic　animals　those　points；　which　at　the　present　time　are　undergoing　rapid　change　by　continued　selection；　are　also　eminently　liable　to　variation。　Look　at　the　breeds　of　the　pigeon；　see　what　a　prodigious　amount　of　difference　there　is　in　the　beak　of　the　different　tumblers；　in　the　beak　and　wattle　of　the　different　carriers；　in　the　carriage　and　tail　of　our　fantails；　&c。；　these　being　the　points　now　mainly　attended　to　by　English　fanciers。　Even　in　the　sub…breeds；　as　in　the　short…faced　tumbler；　it　is　notoriously　difficult　to　breed　them　nearly　to　perfection；　and　frequently　individuals　are　born　which　depart　widely　from　the　standard。　There　may　be　truly　said　to　be　a　constant　struggle　going　on　between；　on　the　one　hand；　the　tendency　to　reversion　to　a　less　modified　state；　as　well　as　an　innate　tendency　to　further　variability　of　all　kinds；　and；　on　the　other　hand；　the　power　of　steady　selection　to　keep　the　breed　true。　In　the　long　run　selection　gains　the　day；　and　we