darwin and modern science-第86节

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l's　statement　the　following　passage　from　a　letter　written　by　Darwin；　Aug。　7；　1868；　to　G。H。　Lewes；　　〃That　Natural　Selection　would　tend　to　produce　the　most　formidable　thorns　will　be　admitted　by　every　one　who　has　observed　the　distribution　in　South　America　and　Africa　（vide　Livingstone）　of　thorn…　bearing　plants；　for　they　always　appear　where　the　bushes　grow　isolated　and　are　exposed　to　the　attacks　of　mammals。　　Even　in　England　it　has　been　noticed　that　all　spine…bearing　and　sting…bearing　plants　are　palatable　to　quadrupeds；　when　the　thorns　are　crushed。〃　　（〃More　Letters〃；　I。　page　308。）

ADAPTATION　AND　NATURAL　SELECTION。

I　have　preferred　to　show　the　influence　of　the　older　teleology　upon　Natural　History　by　quotations　from　a　single　great　and　insufficiently　appreciated　naturalist。　　It　might　have　been　seen　equally　well　in　the　pages　of　Kirby　and　Spence　and　those　of　many　other　writers。　　If　the　older　naturalists　who　thought　and　spoke　with　Burchell　of　〃the　intention　of　Nature〃　and　the　adaptation　of　beings　〃to　each　other；　and　to　the　situations　in　which　they　are　found；〃　could　have　conceived　the　possibility　of　evolution；　they　must　have　been　led；　as　Darwin　was；　by　the　same　considerations　to　Natural　Selection。　　This　was　impossible　for　them；　because　the　philosophy　which　they　followed　contemplated　the　phenomena　of　adaptation　as　part　of　a　static　immutable　system。　　Darwin；　convinced　that　the　system　is　dynamic　and　mutable；　was　prevented　by　these　very　phenomena　from　accepting　anything　short　of　the　crowning　interpretation　offered　by　Natural　Selection。　　（〃I　had　always　been　much　struck　by　such　adaptations　（e。g。　woodpecker　and　tree…frog　for　climbing；　seeds　for　dispersal）；　and　until　these　could　be　explained　it　seemed　to　me　almost　useless　to　endeavour　to　prove　by　indirect　evidence　that　species　have　been　modified。〃　　〃Autobiography〃　in　〃Life　and　Letters　of　Charles　Darwin〃；　Vol。　I。　page　82。　　The　same　thought　is　repeated　again　and　again　in　Darwin's　letters　to　his　friends。　　It　is　forcibly　urged　in　the　Introduction　to　the　〃Origin〃　（1859）；　page　3。）　　And　the　birth　of　Darwin's　unalterable　conviction　that　adaptation　is　of　dominant　importance　in　the　organic　world；a　conviction　confirmed　and　ever　again　confirmed　by　his　experience　as　a　naturalistmay　probably　be　traced　to　the　influence　of　the　great　theologian。　　Thus　Darwin；　speaking　of　his　Undergraduate　days；　tells　us　in　his　〃Autobiography〃　that　the　logic　of　Paley's　〃Evidences　of　Christianity〃　and　〃Moral　Philosophy〃　gave　him　as　much　delight　as　did　Euclid。

〃The　careful　study　of　these　works；　without　attempting　to　learn　any　part　by　rote；　was　the　only　part　of　the　academical　course　which；　as　I　then　felt　and　as　I　still　believe；　was　of　the　least　use　to　me　in　the　education　of　my　mind。　I　did　not　at　that　time　trouble　myself　about　Paley's　premises；　and　taking　these　on　trust；　I　was　charmed　and　convinced　by　the　long　line　of　argumentation。〃　　（〃Life　and　Letters〃；　I。　page　47。）

When　Darwin　came　to　write　the　〃Origin〃　he　quoted　in　relation　to　Natural　Selection　one　of　Paley's　conclusions。　　〃No　organ　will　be　formed；　as　Paley　has　remarked；　for　the　purpose　of　causing　pain　or　for　doing　an　injury　to　its　possessor。〃　　（〃Origin　of　Species〃　（1st　edition）　1859；　page　201。）

The　study　of　adaptation　always　had　for　Darwin；　as　it　has　for　many；　a　peculiar　charm。　　His　words；　written　Nov。　28；　1880；　to　Sir　W。　Thiselton…　Dyer；　are　by　no　means　inapplicable　to…day：　　〃Many　of　the　Germans　are　very　contemptuous　about　making　out　use　of　organs；　but　they　may　sneer　the　souls　out　of　their　bodies；　and　I　for　one　shall　think　it　the　most　interesting　part　of　natural　history。〃　　（〃More　Letters〃　II。　page　428。）

PROTECTIVE　AND　AGGRESSIVE　RESEMBLANCE：　　PROCRYPTIC　AND　ANTICRYPTIC　COLOURING。

Colouring　for　the　purpose　of　concealment　is　sometimes　included　under　the　head　Mimicry；　a　classification　adopted　by　H。W。　Bates　in　his　classical　paper。　　Such　an　arrangement　is　inconvenient；　and　I　have　followed　Wallace　in　keeping　the　two　categories　distinct。

The　visible　colours　of　animals　are　far　more　commonly　adapted　for　Protective　Resemblance　than　for　any　other　purpose。　　The　concealment　of　animals　by　their　colours；　shapes　and　attitudes；　must　have　been　well　known　from　the　period　at　which　human　beings　first　began　to　take　an　intelligent　interest　in　Nature。　　An　interesting　early　record　is　that　of　Samuel　Felton；　who　（Dec。　2；　1763）　figured　and　gave　some　account　of　an　Acridian　（Phyllotettix）　from　Jamaica。　　Of　this　insect　he　says　〃THE　THORAX　is　like　a　leaf　that　is　raised　perpendicularly　from　the　body。〃　　（〃Phil。　Trans。　Roy。　Soc。〃　Vol。　LIV。　Tab。　VI。　page　55。）

Both　Protective　and　Aggressive　Resemblances　were　appreciated　and　clearly　explained　by　Erasmus　Darwin　in　1794：　　〃The　colours　of　many　animals　seem　adapted　to　their　purposes　of　concealing　themselves　either　to　avoid　danger；　or　to　spring　upon　their　prey。〃　　（〃Zoonomia〃；　Vol。　I。　page　509；　London；　1794。）

Protective　Resemblance　of　a　very　marked　and　beautiful　kind　is　found　in　certain　plants；　inhabitants　of　desert　areas。　　Examples　observed　by　Burchell　almost　exactly　a　hundred　years　ago　have　already　been　mentioned。　　In　addition　to　the　resemblance　to　stones　Burchell　observed；　although　he　did　not　publish　the　fact；　a　South　African　plant　concealed　by　its　likeness　to　the　dung　of　birds。　　（Sir　William　Thiselton…Dyer　has　suggested　the　same　method　of　concealment　（〃Annals　of　Botany〃；　Vol。　XX。　page　123）。　　Referring　to　Anacampseros　papyracea；　figured　on　plate　IX。；　the　author　says　of　its　adaptive　resemblance：　　〃At　the　risk　of　suggesting　one　perhaps　somewhat　far…　fetched；　I　must　confess　that　the　aspect　of　the　plant　always　calls　to　my　mind　the　dejecta　of　some　bird；　and　the　more　so　owing　to　the　whitening　of　the　branches　towards　the　tips〃　（loc。　cit。　page　126）。　　The　student　of　insects；　who　is　so　familiar　with　this　very　form　of　protective　resemblance　in　larvae；　and　even　perfect　insects；　will　not　be　inclined　to　consider　the　suggestion　far…fetched。）　　The　observation　is　recorded　in　one　of　the　manuscript　journals　kept　by　the　great　explorer　during　his　journey。　　I　owe　the　opportunity　of　studying　it　to　the　kindness　of　Mr　Francis　A。　Burchell　of　the　Rhodes　University　College；　Grahamstown。　　The　following　account　is　given　under　the　date　July　5；　1812；　when　Burchell　was　at　the　Makkwarin　River；　about　half…way　between　the　Kuruman　River　and　Litakun　the　old　capital　of　the　Bachapins　（Bechuanas）：　　〃I　found　a　curious　little　Crassula　（not　in　flower）　so　snow　white；　that　I　should　never　has　（have）　distinguished　it　from　the　white　limestones。。。It　was　an　inch　high　and　a　little　branchy；。。。and　was　at　first　mistaken　for　the　dung　of　birds　of　the　passerine　order。　　I　have　often　had　occasion　to　remark　that　in　stony　place（s）　there　grow　many　small　succulent　plants　and　abound　insects　（chiefly　Grylli）　which　have　exactly　the　same　colour　as　the　ground　and　must　for　ever　escape　observation　unless　a　person　sit　on　the　ground　and　observe　very　attentively。〃

The　cryptic　resemblances　of　animals　impressed　Darwin　and　Wallace　in　very　different　degrees；　probably　in　part　due　to　the　fact　that　Wallace's　tropical　experiences　were　so　largely　derived　from　the　insect　world；　in　part　to　the　importance　assigned　by　Darwin　to　Sexual　Selection　〃a　subject　which　had　always　greatly　interested　me；〃　as　he　says　in　his　〃Autobiography〃；　（〃Life　and　Letters〃；　Vol。　I。　page　94。）　　There　is　no　reference　to　Cryptic　Resemblance　in　Darwin's　section　of　the　Joint　Essay；　although　he　gives　an　excellent　short　account　of　Sexual　Selection　（see　page　295）。　　Wallace's　section　on　the　other　hand　contains　the　following　statement：　　〃Even　the　peculiar　colours　of　many　animals；　especially　insects；　so　closely　resembling　the　soil　or　the　leaves　or　the　trunks　on　which　they　habitually　reside；　are　explained　on　the　same　principle；　for　though　in　the　course　of　ages　varieties　of　many　tints　may　have　occurred；　YET　THOSE　RACES　HAVING　COLOURS　BEST　ADAPTED　TO　CONCEALMENT　FROM　THEIR　ENEMIES　WOULD　INEVITABLY　SURVIVE　THE　LONGEST。〃　　（〃Journ。　Proc。　Linn。　Soc。〃　Vol。　III。　1859；　page　61。　　The　italics　are　Wallace's。）

It　would　occupy　too　much　space　to　attempt　any　discussion　of　the　difference　between　the　views　of　these　two　naturalists；　but　it　is　clear　that　Darwin；　although　fully　believing　in　the　efficiency　of　protective　resemblance　and　replying　to　St　George　Mivart's　contention　that　Natural　Selection　was　incompetent　to　produce　it　（〃Origin〃　（6th　edition）　London；　1872；　pages　181；　182；　see　also　page　66。）；　never　entirely　agreed　with　Wallace's　estimate　of　its　importance。　　Thus　the　following　extract　from　a　letter　to　Sir　Joseph　Hooker；　May　21；　1868；　refers　to　Wallace：　　〃I　find　I　must　（and　I　always　distrust　myself　when　I　differ　from　him）　separate　rather　widely　from　him　all　about　birds'　nests　and　protection；　he　is　riding　that　hobby　to　death。〃　　（〃More　Letters〃；　I。　page　304。）　　It　is　clear　from　the　account　given　in　〃The　Descent　of　Man〃；　（London；　1874；　pages　452…458。　　See　also　〃Life　and　Letters〃；　III。　pages　123…125；　and　〃More　Letters〃；　II。　pages　59…63；　72…74；　76…78；　84…90；　92；　93。）；　that　the　divergence　was　due　to　the　fact　that　Darwin　ascribed　more　importance　to　Sexual　Selection　than　did　Wallace；　and　Wallace　more　importance　to　Protective　Resemblance　than　Darwin。　　Thus　Darwin　wrote　to　Wallace；　Oct。　12　and　13；　1867：　　〃By　the　way；　I　cannot　but　think　that　you　push　protection　too　far　in　some　cases；　as　with　th