darwin and modern science-第85节

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　to　render　the　brief　summary　of　Darwin's　thoughts　and　opinions　on　the　subject　in　any　way　complete；　it　was　found　necessary　to　say　again　much　that　has　often　been　said　before。　　No　attempt　has　been　made　to　display　as　a　whole　the　vast　contribution　of　Wallace；　but　certain　of　its　features　are　incidentally　revealed　in　passages　quoted　from　Darwin's　letters。　　It　is　assumed　that　the　reader　is　familiar　with　the　well…known　theories　of　Protective　Resemblance；　Warning　Colours；　and　Mimicry　both　Batesian　and　Mullerian。　　It　would　have　been　superfluous　to　explain　these　on　the　present　occasion；　for　a　far　more　detailed　account　than　could　have　been　attempted　in　these　pages　has　recently　appeared。　　（Poulton；　〃Essays　on　Evolution〃　Oxford；　1908；　pages　293…382。）　　Among　the　older　records　I　have　made　a　point　of　bringing　together　the　principal　observations　scattered　through　the　note…　books　and　collections　of　W。J。　Burchell。　　These　have　never　hitherto　found　a　place　in　any　memoir　dealing　with　the　significance　of　the　colours　of　animals。

INCIDENTAL　COLOURS。

Darwin　fully　recognised　that　the　colours　of　living　beings　are　not　necessarily　of　value　as　colours；　but　that　they　may　be　an　incidental　result　of　chemical　or　physical　structure。　　Thus　he　wrote　to　T。　Meehan；　Oct。　9；　1874：　　〃I　am　glad　that　you　are　attending　to　the　colours　of　dioecious　flowers；　but　it　is　well　to　remember　that　their　colours　may　be　as　unimportant　to　them　as　those　of　a　gall；　or；　indeed；　as　the　colour　of　an　amethyst　or　ruby　is　to　these　gems。〃　　（〃More　Letters　of　Charles　Darwin〃；　Vol。　I。　pages　354；　355。　　See　also　the　admirable　account　of　incidental　colours　in　〃Descent　of　Man〃　（2nd　edition）；　1874；　pages　261；　262。）

Incidental　colours　remain　as　available　assets　of　the　organism　ready　to　be　turned　to　account　by　natural　selection。　　It　is　a　probable　speculation　that　all　pigmentary　colours　were　originally　incidental；　but　now　and　for　immense　periods　of　time　the　visible　tints　of　animals　have　been　modified　and　arranged　so　as　to　assist　in　the　struggle　with　other　organisms　or　in　courtship。　　The　dominant　colouring　of　plants；　on　the　other　hand；　is　an　essential　element　in　the　paramount　physiological　activity　of　chlorophyll。　　In　exceptional　instances；　however；　the　shapes　and　visible　colours　of　plants　may　be　modified　in　order　to　promote　concealment。

TELEOLOGY　AND　ADAPTATION。

In　the　department　of　Biology　which　forms　the　subject　of　this　essay；　the　adaptation　of　means　to　an　end　is　probably　more　evident　than　in　any　other；　and　it　is　therefore　of　interest　to　compare；　in　a　brief　introductory　section；　the　older　with　the　newer　teleological　views。

The　distinctive　feature　of　Natural　Selection　as　contrasted　with　other　attempts　to　explain　the　process　of　Evolution　is　the　part　played　by　the　struggle　for　existence。　　All　naturalists　in　all　ages　must　have　known　something　of　the　operations　of　〃Nature　red　in　tooth　and　claw〃；　but　it　was　left　for　this　great　theory　to　suggest　that　vast　extermination　is　a　necessary　condition　of　progress；　and　even　of　maintaining　the　ground　already　gained。

Realising　that　fitness　is　the　outcome　of　this　fierce　struggle；　thus　turned　to　account　for　the　first　time；　we　are　sometimes　led　to　associate　the　recognition　of　adaptation　itself　too　exclusively　with　Natural　Selection。　　Adaptation　had　been　studied　with　the　warmest　enthusiasm　nearly　forty　years　before　this　great　theory　was　given　to　the　scientific　world；　and　it　is　difficult　now　to　realise　the　impetus　which　the　works　of　Paley　gave　to　the　study　of　Natural　History。　　That　they　did　inspire　the　naturalists　of　the　early　part　of　the　last　century　is　clearly　shown　in　the　following　passages。

In　the　year　1824　the　Ashmolean　Museum　at　Oxford　was　intrusted　to　the　care　of　J。S。　Duncan　of　New　College。　　He　was　succeeded　in　this　office　by　his　brother；　P。B。　Duncan；　of　the　same　College；　author　of　a　History　of　the　Museum；　which　shows　very　clearly　the　influence　of　Paley　upon　the　study　of　nature；　and　the　dominant　position　given　to　his　teachings：　　〃Happily　at　this　time　（1824）　a　taste　for　the　study　of　natural　history　had　been　excited　in　the　University　by　Dr　Paley's　very　interesting　work　on　Natural　Theology；　and　the　very　popular　lectures　of　Dr　Kidd　on　Comparative　Anatomy；　and　Dr　Buckland　on　Geology。〃　　In　the　arrangement　of　the　contents　of　the　Museum　the　illustration　of　Paley's　work　was　given　the　foremost　place　by　J。S。　Duncan：　　〃The　first　division　proposes　to　familiarize　the　eye　to　those　relations　of　all　natural　objects　which　form　the　basis　of　argument　in　Dr　Paley's　Natural　Theology；　to　induce　a　mental　habit　of　associating　the　view　of　natural　phenomena　with　the　conviction　that　they　are　the　media　of　Divine　manifestation；　and　by　such　association　to　give　proper　dignity　to　every　branch　of　natural　science。〃　　（（From　〃History　and　Arrangement　of　the　Ashmolean　Museum〃　by　P。B。　Duncan：　　see　pages　vi；　vii　of　〃A　Catalogue　of　the　Ashmolean　Museum〃；　Oxford；　1836。）

The　great　naturalist；　W。J。　Burchell；　in　his　classical　work　shows　the　same　recognition　of　adaptation　in　nature　at　a　still　earlier　date。　　Upon　the　subject　of　collections　he　wrote　（〃Travels　in　the　Interior　of　Southern　Africa〃；　London；　Vol。　I。　1822；　page　505。　　The　references　to　Burchell's　observations　in　the　present　essay　are　adapted　from　the　author's　article　in　〃Report　of　the　British　and　South　African　Associations〃；　1905；　Vol。　III。　pages　57…110。）：　　〃It　must　not　be　supposed　that　these　charms　（the　pleasures　of　Nature）　are　produced　by　the　mere　discovery　of　new　objects：　　it　is　the　harmony　with　which　they　have　been　adapted　by　the　Creator　to　each　other；　and　to　the　situations　in　which　they　are　found；　which　delights　the　observer　in　countries　where　Art　has　not　yet　introduced　her　discords。〃　　The　remainder　of　the　passage　is　so　admirable　that　I　venture　to　quote　it：　　〃To　him　who　is　satisfied　with　amassing　collections　of　curious　objects；　simply　for　the　pleasure　of　possessing　them；　such　objects　can　afford；　at　best；　but　a　childish　gratification；　faint　and　fleeting；　while　he　who　extends　his　view　beyond　the　narrow　field　of　nomenclature；　beholds　a　boundless　expanse；　the　exploring　of　which　is　worthy　of　the　philosopher；　and　of　the　best　talents　of　a　reasonable　being。〃

On　September　14；　1811；　Burchell　was　at　Zand　Valley　（Vlei）；　or　Sand　Pool；　a　few　miles　south…west　of　the　site　of　Prieska；　on　the　Orange　River。　　Here　he　found　a　Mesembryanthemum　（M。　turbiniforme；　now　M。　truncatum）　and　also　a　〃Gryllus〃　（Acridian）；　closely　resembling　the　pebbles　with　which　their　locality　was　strewn。　　He　says　of　both　of　these；　〃The　intention　of　Nature；　in　these　instances；　seems　to　have　been　the　same　as　when　she　gave　to　the　Chameleon　the　power　of　accommodating　its　color；　in　a　certain　degree；　to　that　of　the　object　nearest　to　it；　in　order　to　compensate　for　the　deficiency　of　its　locomotive　powers。　　By　their　form　and　colour；　this　insect　may　pass　unobserved　by　those　birds；　which　otherwise　would　soon　extirpate　a　species　so　little　able　to　elude　its　pursuers；　and　this　juicy　little　Mesembryanthemum　may　generally　escape　the　notice　of　cattle　and　wild　animals。〃　　（Loc。　cit。　pages　310；　311。　　See　Sir　William　Thiselton…Dyer　〃Morphological　Notes〃；　XI。；　〃Protective　Adaptations〃；　I。；　〃Annals　of　Botany〃；　Vol。　XX。　page　124。　　In　plates　VII。；　VIII。　and　IX。　accompanying　this　article　the　author　represents　the　species　observed　by　Burchell；　together　with　others　in　which　analogous　adaptations　exist。　　He　writes：　　〃Burchell　was　clearly　on　the　track　on　which　Darwin　reached　the　goal。　　But　the　time　had　not　come　for　emancipation　from　the　old　teleology。　　This；　however；　in　no　respect　detracts　from　the　merit　or　value　of　his　work。　　For；　as　Huxley　has　pointed　out　（〃Life　and　Letters　of　Thomas　Henry　Huxley〃；　London；　1900；　I。　page　457）；　the　facts　of　the　old　teleology　are　immediately　transferable　to　Darwinism；　which　simply　supplies　them　with　a　natural　in　place　of　a　supernatural　explanation。〃）　　Burchell　here　seems　to　miss；　at　least　in　part；　the　meaning　of　the　relationship　between　the　quiescence　of　the　Acridian　and　its　cryptic　colouring。　　Quiescence　is　an　essential　element　in　the　protective　resemblance　to　a　stoneprobably　even　more　indispensable　than　the　details　of　the　form　and　colouring。　　Although　Burchell　appears　to　overlook　this　point　he　fully　recognised　the　community　between　protection　by　concealment　and　more　aggressive　modes　of　defence；　for；　in　the　passage　of　which　a　part　is　quoted　above；　he　specially　refers　to　some　earlier　remarks　on　page　226　of　his　Vol。　I。　　We　here　find　that　even　when　the　oxen　were　resting　by　the　Juk　rivier　（Yoke　river）；　on　July　19；　1811；　Burchell　observed　〃Geranium　spinosum；　with　a　fleshy　stem　and　large　white　flowers。。。；　and　a　succulent　species　of　Pelargonium。。。so　defended　by　the　old　panicles；　grown　to　hard　woody　thorns；　that　no　cattle　could　browze　upon　it。〃　　He　goes　on　to　say；　〃In　this　arid　country；　where　every　juicy　vegetable　would　soon　be　eaten　up　by　the　wild　animals；　the　Great　Creating　Power；　with　all…provident　wisdom；　has　given　to　such　plants　either　an　acrid　or　poisonous　juice；　or　sharp　thorns；　to　preserve　the　species　from　annihilation。。。〃　　All　these　modes　of　defence；　especially　adapted　to　a　desert　environment；　have　since　been　generally　recognised；　and　it　is　very　interesting　to　place　beside　Burchell's　statement　the　following　passage　from　a　letter　written　by　Darwin；　Aug。　7；　1868；　to　G。H。　Lewes；　　〃