darwin and modern science-第132节

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　here　introduce　the　mental　factor　because　the　facts　seem　to　justify　the　inference。　　Thus　there　are　some　modes　of　behaviour　which　are　wholly　and　solely　dependent　upon　inherited　racial　preparation；　there　are　other　modes　of　behaviour　which　are　also　dependent；　in　part　at　least；　on　individual　preparation。　　In　the　former　case　the　behaviour　is　adaptive　on　the　first　occurrence　of　the　appropriate　presentation；　in　the　latter　case　accommodation　to　circumstances　is　only　reached　after　a　greater　or　less　amount　of　acquired　organic　modification　of　structure；　often　accompanied　（as　we　assume）　in　the　higher　animals　by　acquired　experience。　　Logically　and　biologically　the　two　classes　of　behaviour　are　clearly　distinguishable：　　but　the　analysis　of　complex　cases　of　behaviour　where　the　two　factors　cooperate；　is　difficult　and　requires　careful　and　critical　study　of　life…history。

The　foundations　of　the　mental　life　are　laid　in　the　conscious　experience　that　accompanies　those　modes　of　behaviour；　dependent　entirely　on　racial　preparation；　which　may　broadly　be　described　as　instinctive。　　In　the　eighth　chapter　of　〃The　Origin　of　Species〃　Darwin　says　（〃Origin　of　Species〃　（6th　edition）；　page　205。）；　〃I　will　not　attempt　any　definition　of　instinct。。。Every　one　understands　what　is　meant；　when　it　is　said　that　instinct　impels　the　cuckoo　to　migrate　and　to　lay　her　eggs　in　other　birds'　nests。　　An　action；　which　we　ourselves　require　experience　to　enable　us　to　perform；　when　performed　by　an　animal；　more　especially　by　a　very　young　one；　without　experience；　and　when　performed　by　many　individuals　in　the　same　way；　without　their　knowing　for　what　purpose　it　is　performed；　is　usually　said　to　be　instinctive。〃　　And　in　the　summary　at　the　close　of　the　chapter　he　says　（〃Origin　of　Species〃　（6th　edition）；　page　233。）；　〃I　have　endeavoured　briefly　to　show　that　the　mental　qualities　of　our　domestic　animals　vary；　and　that　the　variations　are　inherited。　　Still　more　briefly　I　have　attempted　to　show　that　instincts　vary　slightly　in　a　state　of　nature。　　No　one　will　dispute　that　instincts　are　of　the　highest　importance　to　each　animal。　　Therefore　there　is　no　real　difficulty；　under　changing　conditions　of　life；　in　natural　selection　accumulating　to　any　extent　slight　modifications　of　instinct　which　are　in　any　way　useful。　　In　many　cases　habit　or　use　and　disuse　have　probably　come　into　play。〃

Into　the　details　of　Darwin's　treatment　there　is　neither　space　nor　need　to　enter。　　There　are　some　ambiguous　passages；　but　it　may　be　said　that　for　him；　as　for　his　followers　to…day；　instinctive　behaviour　is　wholly　the　result　of　racial　preparation　transmitted　through　organic　heredity。　　For　the　performance　of　the　instinctive　act　no　individual　preparation　under　the　guidance　of　personal　experience　is　necessary。　　It　is　true　that　Darwin　quotes　with　approval　Huber's　saying　that　〃a　little　dose　of　judgment　or　reason　often　comes　into　play；　even　with　animals　low　in　the　scale　of　nature。〃　　（Ibid。　page　205。）　　But　we　may　fairly　interpret　his　meaning　to　be　that　in　behaviour；　which　is　commonly　called　instinctive；　some　element　of　intelligent　guidance　is　often　combined。　　If　this　be　conceded　the　strictly　instinctive　performance　（or　part　of　the　performance）　is　the　outcome　of　heredity　and　due　to　the　direct　transmission　of　parental　or　ancestral　aptitudes。　　Hence　the　instinctive　response　as　such　depends　entirely　on　how　the　nervous　mechanism　has　been　built　up　through　heredity；　while　intelligent　behaviour；　or　the　intelligent　factor　in　behaviour；　depends　also　on　how　the　nervous　mechanism　has　been　modified　and　moulded　by　use　during　its　development　and　concurrently　with　the　growth　of　individual　experience　in　the　customary　situations　of　daily　life。　　Of　course　it　is　essential　to　the　Darwinian　thesis　that　what　Sir　E。　Ray　Lankester　has　termed　〃educability；〃　not　less　than　instinct；　is　hereditary。　　But　it　is　also　essential　to　the　understanding　of　this　thesis　that　the　differentiae　of　the　hereditary　factors　should　be　clearly　grasped。

For　Darwin　there　were　two　modes　of　racial　preparation；　（1）　natural　selection；　and　（2）　the　establishment　of　individually　acquired　habit。　　He　showed　that　instincts　are　subject　to　hereditary　variation；　he　saw　that　instincts　are　also　subject　to　modification　through　acquisition　in　the　course　of　individual　life。　　He　believed　that　not　only　the　variations　but　also；　to　some　extent；　the　modifications　are　inherited。　　He　therefore　held　that　some　instincts　（the　greater　number）　are　due　to　natural　selection　but　that　others　（less　numerous）　are　due；　or　partly　due；　to　the　inheritance　of　acquired　habits。　　The　latter　involve　Lamarckian　inheritance；　which　of　late　years　has　been　the　centre　of　so　much　controversy。　　It　is　noteworthy　however　that　Darwin　laid　especial　emphasis　on　the　fact　that　many　of　the　most　typical　and　also　the　most　complex　instinctsthose　of　neuter　insectsdo　not　admit　of　such　an　interpretation。　　〃I　am　surprised；〃　he　says　（〃Origin　of　Species〃　（6th　edition）；　page　233。）；　〃that　no　one　has　hitherto　advanced　this　demonstrative　case　of　neuter　insects；　against　the　well…known　doctrine　of　inherited　habit；　as　advanced　by　Lamarck。〃　　None　the　less　Darwin　admitted　this　doctrine　as　supplementary　to　that　which　was　more　distinctively　his　ownfor　example　in　the　case　of　the　instincts　of　domesticated　animals。　　Still；　even　in　such　cases；　〃it　may　be　doubted；〃　he　says　（Ibid。　pages　210；　211。）；　〃whether　any　one　would　have　thought　of　training　a　dog　to　point；　had　not　some　one　dog　naturally　shown　a　tendency　in　this　line。。。so　that　habit　and　some　degree　of　selection　have　probably　concurred　in　civilising　by　inheritance　our　dogs。〃　　But　in　the　interpretation　of　the　instincts　of　domesticated　animals；　a　more　recently　suggested　hypothesis；　that　of　organic　selection　（Independently　suggested；　on　somewhat　different　lines；　by　Profs。　J。　Mark　Baldwin；　Henry　F。　Osborn　and　the　writer。）；　may　be　helpful。　　According　to　this　hypothesis　any　intelligent　modification　of　behaviour　which　is　subject　to　selection　is　probably　coincident　in　direction　with　an　inherited　tendency　to　behave　in　this　fashion。　　Hence　in　such　behaviour　there　are　two　factors：　　（1）　an　incipient　variation　in　the　line　of　such　behaviour；　and　（2）　an　acquired　modification　by　which　the　behaviour　is　carried　further　along　the　same　line。　　Under　natural　selection　those　organisms　in　which　the　two　factors　cooperate　are　likely　to　survive。　　Under　artificial　selection　they　are　deliberately　chosen　out　from　among　the　rest。

Organic　selection　has　been　termed　a　compromise　between　the　more　strictly　Darwinian　and　the　Lamarckian　principles　of　interpretation。　　But　it　is　not　in　any　sense　a　compromise。　　The　principle　of　interpretation　of　that　which　is　instinctive　and　hereditary　is　wholly　Darwinian。　　It　is　true　that　some　of　the　facts　of　observation　relied　upon　by　Lamarckians　are　introduced。　　For　Lamarckians　however　the　modifications　which　are　admittedly　factors　in　survival；　are　regarded　as　the　parents　of　inherited　variations；　for　believers　in　organic　selection　they　are　only　the　foster　parents　or　nurses。　It　is　because　organic　selection　is　the　direct　outcome　of　and　a　natural　extension　of　Darwin's　cardinal　thesis　that　some　reference　to　it　here　is　justifiable。　　The　matter　may　be　put　with　the　utmost　brevity　as　follows。　　（1）　Variations　（V）　occur；　some　of　which　are　in　the　direction　of　increased　adaptation　（＋）；　others　in　the　direction　of　decreased　adaptation　（…）。　　（2）　Acquired　modifications　（M）　also　occur。　　Some　of　these　are　in　the　direction　of　increased　accommodation　to　circumstances　（＋）；　while　others　are　in　the　direction　of　diminished　accommodation　（…）。　　Four　major　combinations　are　

（a）　　＋　V　with　＋　M；　（b）　　＋　V　with　…　M；　（c）　　…　V　with　＋　M；　（d）　　…　V　with　…　M。

Of　these　（d）　must　inevitably　be　eliminated　while　（a）　are　selected。　　The　predominant　survival　of　（a）　entails　the　survival　of　the　adaptive　variations　which　are　inherited。　　The　contributory　acquisitions　（＋M）　are　not　inherited；　but　they　are　none　the　less　factors　in　determining　the　survival　of　the　coincident　variations。　　It　is　surely　abundantly　clear　that　this　is　Darwinism　and　has　no　tincture　of　Lamarck's　essential　principle；　the　inheritance　of　acquired　characters。

Whether　Darwin　himself　would　have　accepted　this　interpretation　of　some　at　least　of　the　evidence　put　forward　by　Lamarckians　is　unfortunately　a　matter　of　conjecture。　　The　fact　remains　that　in　his　interpretation　of　instinct　and　in　allied　questions　he　accepted　the　inheritance　of　individually　acquired　modifications　of　behaviour　and　structure。

Darwin　was　chiefly　concerned　with　instinct　from　the　biological　rather　than　from　the　psychological　point　of　view。　　Indeed　it　must　be　confessed　that；　from　the　latter　standpoint；　his　conception　of　instinct　as　a　〃mental　faculty〃　which　〃impels〃　an　animal　to　the　performance　of　certain　actions；　scarcely　affords　a　satisfactory　basis　for　genetic　treatment。　　To　carry　out　the　spirit　of　Darwin's　teaching　it　is　necessary　to　link　more　closely　biological　and　psychological　evolution。　　The　first　step　towards　this　is　to　interpret　the　phenomena　of　instinctive　behaviour　in　terms　of　stimulation　and　response。　　It　may　be　well　to　take　a　particular　case。　　Swimming　on　the　part　of　a　duckling　is；　from　the　biological　point　of　view；　a　typical　example　of　instinctive　behaviour。　　Gently　lower　a　recently　hatched　bird　into　water：　　coordinat