darwin and modern science-第16章

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ecover　themselves。　　But；　as　will　presently　be　shown；　a　change　of　direction　cannot　take　place　at　EVERY　stage　of　the　degenerative　process。　　If　a　certain　critical　stage　of　downward　progress　be　passed；　even　favourable　conditions　of　food…supply　will　no　longer　suffice　permanently　to　change　the　direction　of　the　variation。　　Only　two　cases　are　conceivable；　if　the　determinant　corresponds　to　a　USEFUL　organ；　only　its　removal　can　bring　back　the　germ…plasm　to　its　former　level；　therefore　personal　selection　removes　the　id　in　question；　with　its　determinants；　from　the　germ…plasm；　by　causing　the　elimination　of　the　individual　in　the　struggle　for　existence。　　But　there　is　another　conceivable　case；　the　determinants　concerned　may　be　those　of　an　organ　which　has　become　USELESS；　and　they　will　then　continue　unobstructed；　but　with　exceeding　slowness；　along　the　downward　path；　until　the　organ　becomes　vestigial；　and　finally　disappears　altogether。

The　fluctuations　of　the　determinants　hither　and　thither　may　thus　be　transformed　into　a　lasting　ascending　or　descending　movement；　and　THIS　IS　THE　CRUCIAL　POINT　OF　THESE　GERMINAL　PROCESSES。

This　is　not　a　fantastic　assumption；　we　can　read　it　in　the　fact　of　the　degeneration　of　disused　parts。　　USELESS　ORGANS　ARE　THE　ONLY　ONES　WHICH　ARE　NOT　HELPED　TO　ASCEND　AGAIN　BY　PERSONAL　SELECTION；　AND　THEREFORE　IN　THEIR　CASE　ALONE　CAN　WE　FORM　ANY　IDEA　OF　HOW　THE　PRIMARY　CONSTITUENTS　BEHAVE；　WHEN　THEY　ARE　SUBJECT　SOLELY　TO　INTRA…GERMINAL　FORCES。

The　whole　determinant　system　of　an　id；　as　I　conceive　it；　is　in　a　state　of　continual　fluctuation　upwards　and　downwards。　　In　most　cases　the　fluctuations　will　counteract　one　another；　because　the　passive　streams　of　nutriment　soon　change；　but　in　many　cases　the　limit　from　which　a　return　is　possible　will　be　passed；　and　then　the　determinants　concerned　will　continue　to　vary　in　the　same　direction；　till　they　attain　positive　or　negative　selection…value。　　At　this　stage　personal　selection　intervenes　and　sets　aside　the　variation　if　it　is　disadvantageous；　or　favoursthat　is　to　say；　preservesit　if　it　is　advantageous。　　Only　THE　DETERMINANT　OF　A　USELESS　ORGAN　IS　UNINFLUENCED　BY　PERSONAL　SELECTION；　and；　as　experience　shows；　it　sinks　downwards；　that　is；　the　organ　that　corresponds　to　it　degenerates　very　slowly　but　uninterruptedly　till；　after　what　must　obviously　be　an　immense　stretch　of　time；　it　disappears　from　the　germ…plasm　altogether。

Thus　we　find　in　the　fact　of　the　degeneration　of　disused　parts　the　proof　that　not　all　the　fluctuations　of　a　determinant　return　to　equilibrium　again；　but　that；　when　the　movement　has　attained　to　a　certain　strength；　it　continues　IN　THE　SAME　DIRECTION。　　We　have　entire　certainty　in　regard　to　this　as　far　as　the　downward　progress　is　concerned；　and　we　must　assume　it　also　in　regard　to　ascending　variations；　as　the　phenomena　of　artificial　selection　certainly　justify　us　in　doing。　　If　the　Japanese　breeders　were　able　to　lengthen　the　tail　feathers　of　the　cock　to　six　feet；　it　can　only　have　been　because　the　determinants　of　the　tail…feathers　in　the　germ…plasm　had　already　struck　out　a　path　of　ascending　variation；　and　this　movement　was　taken　advantage　of　by　the　breeder；　who　continually　selected　for　reproduction　the　individuals　in　which　the　ascending　variation　was　most　marked。　　For　all　breeding　depends　upon　the　unconscious　selection　of　germinal　variations。

Of　course　these　germinal　processes　cannot　be　proved　mathematically；　since　we　cannot　actually　see　the　play　of　forces　of　the　passive　fluctuations　and　their　causes。　　We　cannot　say　how　great　these　fluctuations　are；　and　how　quickly　or　slowly；　how　regularly　or　irregularly　they　change。　　Nor　do　we　know　how　far　a　determinant　must　be　strengthened　by　the　passive　flow　of　the　nutritive　stream　if　it　is　to　be　beyond　the　danger　of　unfavourable　variations；　or　how　far　it　must　be　weakened　passively　before　it　loses　the　power　of　recovering　itself　by　its　own　strength。　　It　is　no　more　possible　to　bring　forward　actual　proofs　in　this　case　than　it　was　in　regard　to　the　selection…value　of　the　initial　stages　of　an　adaptation。　　But　if　we　consider　that　all　heritable　variations　must　have　their　roots　in　the　germ…plasm；　and　further；　that　when　personal　selection　does　not　intervene；　that　is　to　say；　in　the　case　of　parts　which　have　become　useless；　a　degeneration　of　the　part；　and　therefore　also　of　its　determinant　must　inevitably　take　place；　then　we　must　conclude　that　processes　such　as　I　have　assumed　are　running　their　course　within　the　germ…plasm；　and　we　can　do　this　with　as　much　certainty　as　we　were　able　to　infer；　from　the　phenomena　of　adaptation；　the　selection…　value　of　their　initial　stages。　　The　fact　of　the　degeneration　of　disused　parts　seems　to　me　to　afford　irrefutable　proof　that　the　fluctuations　within　the　germ…plasm　ARE　THE　REAL　ROOT　OF　ALL　HEREDITARY　VARIATION；　and　the　preliminary　condition　for　the　occurrence　of　the　Darwin…Wallace　factor　of　selection。　　Germinal　selection　supplies　the　stones　out　of　which　personal　selection　builds　her　temples　and　palaces：　　ADAPTATIONS。　　The　importance　for　the　theory　of　the　process　of　degeneration　of　disused　parts　cannot　be　over…　estimated；　especially　when　it　occurs　in　sterile　animal　forms；　where　we　are　free　from　the　doubt　as　to　the　alleged　LAMARCKIAN　FACTOR　which　is　apt　to　confuse　our　ideas　in　regard　to　other　cases。

If　we　regard　the　variation　of　the　many　determinants　concerned　in　the　transformation　of　the　female　into　the　sterile　worker　as　having　come　about　through　the　gradual　transformation　of　the　ids　into　worker…ids；　we　shall　see　that　the　germ…plasm　of　the　sexual　ants　must　contain　three　kinds　of　ids；　male；　female；　and　worker　ids；　or　if　the　workers　have　diverged　into　soldiers　and　nest…builders；　then　four　kinds。　　We　understand　that　the　worker…ids　arose　because　their　determinants　struck　out　a　useful　path　of　variation；　whether　upward　or　downward；　and　that　they　continued　in　this　path　until　the　highest　attainable　degree　of　utility　of　the　parts　determined　was　reached。　　But　in　addition　to　the　organs　of　positive　or　negative　selection…value；　there　were　some　which　were　indifferent　as　far　as　the　success　and　especially　the　functional　capacity　of　the　workers　was　concerned：　　wings；　ovarian　tubes；　receptaculum　seminis；　a　number　of　the　facets　of　the　eye；　perhaps　even　the　whole　eye。　　As　to　the　ovarian　tubes　it　is　possible　that　their　degeneration　was　an　advantage　for　the　workers；　in　saving　energy；　and　if　so　selection　would　favour　the　degeneration；　but　how　could　the　presence　of　eyes　diminish　the　usefulness　of　the　workers　to　the　colony？　or　the　minute　receptaculum　seminis；　or　even　the　wings？　　These　parts　have　therefore　degenerated　BECAUSE　THEY　WERE　OF　NO　FURTHER　VALUE　TO　THE　INSECT。　　But　if　selection　did　not　influence　the　setting　aside　of　these　parts　because　they　were　neither　of　advantage　nor　of　disadvantage　to　the　species；　then　the　Darwinian　factor　of　selection　is　here　confronted　with　a　puzzle　which　it　cannot　solve　alone；　but　which　at　once　becomes　clear　when　germinal　selection　is　added。　　For　the　determinants　of　organs　that　have　no　further　value　for　the　organism；　must；　as　we　have　already　explained；　embark　on　a　gradual　course　of　retrograde　development。

In　ants　the　degeneration　has　gone　so　far　that　there　are　no　wing…rudiments　present　in　ANY　species；　as　is　the　case　with　so　many　butterflies；　flies；　and　locusts；　but　in　the　larvae　the　imaginal　discs　of　the　wings　are　still　laid　down。　　With　regard　to　the　ovaries；　degeneration　has　reached　different　levels　in　different　species　of　ants；　as　has　been　shown　by　the　researches　of　my　former　pupil；　Elizabeth　Bickford。　　In　many　species　there　are　twelve　ovarian　tubes；　and　they　decrease　from　that　number　to　one；　indeed；　in　one　species　no　ovarian　tube　at　all　is　present。　　So　much　at　least　is　certain　from　what　has　been　said；　that　in　this　case　EVERYTHING　depends　on　the　fluctuations　of　the　elements　of　the　germ…plasm。　　Germinal　selection；　here　as　elsewhere；　presents　the　variations　of　the　determinants；　and　personal　selection　favours　or　rejects　these；　or；if　it　be　a　question　of　organs　which　have　become　useless；it　does　not　come　into　play　at　all；　and　allows　the　descending　variation　free　course。

It　is　obvious　that　even　the　problem　of　COADAPTATION　IN　STERILE　ANIMALS　can　thus　be　satisfactorily　explained。　　If　the　determinants　are　oscillating　upwar