darwin and modern science-第15节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


rkers　was　concerned：　　wings；　ovarian　tubes；　receptaculum　seminis；　a　number　of　the　facets　of　the　eye；　perhaps　even　the　whole　eye。　　As　to　the　ovarian　tubes　it　is　possible　that　their　degeneration　was　an　advantage　for　the　workers；　in　saving　energy；　and　if　so　selection　would　favour　the　degeneration；　but　how　could　the　presence　of　eyes　diminish　the　usefulness　of　the　workers　to　the　colony？　or　the　minute　receptaculum　seminis；　or　even　the　wings？　　These　parts　have　therefore　degenerated　BECAUSE　THEY　WERE　OF　NO　FURTHER　VALUE　TO　THE　INSECT。　　But　if　selection　did　not　influence　the　setting　aside　of　these　parts　because　they　were　neither　of　advantage　nor　of　disadvantage　to　the　species；　then　the　Darwinian　factor　of　selection　is　here　confronted　with　a　puzzle　which　it　cannot　solve　alone；　but　which　at　once　becomes　clear　when　germinal　selection　is　added。　　For　the　determinants　of　organs　that　have　no　further　value　for　the　organism；　must；　as　we　have　already　explained；　embark　on　a　gradual　course　of　retrograde　development。

In　ants　the　degeneration　has　gone　so　far　that　there　are　no　wing…rudiments　present　in　ANY　species；　as　is　the　case　with　so　many　butterflies；　flies；　and　locusts；　but　in　the　larvae　the　imaginal　discs　of　the　wings　are　still　laid　down。　　With　regard　to　the　ovaries；　degeneration　has　reached　different　levels　in　different　species　of　ants；　as　has　been　shown　by　the　researches　of　my　former　pupil；　Elizabeth　Bickford。　　In　many　species　there　are　twelve　ovarian　tubes；　and　they　decrease　from　that　number　to　one；　indeed；　in　one　species　no　ovarian　tube　at　all　is　present。　　So　much　at　least　is　certain　from　what　has　been　said；　that　in　this　case　EVERYTHING　depends　on　the　fluctuations　of　the　elements　of　the　germ…plasm。　　Germinal　selection；　here　as　elsewhere；　presents　the　variations　of　the　determinants；　and　personal　selection　favours　or　rejects　these；　or；if　it　be　a　question　of　organs　which　have　become　useless；it　does　not　come　into　play　at　all；　and　allows　the　descending　variation　free　course。

It　is　obvious　that　even　the　problem　of　COADAPTATION　IN　STERILE　ANIMALS　can　thus　be　satisfactorily　explained。　　If　the　determinants　are　oscillating　upwards　and　downwards　in　continual　fluctuation；　and　varying　more　pronouncedly　now　in　one　direction　now　in　the　other；　useful　variations　of　every　determinant　will　continually　present　themselves　anew；　and　may；　in　the　course　of　generations；　be　combined　with　one　another　in　various　ways。　　But　there　is　one　character　of　the　determinants　that　greatly　facilitates　this　complex　process　of　selection；　that；　after　a　certain　limit　has　been　reached；　they　go　on　varying　in　the　same　direction。　　From　this　it　follows　that　development　along　a　path　once　struck　out　may　proceed　without　the　continual　intervention　of　personal　selection。　　This　factor　only　operates；　so　to　speak；　at　the　beginning；　when　it　selects　the　determinants　which　are　varying　in　the　right　direction；　and　again　at　the　end；　when　it　is　necessary　to　put　a　check　upon　further　variation。　　In　addition　to　this；　enormously　long　periods　have　been　available　for　all　these　adaptations；　as　the　very　gradual　transition　stages　between　females　and　workers　in　many　species　plainly　show；　and　thus　this　process　of　transformation　loses　the　marvellous　and　mysterious　character　that　seemed　at　the　first　glance　to　invest　it；　and　takes　rank；　without　any　straining；　among　the　other　processes　of　selection。　　It　seems　to　me　that；　from　the　facts　that　sterile　animal　forms　can　adapt　themselves　to　new　vital　functions；　their　superfluous　parts　degenerate；　and　the　parts　more　used　adapt　themselves　in　an　ascending　direction；　those　less　used　in　a　descending　direction；　we　must　draw　the　conclusion　that　harmonious　adaptation　here　comes　about　WITHOUT　THE　COOPERATION　OF　THE　LAMARCKIAN　PRINCIPLE。　　This　conclusion　once　established；　however；　we　have　no　reason　to　refer　the　thousands　of　cases　of　harmonious　adaptation；　which　occur　in　exactly　the　same　way　among　other　animals　or　plants；　to　a　principle；　the　ACTIVE　INTERVENTION　OF　WHICH　IN　THE　TRANSFORMATION　OF　SPECIES　IS　NOWHERE　PROVED。　　WE　DO　NOT　REQUIRE　IT　TO　EXPLAIN　THE　FACTS；　AND　THEREFORE　WE　MUST　NOT　ASSUME　IT。

The　fact　of　coadaptation；　which　was　supposed　to　furnish　the　strongest　argument　against　the　principle　of　selection；　in　reality　yields　the　clearest　evidence　in　favour　of　it。　　We　MUST　assume　it；　BECAUSE　NO　OTHER　POSSIBILITY　OF　EXPLANATION　IS　OPEN　TO　US；　AND　BECAUSE　THESE　ADAPTATIONS　ACTUALLY　EXIST；　THAT　IS　TO　SAY；　HAVE　REALLY　TAKEN　PLACE。　　With　this　conviction　I　attempted；　as　far　back　as　1894；　when　the　idea　of　germinal　selection　had　not　yet　occurred　to　me；　to　make　〃harmonious　adaptation〃　（coadaptation）　more　easily　intelligible　in　some　way　or　other；　and　so　I　was　led　to　the　idea；　which　was　subsequently　expounded　in　detail　by　Baldwin；　and　Lloyd　Morgan；　and　also　by　Osborn；　and　Gulick　as　ORGANIC　SELECTION。　　It　seemed　to　me　that　it　was　not　necessary　that　all　the　germinal　variations　required　for　secondary　variations　should　have　occurred　SIMULTANEOUSLY；　since；　for　instance；　in　the　case　of　the　stag；　the　bones；　muscles；　sinews；　and　nerves　would　be　incited　by　the　increasing　heaviness　of　the　antlers　to　greater　activity　in　THE　INDIVIDUAL　LIFE；　and　so　would　be　strengthened。　　The　antlers　can　only　have　increased　in　size　by　very　slow　degrees；　so　that　the　muscles　and　bones　may　have　been　able　to　keep　pace　with　their　growth　in　the　individual　life；　until　the　requisite　germinal　variations　presented　themselves。　　In　this　way　a　disharmony　between　the　increasing　weight　of　the　antlers　and　the　parts　which　support　and　move　them　would　be　avoided；　since　time　would　be　given　for　the　appropriate　germinal　variations　to　occur；　and　so　to　set　agoing　the　HEREDITARY　variation　of　the　muscles；　sinews；　and　bones。　　（〃The　Effect　of　External　Influences　upon　Development〃；　Romanes　Lecture；　Oxford；　1894。）

I　still　regard　this　idea　as　correct；　but　I　attribute　less　importance　to　〃organic　selection〃　than　I　did　at　that　time；　in　so　far　that　I　do　not　believe　that　it　ALONE　could　effect　complex　harmonious　adaptations。　　Germinal　selection　now　seems　to　me　to　play　the　chief　part　in　bringing　about　such　adaptations。　　Something　the　same　is　true　of　the　principle　I　have　called　〃Panmixia〃。　　As　I　became　more　and　more　convinced；　in　the　course　of　years；　that　the　LAMARCKIAN　PRINCIPLE　ought　not　to　be　called　in　to　explain　the　dwindling　of　disused　parts；　I　believed　that　this　process　might　be　simply　explained　as　due　to　the　cessation　of　the　conservative　effect　of　natural　selection。　　I　said　to　myself　that；　from　the　moment　in　which　a　part　ceases　to　be　of　use；　natural　selection　withdraws　its　hand　from　it；　and　then　it　must　inevitably　fall　from　the　height　of　its　adaptiveness；　because　inferior　variants　would　have　as　good　a　chance　of　persisting　as　better　ones；　since　all　grades　of　fitness　of　the　part　in　question　would　be　mingled　with　one　another　indiscriminately。　　This　is　undoubtedly　true；　as　Romanes　pointed　out　ten　years　before　I　did；　and　this　mingling　of　the　bad　with　the　good　probably　does　bring　about　a　deterioration　of　the　part　concerned。　　But　it　cannot　account　for　the　steady　diminution；　which　always　occurs　when　a　part　is　in　process　of　becoming　rudimentary；　and　which　goes　on　until　it　ultimately　disappears　altogether。　　The　process　of　dwindling　cannot　therefore　be　explained　as　due　to　panmixia　alone；　we　can　only　find　a　sufficient　explanation　in　germinal　selection。

IV。　　DERIVATIVES　OF　THE　THEORY　OF　SELECTION。

The　impetus　in　all　directions　given　by　Darwin　through　his　theory　of　selection　has　been　an　immeasurable　one；　and　its　influence　is　still　felt。　It　falls　within　the　province　of　the　historian　of　science　to　enumerate　all　the　ideas　which；　in　the　last　quarter　of　the　nineteenth　century；　grew　out　of　Darwin's　theories；　in　the　endeavour　to　penetrate　more　deeply　into　the　problem　of　the　evolution　of　the　organic　world。　　Within　the　narrow　limits　to　which　this　paper　is　restricted；　I　cannot　attempt　to　discuss　any　of　these。

V。　　ARGUMENTS　FOR　THE　REALITY　OF　THE　PROCESSES　OF　SELECTION。

（a）　　SEXUAL　SELECTION。

Sexual　selection　goes　hand　in　hand　with　natural　selection。　　From　the　very　first　I　have　regarded　sexual　selection　as　affording　an　extremely　important　and　interesting　corroboration　of　natural　selection；　but；　singularly　enough；　it　is　precisely　against　this　theory　that　an　adverse　judgment　has　been　pronounced　in　so　many　quarters；　and　it　is　only　quite　recently；　and　probably　in　proportion　as　the　wealth　of　facts　in　proof　of　it　penetrates　into　a　wider　circle；　that　we　seem　to　be　approaching　a　more　general　recognition　of　this　side　of　the　problem　of　adaptation。　　Thus　Darwin's　words　in　his　preface　to　the　second　edition　（1874）　of　his　book；　〃The　Descent　of　Man　and　Sexual　Selection〃；　are　being　justified：　　〃My　conviction　as　to　the　operation　of　natural　selection　remains　unshaken；〃　and　further；　〃If　naturalists　were　to　become　more　familiar　with　the　idea　of　sexual　selection；　it　would；　I　think；　be　accepted　to　a　much　greater　extent；　and　already　it　is　fully　and　favourably　accepted　by　many　competent　judges。〃　　Darwin　was　able　to　speak　thus　because　he　was　already　acquainted　with　an　immense　mass　of　facts；　which；　taken　together；　yield　overwhelming　evidence　of　the　validity　of　the　principle　of　sexual　selection。

NATURAL　SELECTION　chooses　out　for　reproduction　the　individuals　that　are　best　equipped　for