darwin and modern science-第15章

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ection…value　of　the　initial　stages；　and　the　stages　in　the　increase　of　variations；　as　has　been　already　shown。　　But　the　selection…value　of　a　finished　adaptation　can　in　many　cases　be　statistically　determined。　　Cesnola　and　Poulton　have　made　valuable　experiments　in　this　direction。　　The　former　attached　forty…five　individuals　of　the　green；　and　sixty…five　of　the　brown　variety　of　the　praying　mantis　（Mantis　religiosa）；　by　a　silk　thread　to　plants；　and　watched　them　for　seventeen　days。　　The　insects　which　were　on　a　surface　of　a　colour　similar　to　their　own　remained　uneaten；　while　twenty…five　green　insects　on　brown　parts　of　plants　had　all　disappeared　in　eleven　days。

The　experiments　of　Poulton　and　Sanders　（〃Report　of　the　British　Association〃　（Bristol；　1898）；　London；　1899；　pages　906…909。）　were　made　with　600　pupae　of　Vanessa　urticae；　the　〃tortoise…shell　butterfly。〃　　The　pupae　were　artificially　attached　to　nettles；　tree…trunks；　fences；　walls；　and　to　the　ground；　some　at　Oxford；　some　at　St　Helens　in　the　Isle　of　Wight。　　In　the　course　of　a　month　93　per　cent　of　the　pupae　at　Oxford　were　killed；　chiefly　by　small　birds；　while　at　St　Helens　68　per　cent　perished。　　The　experiments　showed　very　clearly　that　the　colour　and　character　of　the　surface　on　which　the　pupa　restsand　thus　its　own　conspicuousnessare　of　the　greatest　importance。　　At　Oxford　only　the　four　pupae　which　were　fastened　to　nettles　emerged；　all　the　reston　bark；　stones　and　the　likeperished。　　At　St　Helens　the　elimination　was　as　follows：　　on　fences　where　the　pupae　were　conspicuous；　92　per　cent；　on　bark；　66　per　cent；　on　walls；　54　per　cent；　and　among　nettles；　57　per　cent。　　These　interesting　experiments　confirm　our　views　as　to　protective　coloration；　and　show　further；　THAT　THE　RATIO　OF　ELIMINATION　IN　THE　SPECIES　IS　A　VERY　HIGH　ONE；　AND　THAT　THEREFORE　SELECTION　MUST　BE　VERY　KEEN。

We　may　say　that　the　process　of　selection　follows　as　a　logical　necessity　from　the　fulfilment　of　the　three　preliminary　postulates　of　the　theory：　　variability；　heredity；　and　the　struggle　for　existence；　with　its　enormous　ratio　of　elimination　in　all　species。　　To　this　we　must　add　a　fourth　factor；　the　INTENSIFICATION　of　variations　which　Darwin　established　as　a　fact；　and　which　we　are　now　able　to　account　for　theoretically　on　the　basis　of　germinal　selection。　　It　may　be　objected　that　there　is　considerable　uncertainty　about　this　LOGICAL　proof；　because　of　our　inability　to　demonstrate　the　selection…　value　of　the　initial　stages　and　the　individual　stages　of　increase。　　We　have　therefore　to　fall　back　on　PRESUMPTIVE　EVIDENCE。　　This　is　to　be　found　in　THE　INTERPRETATIVE　VALUE　OF　THE　THEORY。　　Let　us　consider　this　point　in　greater　detail。

In　the　first　place；　it　is　necessary　to　emphasise　what　is　often　overlooked；　namely；　that　the　theory　not　only　explains　the　TRANSFORMATIONS　of　species；　it　also　explains　THEIR　REMAINING　THE　SAME；　in　addition　to　the　principle　of　varying；　it　contains　within　itself　that　of　PERSISTING。　　It　is　part　of　the　essence　of　selection；　that　it　not　only　causes　a　part　to　VARY　till　it　has　reached　its　highest　pitch　of　adaptation；　but　that　it　MAINTAINS　IT　AT　THIS　PITCH。　　THIS　CONSERVING　INFLUENCE　OF　NATURAL　SELECTION　is　of　great　importance；　and　was　early　recognised　by　Darwin；　it　follows　naturally　from　the　principle　of　the　survival　of　the　fittest。

We　understand　from　this　how　it　is　that　a　species　which　has　become　fully　adapted　to　certain　conditions　of　life　ceases　to　vary；　but　remains　〃constant；〃　as　long　as　the　conditions　of　life　FOR　IT　remain　unchanged；　whether　this　be　for　thousands　of　years；　or　for　whole　geological　epochs。　　But　the　most　convincing　proof　of　the　power　of　the　principle　of　selection　lies　in　the　innumerable　multitude　of　phenomena　which　cannot　be　explained　in　any　other　way。　　To　this　category　belong　all　structures　which　are　only　PASSIVELY　of　advantage　to　the　organism；　because　none　of　these　can　have　arisen　by　the　alleged　LAMARCKIAN　PRINCIPLE。　　These　have　been　so　often　discussed　that　we　need　do　no　more　than　indicate　them　here。　　Until　quite　recently　the　sympathetic　coloration　of　animalsfor　instance；　the　whiteness　of　Arctic　animalswas　referred；　at　least　in　part；　to　the　DIRECT　influence　of　external　factors；　but　the　facts　can　best　be　explained　by　referring　them　to　the　processes　of　selection；　for　then　it　is　unnecessary　to　make　the　gratuitous　assumption　that　many　species　are　sensitive　to　the　stimulus　of　cold　and　that　others　are　not。　　The　great　majority　of　Arctic　land…animals；　mammals　and　birds；　are　white；　and　this　proves　that　they　were　all　able　to　present　the　variation　which　was　most　useful　for　them。　　The　sable　is　brown；　but　it　lives　in　trees；　where　the　brown　colouring　protects　and　conceals　it　more　effectively。　　The　musk…sheep　（Ovibos　moschatus）　is　also　brown；　and　contrasts　sharply　with　the　ice　and　snow；　but　it　is　protected　from　beasts　of　prey　by　its　gregarious　habit；　and　therefore　it　is　of　advantage　to　be　visible　from　as　great　a　distance　as　possible。　　That　so　many　species　have　been　able　to　give　rise　to　white　varieties　does　not　depend　on　a　special　sensitiveness　of　the　skin　to　the　influence　of　cold；　but　to　the　fact　that　Mammals　and　Birds　have　a　general　tendency　to　vary　towards　white。　　Even　with　us；　many　birdsstarlings；　blackbirds；　swallows；　etc。occasionally　produce　white　individuals；　but　the　white　variety　does　not　persist；　because　it　readily　falls　a　victim　to　the　carnivores。　　This　is　true　of　white　fawns；　foxes；　deer；　etc。　　The　whiteness；　therefore；　arises　from　internal　causes；　and　only　persists　when　it　is　useful。　　A　great　many　animals　living　in　a　GREEN　ENVIRONMENT　have　become　clothed　in　green；　especially　insects；　caterpillars；　and　Mantidae；　both　persecuted　and　persecutors。

That　it　is　not　the　direct　effect　of　the　environment　which　calls　forth　the　green　colour　is　shown　by　the　many　kinds　of　caterpillar　which　rest　on　leaves　and　feed　on　them；　but　are　nevertheless　brown。　　These　feed　by　night　and　betake　themselves　through　the　day　to　the　trunk　of　the　tree；　and　hide　in　the　furrows　of　the　bark。　　We　cannot；　however；　conclude　from　this　that　they　were　UNABLE　to　vary　towards　green；　for　there　are　Arctic　animals　which　are　white　only　in　winter　and　brown　in　summer　（Alpine　hare；　and　the　ptarmigan　of　the　Alps）；　and　there　are　also　green　leaf…insects　which　remain　green　only　while　they　are　young　and　difficult　to　see　on　the　leaf；　but　which　become　brown　again　in　the　last　stage　of　larval　life；　when　they　have　outgrown　the　leaf。　They　then　conceal　themselves　by　day；　sometimes　only　among　withered　leaves　on　the　ground；　sometimes　in　the　earth　itself。　　It　is　interesting　that　in　one　genus；　Chaerocampa；　one　species　is　brown　in　the　last　stage　of　larval　life；　another　becomes　brown　earlier；　and　in　many　species　the　last　stage　is　not　wholly　brown；　a　part　remaining　green。　　Whether　this　is　a　case　of　a　double　adaptation；　or　whether　the　green　is　being　gradually　crowded　out　by　the　brown；　the　fact　remains　that　the　same　species；　even　the　same　individual；　can　exhibit　both　variations。　　The　case　is　the　same　with　many　of　the　leaf…like　Orthoptera；　as；　for　instance；　the　praying　mantis　（Mantis　religiosa）　which　we　have　already　mentioned。

But　the　best　proofs　are　furnished　by　those　often…cited　cases　in　which　the　insect　bears　a　deceptive　resemblance　to　another　object。　　We　now　know　many　such　cases；　such　as　the　numerous　imitations　of　green　or　withered　leaves；　which　are　brought　about　in　the　most　diverse　ways；　sometimes　by　mere　variations　in　the　form　of　the　insect　and　in　its　colour；　sometimes　by　an　elaborate　marking；　like　that　which　occurs　in　the　Indian　leaf…butterflies；　Kallima　inachis。　　In　the　single　butterfly…genus　Anaea；　in　the　woods　of　South　America；　there　are　about　a　hundred　species　which　are　all　gaily　coloured　on　the　upper　surface；　and　on　the　reverse　side　exhibit　the　most　delicate　imitation　of　the　colouring　and　pattern　of　a　leaf；　generally　without　any　indication　of　the　leaf…ribs；　but　extremely　deceptive　nevertheless。　　Anyone　who　has　seen　only　one　such　butterfly　may　doubt　whether　many　of　the　insignificant　details　of　the　marking　can　really　be　of　advantage　to　the　insect。　　Such　details　are　for　instance　the　apparent　holes　and　splits　in　the　apparently　dry　or　half…rotten　leaf；　which　are　usually　due　to　the　fact　that　the　scales　are　absent　on　a　circular　or　oval　patch　so　that　the　colourless　wing…membrane　lies　bare；　and　one　can　look　through　the　spot　as　through　a　window。　　Whether　the　bird　which　is　seeking　or　pursuing　the　butterflies　takes　these　holes　for　dewdrops；　or　for　the　work　of　a　devouring　insect；　does　not　affect　the　question；　the　mirror…like　spot　undoubtedly　increases　the　general　deceptiveness；　for　the　same　thing　occurs　in　many　leaf…butterflies；　though　not　in　all；　and　in　some　cases　it　is　replaced　in　quite　a　peculiar　manner。　　In　one　species　of　Anaea　（A。　divina）；　the　resting　butterfly　looks　exactly　like　a　leaf　out　of　the　outer　edge　of　which　a　large　semicircular　piece　has　been　eaten；　possibly　by　a　caterpillar；　but　if　we　look　more　closely　it　is　obvious　that　there　is　no　part　of　the　wing　absent；　and　that　the　semicircular　piece　is　of　a　clear；　pale　yellow　colour；　while　the　rest　of　the　wing　is　of　a　strongly　contrasted　dark　brown。

But　the　deceptive　resemblance　may　be　caused　in　quite　a　different　manner。　　I　have　often　speculated　as　to　what　advantage　the　brilliant　white　C　could　give　to　the　otherwise　dusky…coloured　〃Comma　butterfly〃　（Grapta　C。　album）。　　Poulton's　recent　observations　（〃Proc。　Ent。　Soc〃。；　London；　May　6；　1903。）　have　shown　that　this　represents　the　imitation　of　a　crack　such　as　is　often　seen　in　dry　leaves；　and　is　very　conspicuous　because　the　light　shines　through　it。

The　utility　obviously　lies　in　presenting　to　the　bird　the　very　familiar　picture　of　a　broken　leaf　with　a　clear　shining　slit；　and　we　may　conclude；　from　the　imitation　of　such　small　details；　that　the　birds　are　very　sharp　observers　and　that　the　smallest　deviation　from　the　usual　arrests　their　attention　and　incites　them　to　closer　investigation。　　It　is　obvious　that　such　detailedwe　might　almost　say　such　subtledeceptive　resemblances　could　only　have　come　about　in　the　course　of　long　ages　through　the　acquirement　from　time　to　time　of　something　new　which　heightened　the　already　existing　resemblance。

In　face　of　facts　like　these　there　can　be　no　question　of　chance；　and　no　one　has　succeeded　so　far　in　finding　any　other　explanation　to　replace　that　by　selection。　　For　the　rest；　the　apparent　leaves　are　by　no　means　perfect　copies　of　a　leaf；　many　of　them　only　represent　the　torn　or　broken　piece；　or　the　half　or　two…thirds　of　a　leaf；　but　then　the　leaves　themselves　frequently　do　not　present　themselves　to　the　eye　as　a　whole；　but　partially　concealed　among　other　leaves。　　Even　those　butterflies　which；　like　the　species　of　Kallima　and　Anaea；　represent　the　whole　of　a　leaf　with　stalk；　ribs；　apex；　and　the　whole　breadth；　are　not　actual　copies　which　would　satisfy　a　botanist；　there　is　often　much　wanting。　　In　Kallima　the　lateral　ribs　of　the　leaf　are　never　all　included　in　the　markings；　there　are　only　two　or　three　on　the　left　side　and　at　most　four　or　five　on　the　right；　and　in　many　individuals　these　are　rather　obscure；　while　in　others　they　are　comparatively　distinct。　　This　furnishes　us　with　fresh　evidence　in　favour　of　their　origin　through　processes　of　selection；　for　a　botanically　perfect　picture　could　not　arise　in　this　way；　there　could　only　be　a　fixing　of　such　details　as　heightened　the