darwin and modern science-第11部分

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make　the　skin　tough　and　resistant。　　In　a　small　group　of　themthe　species　of　Synaptathe　calcareous　bodies　occur　in　the　form　of　delicate　anchors　of　microscopic　size。　　Up　till　1897　these　anchors；　like　many　other　delicate　microscopic　structures；　were　regarded　as　curiosities；　as　natural　marvels。　　But　a　Swedish　observer；　Oestergren；　has　recently　shown　that　they　have　a　biological　significance：　　they　serve　the　footless　Synapta　as　auxiliary　organs　of　locomotion；　since；　when　the　body　swells　up　in　the　act　of　creeping；　they　press　firmly　with　their　tips；　which　are　embedded　in　the　skin；　against　the　substratum　on　which　the　animal　creeps；　and　thus　prevent　slipping　backwards。　　In　other　Holothurians　this　slipping　is　made　impossible　by　the　fixing　of　the　tube…feet。　　The　anchors　act　automatically；　sinking　their　tips　towards　the　ground　when　the　corresponding　part　of　the　body　thickens；　and　returning　to　the　original　position　at　an　angle　of　45　degrees　to　the　upper　surface　when　the　part　becomes　thin　again。　　The　arms　of　the　anchor　do　not　lie　in　the　same　plane　as　the　shaft；　and　thus　the　curve　of　the　arms　forms　the　outermost　part　of　the　anchor；　and　offers　no　further　resistance　to　the　gliding　of　the　animal。　　Every　detail　of　the　anchor；　the　curved　portion；　the　little　teeth　at　the　head；　the　arms；　etc。；　can　be　interpreted　in　the　most　beautiful　way；　above　all　the　form　of　the　anchor　itself；　for　the　two　arms　prevent　it　from　swaying　round　to　the　side。　　The　position　of　the　anchors；　too；　is　definite　and　significant；　they　lie　obliquely　to　the　longitudinal　axis　of　the　animal；　and　therefore　they　act　alike　whether　the　animal　is　creeping　backwards　or　forwards。　　Moreover；　the　tips　would　pierce　through　the　skin　if　the　anchors　lay　in　the　longitudinal　direction。　　Synapta　burrows　in　the　sand；　it　first　pushes　in　the　thin　anterior　end；　and　thickens　this　again；　thus　enlarging　the　hole；　then　the　anterior　tentacles　displace　more　sand；　the　body　is　worked　in　a　little　farther；　and　the　process　begins　anew。　　In　the　first　act　the　anchors　are　passive；　but　they　begin　to　take　an　active　share　in　the　forward　movement　when　the　body　is　contracted　again。　　Frequently　the　animal　retains　only　the　posterior　end　buried　in　the　sand；　and　then　the　anchors　keep　it　in　position；　and　make　rapid　withdrawal　possible。

Thus　we　have　in　these　apparently　random　forms　of　the　calcareous　bodies；　complex　adaptations　in　which　every　little　detail　as　to　direction；　curve；　and　pointing　is　exactly　determined。　　That　they　have　selection…value　in　their　present　perfected　form　is　beyond　all　doubt；　since　the　animals　are　enabled　by　means　of　them　to　bore　rapidly　into　the　ground　and　so　to　escape　from　enemies。　　We　do　not　know　what　the　initial　stages　were；　but　we　cannot　doubt　that　the　little　improvements；　which　occurred　as　variations　of　the　originally　simple　slimy　bodies　of　the　Holothurians；　were　preserved　because　they　already　possessed　selection…value　for　the　Synaptidae。　　For　such　minute　microscopic　structures　whose　form　is　so　delicately　adapted　to　the　role　they　have　to　play　in　the　life　of　the　animal；　cannot　have　arisen　suddenly　and　as　a　whole；　and　every　new　variation　of　the　anchor；　that　is；　in　the　direction　of　the　development　of　the　two　arms；　and　every　curving　of　the　shaft　which　prevented　the　tips　from　projecting　at　the　wrong　time；　in　short；　every　little　adaptation　in　the　modelling　of　the　anchor　must　have　possessed　selection…value。　　And　that　such　minute　changes　of　form　fall　within　the　sphere　of　fluctuating　variations；　that　is　to　say；　THAT　THEY　OCCUR　is　beyond　all　doubt。

In　many　of　the　Synaptidae　the　anchors　are　replaced　by　calcareous　rods　bent　in　the　form　of　an　S；　which　are　said　to　act　in　the　same　way。　　Others；　such　as　those　of　the　genus　Ankyroderma；　have　anchors　which　project　considerably　beyond　the　skin；　and；　according　to　Oestergren；　serve　〃to　catch　plant…　particles　and　other　substances〃　and　so　mask　the　animal。　　Thus　we　see　that　in　the　Synaptidae　the　thick　and　irregular　calcareous　bodies　of　the　Holothurians　have　been　modified　and　transformed　in　various　ways　in　adaptation　to　the　footlessness　of　these　animals；　and　to　the　peculiar　conditions　of　their　life；　and　we　must　conclude　that　the　earlier　stages　of　these　changes　presented　themselves　to　the　processes　of　selection　in　the　form　of　microscopic　variations。　　For　it　is　as　impossible　to　think　of　any　origin　other　than　through　selection　in　this　case　as　in　the　case　of　the　toughness；　and　the　〃drip…tips〃　of　tropical　leaves。　　And　as　these　last　could　not　have　been　produced　directly　by　the　beating　of　the　heavy　rain…drops　upon　them；　so　the　calcareous　anchors　of　Synapta　cannot　have　been　produced　directly　by　the　friction　of　the　sand　and　mud　at　the　bottom　of　the　sea；　and；　since　they　are　parts　whose　function　is　PASSIVE　the　Lamarckian　factor　of　use　and　disuse　does　not　come　into　question。　　The　conclusion　is　unavoidable；　that　the　microscopically　small　variations　of　the　calcareous　bodies　in　the　ancestral　forms　have　been　intensified　and　accumulated　in　a　particular　direction；　till　they　have　led　to　the　formation　of　the　anchor。　　Whether　this　has　taken　place　by　the　action　of　natural　selection　alone；　or　whether　the　laws　of　variation　and　the　intimate　processes　within　the　germ…plasm　have　cooperated　will　become　clear　in　the　discussion　of　germinal　selection。　　This　whole　process　of　adaptation　has　obviously　taken　place　within　the　time　that　has　elapsed　since　this　group　of　sea…cucumbers　lost　their　tube…feet；　those　characteristic　organs　of　locomotion　which　occur　in　no　group　except　the　Echinoderms；　and　yet　have　totally　disappeared　in　the　Synaptidae。　　And　after　all　what　would　animals　that　live　in　sand　and　mud　do　with　tube…feet？

（c）　　COADAPTATION。

Darwin　pointed　out　that　one　of　the　essential　differences　between　artificial　and　natural　selection　lies　in　the　fact　that　the　former　can　modify　only　a　few　characters；　usually　only　one　at　a　time；　while　Nature　preserves　in　the　struggle　for　existence　all　the　variations　of　a　species；　at　the　same　time　and　in　a　purely　mechanical　way；　if　they　possess　selection…value。

Herbert　Spencer；　though　himself　an　adherent　of　the　theory　of　selection；　declared　in　the　beginning　of　the　nineties　that　in　his　opinion　the　range　of　this　principle　was　greatly　over…estimated；　if　the　great　changes　which　have　taken　place　in　so　many　organisms　in　the　course　of　ages　are　to　be　interpreted　as　due　to　this　process　of　selection　alone；　since　no　transformation　of　any　importance　can　be　evolved　by　itself；　it　is　always　accompanied　by　a　host　of　secondary　changes。　　He　gives　the　familiar　example　of　the　Giant　Stag　of　the　Irish　peat；　the　enormous　antlers　of　which　required　not　only　a　much　stronger　skull　cap；　but　also　greater　strength　of　the　sinews；　muscles；　nerves　and　bones　of　the　whole　anterior　half　of　the　animal；　if　their　mass　was　not　to　weigh　down　the　animal　altogether。　　It　is　inconceivable；　he　says；　that　so　many　processes　of　selection　should　take　place　SIMULTANEOUSLY；　and　we　are　therefore　forced　to　fall　back　on　the　Lamarckian　factor　of　the　use　and　disuse　of　functional　parts。　　And　how；　he　asks；　could　natural　selection　follow　two　opposite　directions　of　evolution　in　different　parts　of　the　body　at　the　same　time；　as　for　instance　in　the　case　of　the　kangaroo；　in　which　the　forelegs　must　have　become　shorter；　while　the　hind　legs　and　the　tail　were　becoming　longer　and　stronger？

Spencer's　main　object　was　to　substantiate　the　validity　of　the　Lamarckian　principle；　the　cooperation　of　which　with　selection　had　been　doubted　by　many。　　And　it　does　seem　as　though　this　principle；　if　it　operates　in　nature　at　all；　offers　a　ready　and　simple　explanation　of　all　such　secondary　variations。　　Not　only　muscles；　but　nerves；　bones；　sinews；　in　short　all　tissues　which　function　actively；　increase　in　strength　in　proportion　as　they　are　used；　and　conversely　they　decrease　when　the　claims　on　them　diminish。　　All　the　parts；　therefore；　which　depend　on　the　part　that　varied　first；　as　for　instance　the　enlarged　antlers　of　the　Irish　Elk；　must　have　been　increased　or　decreased　in　strength；　in　exact　proportion　to　the　claims　made　upon　them；just　as　is　actually　the　case。

But　beautiful　as　this　explanation　would　be；　I　regard　it　as　untenable；　because　it　assumes　the　TRANSMISSIBILITY　OF　FUNCTIONAL　MODIFICATIONS　（so…　called　〃acquired〃　characters）；　and　this　is　not　only　undemonstrable；　but　is　scarcely　theoretically　conceivable；　for　the　secondary　variations　which　accompany　or　follow　the　first　as　correlative　variations；　occur　also　in　cases　in　which　the　animals　concerned　are　sterile　and　THEREFORE　CANNOT　TRANSMIT　ANYTHING　TO　THEIR　DESCENDANTS。　　This　is　true　of　WORKER　BEES；　and　particularly　of　ANTS；　and　I　shall　here　give　a　brief　survey　of　the　present　state　of　the　problem　as　it　appears　to　me。

Much　has　been　written　on　both　sides　of　this　question　since　the　published　controversy　on　the　subject　in　the　nineties　between　Herbert　Spencer　and　myself。　　I　should　like　to　return　to　the　matter　in　detail；　if　the　space　at　my　disposal　permitted；　because　it　seems　to　me　that　the　arguments　I　advanced　at　that　time　are　equally　cogent　to…day；　notwithstanding　all　the　objections　that　have　since　been　urged　against　them。　　Moreover；　the　matter　is　by　no　means　one　of　subordinate　interest；　it　is　the　very　kernel　of　the　whole　question　of　the　reality　and　value　of　the　principle　of　selection。　　For　if　selection　alone　does　not　suffice　to　explain　〃HARMONIOUS　ADAPTATION〃　as　I　have　called　Spencer's　COADAPTATION；　and　if　we　require　to　call　in　the　aid　of　the　Lamarckian　factor　it　would　be　questionable　whether　selection　could　explain　any　adaptations　whatever。　　In　this　particular　caseof　worker　bees　the　Lamarckian　factor　may　be　excluded　altogether；　for　it　can　be　demonstrated　that　here　at　any　rate　the　effects　of　use　and　disuse　cannot　be　transmitted。

But　if　it　be　asked　why　we　are　unwilling　to　admit　the　cooperation　of　the　Darwinian　factor　of　selection　and　the　Lamarckian　factor；　since　this　would　afford　us　an　easy　and　satisfactory　explanation　of　the　phenomena；　I　answer：　　BECAUSE　THE　LAMARCKIAN　PRINCIPLE　IS　FALLACIOUS；　AND　BECAUSE　BY　ACCEPTING　IT　WE　CLOSE　THE　WAY　TOWARDS　DEEPER　INSIGHT。　　It　is　not　a　spirit　of　combativeness　or　a　desire　for　self…vindication　that　induces　me　to　take　the　field　once　more　against　the　Lamarckian　principle；　it　is　the　conviction　that　the　progress　of　our　knowledge　is　being　obstructed　by　the　acceptance　of　this　fallac