darwin and modern science- 第79部分

VARIABLE　PROTECTIVE　RESEMBLANCE。

The　wonderful　power　of　rapid　colour　adjustment　possessed　by　the　cuttle…fish　was　observed　by　Darwin　in　1832　at　St　Jago；　Cape　de　Verd　Islands；　the　first　place　visited　during　the　voyage　of　the　〃Beagle〃。　　From　Rio　he　wrote　to　Henslow；　giving　the　following　account　of　his　observations；　May　18；　1832：　　〃I　took　several　specimens　of　an　Octopus　which　possessed　a　most　marvellous　power　of　changing　its　colours；　equalling　any　chameleon；　and　evidently　accommodating　the　changes　to　the　colour　of　the　ground　which　it　passed　over。　Yellowish　green；　dark　brown；　and　red；　were　the　prevailing　colours；　this　fact　appears　to　be　new；　as　far　as　I　can　find　out。〃　　（〃Life　and　Letters〃；　I。　pages　235；　236。　　See　also　Darwin's　〃Journal　of　Researches〃；　1876；　pages　6…　8；　where　a　far　more　detailed　account　is　given　together　with　a　reference　to　〃Encycl。　of　Anat。　and　Physiol。〃）

Darwin　was　well　aware　of　the　power　of　individual　colour　adjustment；　now　known　to　be　possessed　by　large　numbers　of　lepidopterous　pupae　and　larvae。　　An　excellent　example　was　brought　to　his　notice　by　C。V。　Riley　（〃More　Letters〃　II；　pages　385；　386。）；　while　the　most　striking　of　the　early　results　obtained　with　the　pupae　of　butterfliesthose　of　Mrs　M。E。　Barber　upon　Papilio　nireuswas　communicated　by　him　to　the　Entomological　Society　of　London。　　（〃Trans。　Ent。　Soc。　Lond。〃　1874；　page　519。　　See　also　〃More　Letters〃；　II。　page　403。）

It　is　also　necessary　to　direct　attention　to　C。W。　Beebe's　（〃Zoologica：　　N。Y。　Zool。　Soc。〃　Vol。　I。　No。　1；　Sept。　25；　1907：　〃Geographic　variation　in　birds　with　especial　reference　to　the　effects　of　humidity〃。）　recent　discovery　that　the　pigmentation　of　the　plumage　of　certain　birds　is　increased　by　confinement　in　a　superhumid　atmosphere。　　In　Scardafella　inca；　on　which　the　most　complete　series　of　experiments　was　made；　the　changes　took　place　only　at　the　moults；　whether　normal　and　annual　or　artificially　induced　at　shorter　periods。　　There　was　a　corresponding　increase　in　the　choroidal　pigment　of　the　eye。　　At　a　certain　advanced　stage　of　feather　pigmentation　a　brilliant　iridescent　bronze　or　green　tint　made　its　appearance　on　those　areas　where　iridescence　most　often　occurs　in　allied　genera。　　Thus　in　birds　no　less　than　in　insects；　characters　previously　regarded　as　of　taxonomic　value；　can　be　evoked　or　withheld　by　the　forces　of　the　environment。

WARNING　OR　APOSEMATIC　COLOURS。

From　Darwin's　description　of　the　colours　and　habits　it　is　evident　that　he　observed；　in　1833；　an　excellent　example　of　warning　colouring　in　a　little　South　American　toad　（Phryniscus　nigricans）。　　He　described　it　in　a　letter　to　Henslow；　written　from　Monte　Video；　Nov。　24；　1832：　　〃As　for　one　little　toad；　I　hope　it　may　be　new；　that　it　may　be　christened　'diabolicus。'　　Milton　must　allude　to　this　very　individual　when　he　talks　of　'squat　like　a　toad'；　its　colours　are　by　Werner　（〃Nomenclature　of　Colours〃；　1821）　ink　black；　vermilion　red　and　buff　orange。〃　　（〃More　Letters〃；　I。　page　12。）　　In　the　〃Journal　of　Researches〃　（1876；　page　97。）　its　colours　are　described　as　follows：　　〃If　we　imagine；　first；　that　it　had　been　steeped　in　the　blackest　ink；　and　then；　when　dry；　allowed　to　crawl　over　a　board；　freshly　painted　with　the　brightest　vermilion；　so　as　to　colour　the　soles　of　its　feet　and　parts　of　its　stomach；　a　good　idea　of　its　appearance　will　be　gained。〃　　〃Instead　of　being　nocturnal　in　its　habits；　as　other　toads　are；　and　living　in　damp　obscure　recesses；　it　crawls　during　the　heat　of　the　day　about　the　dry　sand…hillocks　and　arid　plains；。。。〃　　The　appearance　and　habits　recall　T。　Belt's　well…known　description　of　the　conspicuous　little　Nicaraguan　frog　which　he　found　to　be　distasteful　to　a　duck。　　（〃The　Naturalist　in　Nicaragua〃　（2nd　edition）　London；　1888；　page　321。）

The　recognition　of　the　Warning　Colours　of　caterpillars　is　due　in　the　first　instance　to　Darwin；　who；　reflecting　on　Sexual　Selection；　was　puzzled　by　the　splendid　colours　of　sexually　immature　organisms。　　He　applied　to　Wallace　〃who　has　an　innate　genius　for　solving　difficulties。〃　　（〃Descent　of　Man〃；　page　325。　　On　this　and　the　following　page　an　excellent　account　of　the　discovery　will　be　found；　as　well　as　in　Wallace's　〃Natural　Selection〃；　London；　1875；　pages　117…122。）　　Darwin's　original　letter　exists　（〃Life　and　Letters〃；　III。　pages　93；　94。）；　and　in　it　we　are　told　that　he　had　taken　the　advice　given　by　Bates：　　〃You　had　better　ask　Wallace。〃　　After　some　consideration　Wallace　replied　that　he　believed　the　colours　of　conspicuous　caterpillars　and　perfect　insects　were　a　warning　of　distastefulness　and　that　such　forms　would　be　refused　by　birds。　　Darwin's　reply　（〃Life　and　Letters〃；　III。　pages　94；　95。）　is　extremely　interesting　both　for　its　enthusiasm　at　the　brilliancy　of　the　hypothesis　and　its　caution　in　acceptance　without　full　confirmation：

〃Bates　was　quite　right；　you　are　the　man　to　apply　to　in　a　difficulty。　　I　never　heard　anything　more　ingenious　than　your　suggestion；　and　I　hope　you　may　be　able　to　prove　it　true。　　That　is　a　splendid　fact　about　the　white　moths　（A　single　white　moth　which　was　rejected　by　young　turkeys；　while　other　moths　were　greedily　devoured：　　〃Natural　Selection〃；　1875；　page　78。）；　it　warms　one's　very　blood　to　see　a　theory　thus　almost　proved　to　be　true。〃

Two　years　later　the　hypothesis　was　proved　to　hold　for　caterpillars　of　many　kinds　by　J。　Jenner　Weir　and　A。G。　Butler；　whose　observations　have　since　been　abundantly　confirmed　by　many　naturalists。　　Darwin　wrote　to　Weir；　May　13；　1869：　　〃Your　verification　of　Wallace's　suggestion　seems　to　me　to　amount　to　quite　a　discovery。〃　　（〃More　Letters〃；　II。　page　71　（footnote）。）

RECOGNITION　OR　EPISEMATIC　CHARACTERS。

This　principle　does　not　appear　to　have　been　in　any　way　foreseen　by　Darwin；　although　he　draws　special　attention　to　several　elements　of　pattern　which　would　now　be　interpreted　by　many　naturalists　as　epismes。　　He　believed　that　the　markings　in　question　interfered　with　the　cryptic　effect；　and　came　to　the　conclusion　that；　even　when　common　to　both　sexes；　they　〃are　the　result　of　sexual　selection　primarily　applied　to　the　male。〃　　（〃Descent　of　Man〃；　page　544。）　　The　most　familiar　of　all　recognition　characters　was　carefully　explained　by　him；　although　here　too　explained　as　an　ornamental　feature　now　equally　transmitted　to　both　sexes：　　〃The　hare　on　her　form　is　a　familiar　instance　of　concealment　through　colour；　yet　this　principle　partly　fails　in　a　closely…allied　species；　the　rabbit；　for　when　running　to　its　burrow；　it　is　made　conspicuous　to　the　sportsman；　and　no　doubt　to　all　beasts　of　prey；　by　its　upturned　white　tail。〃　　（〃Descent　of　Man〃；　page　542。）

The　analogous　episematic　use　of　the　bright　colours　of　flowers　to　attract　insects　for　effecting　cross…fertilisation　and　of　fruits　to　attract　vertebrates　for　effecting　dispersal　is　very　clearly　explained　in　the　〃Origin〃。　　（Edition　1872；　page　161。　　For　a　good　example　of　Darwin's　caution　in　dealing　with　exceptions　see　the　allusion　to　brightly　coloured　fruit　in　〃More　Letters〃；　II。　page　348。）

It　is　not；　at　this　point；　necessary　to　treat　sematic　characters　at　any　greater　length。　　They　will　form　the　subject　of　a　large　part　of　the　following　section；　where　the　models　of　Batesian　（Pseudaposematic）　mimicry　are　considered　as　well　as　the　Mullerian　（Synaposematic）　combinations　of　Warning　Colours。

MIMICRY；BATESIAN　OR　PSEUDAPOSEMATIC；　MULLERIAN　OR　SYNAPOSEMATIC。

The　existence　of　superficial　resemblances　between　animals　of　various　degrees　of　affinity　must　have　been　observed　for　hundreds　of　years。　　Among　the　early　examples；　the　best　known　to　me　have　been　found　in　the　manuscript　note…books　and　collections　of　W。J。　Burchell；　the　great　traveller　in　Africa　（1810…15）　and　Brazil　（1825…30）。　　The　most　interesting　of　his　records　on　this　subject　are　brought　together　in　the　following　paragraphs。

Conspicuous　among　well…defended　insects　are　the　dark　steely　or　iridescent　greenish　blue　fossorial　wasps　or　sand…wasps；　Sphex　and　the　allied　genera。　　Many　Longicorn　beetles　mimic　these　in　colour；　slender　shape　of　body　and　limbs；　rapid　movements；　and　the　readiness　with　which　they　take　to　flight。　　On　Dec。　21；　1812；　Burchell　captured　one　such　beetle　（Promeces　viridis）　at　Kosi　Fountain　on　the　journey　from　the　source　of　the　Kuruman　River　to　Klaarwater。　　It　is　correctly　placed　among　the　Longicorns　in　his　catalogue；　but　opposite　to　its　number　is　the　comment　〃Sphex！　totus　purpureus。〃

In　our　own　country　the　black…and…yellow　colouring　of　many　stinging　insects；　especially　the　ordinary　wasps；　affords　perhaps　the　commonest　model　for　mimicry。　　It　is　reproduced　with　more　or　less　accuracy　on　moths；　flies　and　beetles。　　Among　the　latter　it　is　again　a　Longicorn　which　offers　one　of　the　best…known；　although　by　no　means　one　of　the　most　perfect；　examples。　　The　appearance　of　the　well…known　〃wasp…beetle〃　（Clytus　arietis）　in　the　living　state　is　sufficiently　suggestive　to　prevent　the　great　majority　of　people　from　touching　it。　　In　Burchell's　Brazilian　collection　there　is　a　nearly　allied　species　（Neoclytus　curvatus）　which　appears　to　be　somewhat　less　wasp…　like　than　the　British　beetle。　　The　specimen　bears　the　number　〃1188；〃　and　the　date　March　27；　1827；　when　Burchell　was　collecting　in　the　neighbourhood　of　San　Paulo。　　Turning　to　the　corresponding　number　in　the　Brazilian　note…　book　we　find　this　record：　　〃It　runs　rapidly　like　an　ichneumon　or　wasp；　of　which　it　has　the　appearance。〃

The　formidable；　well…defended　ants　are　as　freely　mimicked　by　other　insects　as　the　sand…wasps；　ordinary　wasps　and　bees。　　Thus　on　February　17；　1901；　Guy　A。K。　Marshall　captured；　near　Salisbury；　Mashonaland；　three　similar　species　of　ants　（Hymenoptera）　with　a　bug　（Hemiptera）　and　a　Locustid　（Orthoptera）；　the　two　latter　mimicking　the　former。　　All　the　insects；　seven　in　number；　were　caught　on　a　single　plant；　a　small　bushy　vetch。　　（〃Trans。　Ent。　Soc。　Lond。〃　1902；　page　535；　plate　XIX。　figs。　53…59。）

This　is　an　interesting　recent　example　from　South　Africa；　and　large　numbers　of　others　might　be　addedthe　observations　of　many　naturalists　in　many　lands；　but　nearly　all　of　them　known　since　that　general　awakening　of　interest　in　the　subject　which　was　inspired　by　the　great　hypotheses　of　H。W。　Bates　and　Fritz　Muller。　　We　find；　however；　that　Burchell　had　more　than　once　recorded　the　mimetic　resemblance　to　ants。　　An　extremely　ant…like　bug　（the　larva　of　a　species　of　Alydus）　in　his　Brazilian　collection