darwin and modern science-第11章

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e　conviction　that　the　progress　of　our　knowledge　is　being　obstructed　by　the　acceptance　of　this　fallacious　principle；　since　the　facile　explanation　it　apparently　affords　prevents　our　seeking　after　a　truer　explanation　and　a　deeper　analysis。

The　workers　in　the　various　species　of　ants　are　sterile；　that　is　to　say；　they　take　no　regular　part　in　the　reproduction　of　the　species；　although　individuals　among　them　may　occasionally　lay　eggs。　　In　addition　to　this　they　have　lost　the　wings；　and　the　receptaculum　seminis；　and　their　compound　eyes　have　degenerated　to　a　few　facets。　　How　could　this　last　change　have　come　about　through　disuse；　since　the　eyes　of　workers　are　exposed　to　light　in　the　same　way　as　are　those　of　the　sexual　insects　and　thus　in　this　particular　case　are　not　liable　to　〃disuse〃　at　all？　　The　same　is　true　of　the　receptaculum　seminis；　which　can　only　have　been　disused　as　far　as　its　glandular　portion　and　its　stalk　are　concerned；　and　also　of　the　wings；　the　nerves　tracheae　and　epidermal　cells　of　which　could　not　cease　to　function　until　the　whole　wing　had　degenerated；　for　the　chitinous　skeleton　of　the　wing　does　not　function　at　all　in　the　active　sense。

But；　on　the　other　hand；　the　workers　in　all　species　have　undergone　modifications　in　a　positive　direction；　as；　for　instance；　the　greater　development　of　brain。　　In　many　species　large　workers　have　evolved；the　so…　called　SOLDIERS；　with　enormous　jaws　and　teeth；　which　defend　the　colony；　and　in　others　there　are　SMALL　workers　which　have　taken　over　other　special　functions；　such　as　the　rearing　of　the　young　Aphides。　　This　kind　of　division　of　the　workers　into　two　castes　occurs　among　several　tropical　species　of　ants；　but　it　is　also　present　in　the　Italian　species；　Colobopsis　truncata。　　Beautifully　as　the　size　of　the　jaws　could　be　explained　as　due　to　the　increased　use　made　of　them　by　the　〃soldiers；〃　or　the　enlarged　brain　as　due　to　the　mental　activities　of　the　workers；　the　fact　of　the　infertility　of　these　forms　is　an　insurmountable　obstacle　to　accepting　such　an　explanation。　Neither　jaws　nor　brain　can　have　been　evolved　on　the　Lamarckian　principle。

The　problem　of　coadaptation　is　no　easier　in　the　case　of　the　ant　than　in　the　case　of　the　Giant　Stag。　　Darwin　himself　gave　a　pretty　illustration　to　show　how　imposing　the　difference　between　the　two　kinds　of　workers　in　one　species　would　seem　if　we　translated　it　into　human　terms。　　In　regard　to　the　Driver　ants　（Anomma）　we　must　picture　to　ourselves　a　piece　of　work；　〃for　instance　the　building　of　a　house；　being　carried　on　by　two　kinds　of　workers；　of　which　one　group　was　five　feet　four　inches　high；　the　other　sixteen　feet　high。〃　　（〃Origin　of　Species〃　（6th　edition）；　page　232。）

Although　the　ant　is　a　small　animal　as　compared　with　man　or　with　the　Irish　Elk；　the　〃soldier〃　with　its　relatively　enormous　jaws　is　hardly　less　heavily　burdened　than　the　Elk　with　its　antlers；　and　in　the　ant's　case；　too；　a　strengthening　of　the　skeleton；　of　the　muscles；　the　nerves　of　the　head；　and　of　the　legs　must　have　taken　place　parallel　with　the　enlargement　of　the　jaws。　　HARMONIOUS　ADAPTATION　（coadaptation）　has　here　been　active　in　a　high　degree；　and　yet　these　〃soldiers〃　are　sterile！　　There　thus　remains　nothing　for　it　but　to　refer　all　their　adaptations；　positive　and　negative　alike；　to　processes　of　selection　which　have　taken　place　in　the　rudiments　of　the　workers　within　the　egg　and　sperm…cells　of　their　parents。　　There　is　no　way　out　of　the　difficulty　except　the　one　Darwin　pointed　out。　　He　himself　did　not　find　the　solution　of　the　riddle　at　once。　　At　first　he　believed　that　the　case　of　the　workers　among　social　insects　presented　〃the　most　serious　special　difficulty〃　in　the　way　of　his　theory　of　natural　selection；　and　it　was　only　after　it　had　become　clear　to　him；　that　it　was　not　the　sterile　insects　themselves　but　their　parents　that　were　selected；　according　as　they　produced　more　or　less　well　adapted　workers；　that　he　was　able　to　refer　to　this　very　case　of　the　conditions　among　ants　〃IN　ORDER　TO　SHOW　THE　POWER　OF　NATURAL　SELECTION〃　（〃Origin　of　Species〃；　page　233；　see　also　edition　1；　page　242。）。　　He　explains　his　view　by　a　simple　but　interesting　illustration。　　Gardeners　have　produced；　by　means　of　long　continued　artificial　selection；　a　variety　of　Stock；　which　bears　entirely　double；　and　therefore　infertile　flowers　（Ibid。　page　230。）。　　Nevertheless　the　variety　continues　to　be　reproduced　from　seed；　because　in　addition　to　the　double　and　infertile　flowers；　the　seeds　always　produce　a　certain　number　of　single；　fertile　blossoms；　and　these　are　used　to　reproduce　the　double　variety。　　These　single　and　fertile　plants　correspond　〃to　the　males　and　females　of　an　ant…colony；　the　infertile　plants；　which　are　regularly　produced　in　large　numbers；　to　the　neuter　workers　of　the　colony。〃

This　illustration　is　entirely　apt；　the　only　difference　between　the　two　cases　consisting　in　the　fact　that　the　variation　in　the　flower　is　not　a　useful；　but　a　disadvantageous　one；　which　can　only　be　preserved　by　artificial　selection　on　the　part　of　the　gardener；　while　the　transformations　that　have　taken　place　parallel　with　the　sterility　of　the　ants　are　useful；　since　they　procure　for　the　colony　an　advantage　in　the　struggle　for　existence；　and　they　are　therefore　preserved　by　natural　selection。　　Even　the　sterility　itself　in　this　case　is　not　disadvantageous；　since　the　fertility　of　the　true　females　has　at　the　same　time　considerably　increased。　　We　may　therefore　regard　the　sterile　forms　of　ants；　which　have　gradually　been　adapted　in　several　directions　to　varying　functions；　AS　A　CERTAIN　PROOF　that　selection　really　takes　place　in　the　germ…cells　of　the　fathers　and　mothers　of　the　workers；　and　that　SPECIAL　COMPLEXES　OF　PRIMORDIA　（IDS）　are　present　in　the　workers　and　in　the　males　and　females；　and　these　complexes　contain　the　primordia　of　the　individual　parts　（DETERMINANTS）。　　But　since　all　living　entities　vary；　the　determinants　must　also　vary；　now　in　a　favourable；　now　in　an　unfavourable　direction。　　If　a　female　produces　eggs；　which　contain　favourably　varying　determinants　in　the　worker…ids；　then　these　eggs　will　give　rise　to　workers　modified　in　the　favourable　direction；　and　if　this　happens　with　many　females；　the　colony　concerned　will　contain　a　better　kind　of　worker　than　other　colonies。

I　digress　here　in　order　to　give　an　account　of　the　intimate　processes；　which；　according　to　my　view；　take　place　within　the　germ…plasm；　and　which　I　have　called　〃GERMINAL　SELECTION。〃　　These　processes　are　of　importance　since　they　form　the　roots　of　variation；　which　in　its　turn　is　the　root　of　natural　selection。　　I　cannot　here　do　more　than　give　a　brief　outline　of　the　theory　in　order　to　show　how　the　Darwin…Wallace　theory　of　selection　has　gained　support　from　it。

With　others；　I　regard　the　minimal　amount　of　substance　which　is　contained　within　the　nucleus　of　the　germ…cells；　in　the　form　of　rods；　bands；　or　granules；　as　the　GERM…SUBSTANCE　or　GERM…PLASM；　and　I　call　the　individual　granules　IDS。　　There　is　always　a　multiplicity　of　such　ids　present　in　the　nucleus；　either　occurring　individually；　or　united　in　the　form　of　rods　or　bands　（chromosomes）。　　Each　id　contains　the　primary　constituents　of　a　WHOLE　individual；　so　that　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。

T