darwin and modern science-第24章

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ume　selection；　because　it　is　the　only　possible　explanation　applicable　to　whole　classes　of　phenomena；　and　because；　on　the　other　hand；　it　is　made　up　of　factors　which　we　know　can　be　proved　actually　to　exist；　and　which；　IF　they　exist；　must　of　logical　necessity　cooperate　in　the　manner　required　by　the　theory。　　WE　MUST　ACCEPT　IT　BECAUSE　THE　PHENOMENA　OF　EVOLUTION　AND　ADAPTATION　MUST　HAVE　A　NATURAL　BASIS；　AND　BECAUSE　IT　IS　THE　ONLY　POSSIBLE　EXPLANATION　OF　THEM。　　（This　has　been　discussed　in　many　of　my　earlier　works。　　See　for　instance　〃The　All…　Sufficiency　of　Natural　Selection；　a　reply　to　Herbert　Spencer〃；　London；　1893。）

Many　people　are　willing　to　admit　that　selection　explains　adaptations；　but　they　maintain　that　only　a　part　of　the　phenomena　are　thus　explained；　because　everything　does　not　depend　upon　adaptation。　　They　regard　adaptation　as；　so　to　speak；　a　special　effort　on　the　part　of　Nature；　which　she　keeps　in　readiness　to　meet　particularly　difficult　claims　of　the　external　world　on　organisms。　　But　if　we　look　at　the　matter　more　carefully　we　shall　find　that　adaptations　are　by　no　means　exceptional；　but　that　they　are　present　everywhere　in　such　enormous　numbers；　that　it　would　be　difficult　in　regard　to　any　structure　whatever；　to　prove　that　adaptation　had　NOT　played　a　part　in　its　evolution。

How　often　has　the　senseless　objection　been　urged　against　selection　that　it　can　create　nothing；　it　can　only　reject。　　It　is　true　that　it　cannot　create　either　the　living　substance　or　the　variations　of　it；　both　must　be　given。　　But　in　rejecting　one　thing　it　preserves　another；　intensifies　it；　combines　it；　and　in　this　way　CREATES　what　is　new。　　EVERYTHING　in　organisms　depends　on　adaptation；　that　is　to　say；　everything　must　be　admitted　through　the　narrow　door　of　selection；　otherwise　it　can　take　no　part　in　the　building　up　of　the　whole。　　But；　it　is　asked；　what　of　the　direct　effect　of　external　conditions；　temperature；　nutrition；　climate　and　the　like？　　Undoubtedly　these　can　give　rise　to　variations；　but　they　too　must　pass　through　the　door　of　selection；　and　if　they　cannot　do　this　they　are　rejected；　eliminated　from　the　constitution　of　the　species。

It　may；　perhaps；　be　objected　that　such　external　influences　are　often　of　a　compelling　power；　and　that　every　animal　MUST　submit　to　them；　and　that　thus　selection　has　no　choice　and　can　neither　select　nor　reject。　　There　may　be　such　cases；　let　us　assume　for　instance　that　the　effect　of　the　cold　of　the　Arctic　regions　was　to　make　all　the　mammals　become　black；　the　result　would　be　that　they　would　all　be　eliminated　by　selection；　and　that　no　mammals　would　be　able　to　live　there　at　all。　　But　in　most　cases　a　certain　percentage　of　animals　resists　these　strong　influences；　and　thus　selection　secures　a　foothold　on　which　to　work；　eliminating　the　unfavourable　variation；　and　establishing　a　useful　colouring；　consistent　with　what　is　required　for　the　maintenance　of　the　species。

Everything　depends　upon　adaptation！　　We　have　spoken　much　of　adaptation　in　colouring；　in　connection　with　the　examples　brought　into　prominence　by　Darwin；　because　these　are　conspicuous；　easily　verified；　and　at　the　same　time　convincing　for　the　theory　of　selection。　　But　is　it　only　desert　and　polar　animals　whose　colouring　is　determined　through　adaptation？　　Or　the　leaf…butterflies；　and　the　mimetic　species；　or　the　terrifying　markings；　and　〃warning…colours〃　and　a　thousand　other　kinds　of　sympathetic　colouring？　　It　is；　indeed；　never　the　colouring　alone　which　makes　up　the　adaptation；　the　structure　of　the　animal　plays　a　part；　often　a　very　essential　part；　in　the　protective　disguise；　and　thus　MANY　variations　may　cooperate　towards　ONE　common　end。　　And　it　is　to　be　noted　that　it　is　by　no　means　only　external　parts　that　are　changed；　internal　parts　are　ALWAYS　modified　at　the　same　timefor　instance；　the　delicate　elements　of　the　nervous　system　on　which　depend　the　INSTINCT　of　the　insect　to　hold　its　wings；　when　at　rest；　in　a　perfectly　definite　position；　which；　in　the　leaf…butterfly；　has　the　effect　of　bringing　the　two　pieces　on　which　the　marking　occurs　on　the　anterior　and　posterior　wing　into　the　same　direction；　and　thus　displaying　as　a　whole　the　fine　curve　of　the　midrib　on　the　seeming　leaf。　But　the　wing…holding　instinct　is　not　regulated　in　the　same　way　in　all　leaf…butterflies；　even　our　indigenous　species　of　Vanessa；　with　their　protective　ground…colouring；　have　quite　a　distinctive　way　of　holding　their　wings　so　that　the　greater　part　of　the　anterior　wing　is　covered　by　the　posterior　when　the　butterfly　is　at　rest。　　But　the　protective　colouring　appears　on　the　posterior　wing　and　on　the　tip　of　the　anterior；　TO　PRECISELY　THE　DISTANCE　TO　WHICH　IT　IS　LEFT　UNCOVERED。　　This　occurs；　as　Standfuss　has　shown；　in　different　degree　in　our　two　most　nearly　allied　species；　the　uncovered　portion　being　smaller　in　V。　urticae　than　in　V。　polychloros。　　In　this　case；　as　in　most　leaf…butterflies；　the　holding　of　the　wing　was　probably　the　primary　character；　only　after　that　was　thoroughly　established　did　the　protective　marking　develop。　　In　any　case；　the　instinctive　manner　of　holding　the　wings　is　associated　with　the　protective　colouring；　and　must　remain　as　it　is　if　the　latter　is　to　be　effective。　　How　greatly　instincts　may　change；　that　is　to　say；　may　be　adapted；　is　shown　by　the　case　of　the　Noctuid　〃shark〃　moth；　Xylina　vetusta。　　This　form　bears　a　most　deceptive　resemblance　to　a　piece　of　rotten　wood；　and　the　appearance　is　greatly　increased　by　the　modification　of　the　innate　impulse　to　flight　common　to　so　many　animals；　which　has　here　been　transformed　into　an　almost　contrary　instinct。　　This　moth　does　not　fly　away　from　danger；　but　〃feigns　death；〃　that　is；　it　draws　antennae；　legs　and　wings　close　to　the　body；　and　remains　perfectly　motionless。　　It　may　be　touched；　picked　up；　and　thrown　down　again；　and　still　it　does　not　move。　　This　remarkable　instinct　must　surely　have　developed　simultaneously　with　the　wood…colouring；　at　all　events；　both　cooperating　variations　are　now　present；　and　prove　that　both　the　external　and　the　most　minute　internal　structure　have　undergone　a　process　of　adaptation。

The　case　is　the　same　with　all　structural　variations　of　animal　parts；　which　are　not　absolutely　insignificant。　　When　the　insects　acquired　wings　they　must　also　have　acquired　the　mechanism　with　which　to　move　themthe　musculature；　and　the　nervous　apparatus　necessary　for　its　automatic　regulation。　　All　instincts　depend　upon　compound　reflex　mechanisms　and　are　just　as　indispensable　as　the　parts　they　have　to　set　in　motion；　and　all　may　have　arisen　through　processes　of　selection　if　the　reasons　which　I　have　elsewhere　given　for　this　view　are　correct。　　（〃The　Evolution　Theory〃；　London；　1904；　page　144。）

Thus　there　is　no　lack　of　adaptations　within　the　organism；　and　particularly　in　its　most　important　and　complicated　parts；　so　that　we　may　say　that　there　is　no　actively　functional　organ　that　has　not　undergone　a　process　of　adaptation　relative　to　its　function　and　the　requirements　of　the　organism。　　Not　only　is　every　gland　structurally　adapted；　down　to　the　very　minutest　histological　details；　to　its　function；　but　the　function　is　equally　minutely　adapted　to　the　needs　of　the　body。　　Every　cell　in　the　mucous　lining　of　the　intestine　is　exactly　regulated　in　its　relation　to　the　different　nutritive　substances；　and　behaves　in　quite　a　different　way　towards　the　fats；　and　towards　nitrogenous　substances；　or　peptones。

I　have　elsewhere　called　attention　to　the　many　adaptations　of　the　whale　to　the　surrounding　medium；　and　have　pointed　outwhat　has　long　been　known；　but　is　not　universally　admitted；　even　nowthat　in　it　a　great　number　of　important　organs　have　been　transformed　in　adaptation　to　the　peculiar　conditions　of　aquatic　life；　although　the　ancestors　of　the　whale　must　have　lived；　like　other　hair…covered　mammals；　on　land。　　I　cited　a　number　of　these　transformationsthe　fish…like　form　of　the　body；　the　hairlessness　of　the　skin；　the　transformation　of　the　fore…limbs　to　fins；　the　disappearance　of　the　hind…limbs　and　the　development　of　a　tail　fin；　the　layer　of　blubber　under　the　skin；　which　affords　the　protection　from　cold　necessary　to　a　warm…　blooded　animal；　the　disappearance　of　the　ear…muscles　and　the　auditory　passages；　the　displacement　of　the　external　nares　to　the　forehead　for　the　greater　security　of　the　breathing…h