darwin and modern science-第14节

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t　several　ids　are　concerned　in　the　development　of　a　new　individual。

In　every　being　of　complex　structure　thousands　of　primary　constituents　must　go　to　make　up　a　single　id；　these　I　call　DETERMINANTS；　and　I　mean　by　this　name　very　small　individual　particles；　far　below　the　limits　of　microscopic　visibility；　vital　units　which　feed；　grow；　and　multiply　by　division。　　These　determinants　control　the　parts　of　the　developing　embryo；in　what　manner　need　not　here　concern　us。　　The　determinants　differ　among　themselves；　those　of　a　muscle　are　differently　constituted　from　those　of　a　nerve…cell　or　a　glandular　cell；　etc。；　and　every　determinant　is　in　its　turn　made　up　of　minute　vital　units；　which　I　call　BIOPHORS；　or　the　bearers　of　life。　　According　to　my　view；　these　determinants　not　only　assimilate；　like　every　other　living　unit；　but　they　VARY　in　the　course　of　their　growth；　as　every　living　unit　does；　they　may　vary　qualitatively　if　the　elements　of　which　they　are　composed　vary；　they　may　grow　and　divide　more　or　less　rapidly；　and　their　variations　give　rise　to　CORRESPONDING　variations　of　the　organ；　cell；　or　cell…group　which　they　determine。　　That　they　are　undergoing　ceaseless　fluctuations　in　regard　to　size　and　quality　seems　to　me　the　inevitable　consequence　of　their　unequal　nutrition；　for　although　the　germ…cell　as　a　whole　usually　receives　sufficient　nutriment；　minute　fluctuations　in　the　amount　carried　to　different　parts　within　the　germ…plasm　cannot　fail　to　occur。

Now；　if　a　determinant；　for　instance　of　a　sensory　cell；　receives　for　a　considerable　time　more　abundant　nutriment　than　before；　it　will　grow　more　rapidlybecome　bigger；　and　divide　more　quickly；　and；　later；　when　the　id　concerned　develops　into　an　embryo；　this　sensory　cell　will　become　stronger　than　in　the　parents；　possibly　even　twice　as　strong。　　This　is　an　instance　of　a　HEREDITARY　INDIVIDUAL　VARIATION；　arising　from　the　germ。

The　nutritive　stream　which；　according　to　our　hypothesis；　favours　the　determinant　N　by　chance；　that　is；　for　reasons　unknown　to　us；　may　remain　strong　for　a　considerable　time；　or　may　decrease　again；　but　even　in　the　latter　case　it　is　conceivable　that　the　ascending　movement　of　the　determinant　may　continue；　because　the　strengthened　determinant　now　ACTIVELY　nourishes　itself　more　abundantly；that　is　to　say；　it　attracts　the　nutriment　to　itself；　and　to　a　certain　extent　withdraws　it　from　its　fellow…　determinants。　　In　this　way；　it　mayas　it　seems　to　meget　into　PERMANENT　UPWARD　MOVEMENT；　AND　ATTAIN　A　DEGREE　OF　STRENGTH　FROM　WHICH　THERE　IS　NO　FALLING　BACK。　　Then　positive　or　negative　selection　sets　in；　favouring　the　variations　which　are　advantageous；　setting　aside　those　which　are　disadvantageous。

In　a　similar　manner　a　DOWNWARD　variation　of　the　determinants　may　take　place；　if　its　progress　be　started　by　a　diminished　flow　of　nutriment。　　The　determinants　which　are　weakened　by　this　diminished　flow　will　have　less　affinity　for　attracting　nutriment　because　of　their　diminished　strength；　and　they　will　assimilate　more　feebly　and　grow　more　slowly；　unless　chance　streams　of　nutriment　help　them　to　recover　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；