darwin and modern science-第25章

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；　the　displacement　of　the　external　nares　to　the　forehead　for　the　greater　security　of　the　breathing…hole　during　the　brief　appearance　at　the　surface；　and　certain　remarkable　changes　in　the　respiratory　and　circulatory　organs　which　enable　the　animal　to　remain　for　a　long　time　under　water。　　I　might　have　added　many　more；　for　the　list　of　adaptations　in　the　whale　to　aquatic　life　is　by　no　means　exhausted；　they　are　found　in　the　histological　structure　and　in　the　minutest　combinations　in　the　nervous　system。　　For　it　is　obvious　that　a　tail…fin　must　be　used　in　quite　a　different　way　from　a　tail；　which　serves　as　a　fly…brush　in　hoofed　animals；　or　as　an　aid　to　springing　in　the　kangaroo　or　as　a　climbing　organ；　it　will　require　quite　different　reflex…mechanisms　and　nerve…combinations　in　the　motor　centres。

I　used　this　example　in　order　to　show　how　unnecessary　it　is　to　assume　a　special　internal　evolutionary　power　for　the　phylogenesis　of　species；　for　this　whole　order　of　whales　is；　so　to　speak；　MADE　UP　OF　ADAPTATIONS；　it　deviates　in　many　essential　respects　from　the　usual　mammalian　type；　and　all　the　deviations　are　adaptations　to　aquatic　life。　　But　if　precisely　the　most　essential　features　of　the　organisation　thus　depend　upon　adaptation；　what　is　left　for　a　phyletic　force　to　do；　since　it　is　these　essential　features　of　the　structure　it　would　have　to　determine？　　There　are　few　people　now　who　believe　in　a　phyletic　evolutionary　power；　which　is　not　made　up　of　the　forces　known　to　usadaptation　and　hereditybut　the　conviction　that　EVERY　part　of　an　organism　depends　upon　adaptation　has　not　yet　gained　a　firm　footing。　　Nevertheless；　I　must　continue　to　regard　this　conception　as　the　correct　one；　as　I　have　long　done。

I　may　be　permitted　one　more　example。　　The　feather　of　a　bird　is　a　marvellous　structure；　and　no　one　will　deny　that　as　a　whole　it　depends　upon　adaptation。　But　what　part　of　it　DOES　NOT　depend　upon　adaptation？　　The　hollow　quill；　the　shaft　with　its　hard；　thin；　light　cortex；　and　the　spongy　substance　within　it；　its　square　section　compared　with　the　round　section　of　the　quill；　the　flat　barbs；　their　short；　hooked　barbules　which；　in　the　flight…feathers；　hook　into　one　another　with　just　sufficient　firmness　to　resist　the　pressure　of　the　air　at　each　wing…beat；　the　lightness　and　firmness　of　the　whole　apparatus；　the　elasticity　of　the　vane；　and　so　on。　　And　yet　all　this　belongs　to　an　organ　which　is　only　passively　functional；　and　therefore　can　have　nothing　to　do　with　the　LAMARCKIAN　PRINCIPLE。　　Nor　can　the　feather　have　arisen　through　some　magical　effect　of　temperature；　moisture；　electricity；　or　specific　nutrition；　and　thus　selection　is　again　our　only　anchor　of　safety。

Butit　will　be　objectedthe　substance　of　which　the　feather　consists；　this　peculiar　kind　of　horny　substance；　did　not　first　arise　through　selection　in　the　course　of　the　evolution　of　the　birds；　for　it　formed　the　covering　of　the　scales　of　their　reptilian　ancestors。　　It　is　quite　true　that　a　similar　substance　covered　the　scales　of　the　Reptiles；　but　why　should　it　not　have　arisen　among　them　through　selection？　　Or　in　what　other　way　could　it　have　arisen；　since　scales　are　also　passively　useful　parts？　　It　is　true　that　if　we　are　only　to　call　adaptation　what　has　been　acquired　by　the　species　we　happen　to　be　considering；　there　would　remain　a　great　deal　that　could　not　be　referred　to　selection；　but　we　are　postulating　an　evolution　which　has　stretched　back　through　aeons；　and　in　the　course　of　which　innumerable　adaptations　took　place；　which　had　not　merely　ephemeral　persistence　in　a　genus；　a　family　or　a　class；　but　which　was　continued　into　whole　Phyla　of　animals；　with　continual　fresh　adaptations　to　the　special　conditions　of　each　species；　family；　or　class；　yet　with　persistence　of　the　fundamental　elements。　　Thus　the　feather；　once　acquired；　persisted　in　all　birds；　and　the　vertebral　column；　once　gained　by　adaptation　in　the　lowest　forms；　has　persisted　in　all　the　Vertebrates；　from　Amphioxus　upwards；　although　with　constant　readaptation　to　the　conditions　of　each　particular　group。　　Thus　everything　we　can　see　in　animals　is　adaptation；　whether　of　to…day；　or　of　yesterday；　or　of　ages　long　gone　by；　every　kind　of　cell；　whether　glandular；　muscular；　nervous；　epidermic；　or　skeletal；　is　adapted　to　absolutely　definite　and　specific　functions；　and　every　organ　which　is　composed　of　these　different　kinds　of　cells　contains　them　in　the　proper　proportions；　and　in　the　particular　arrangement　which　best　serves　the　function　of　the　organ；　it　is　thus　adapted　to　its　function。

All　parts　of　the　organism　are　tuned　to　one　another；　that　is；　THEY　ARE　ADAPTED　TO　ONE　ANOTHER；　and　in　the　same　way　THE　ORGANISM　AS　A　WHOLE　IS　ADAPTED　TO　THE　CONDITIONS　OF　ITS　LIFE；　AND　IT　IS　SO　AT　EVERY　STAGE　OF　ITS　EVOLUTION。

But　all　adaptations　CAN　be　referred　to　selection；　the　only　point　that　remains　doubtful　is　whether　they　all　MUST　be　referred　to　it。

However　that　may　be；　whether　the　LAMARCKIAN　PRINCIPLE　is　a　factor　that　has　cooperated　with　selection　in　evolution；　or　whether　it　is　altogether　fallacious；　the　fact　remains；　that　selection　is　the　cause　of　a　great　part　of　the　phyletic　evolution　of　organisms　on　our　earth。　　Those　who　agree　with　me　in　rejecting　the　LAMARCKIAN　PRINCIPLE　will　regard　selection　as　the　only　GUIDING　factor　in　evolution；　which　creates　what　is　new　out　of　the　transmissible　variations；　by　ordering　and　arranging　these；　selecting　them　in　relation　to　their　number　and　size；　as　the　architect　does　his　building…　stones　so　that　a　particular　style　must　result。　　（〃Variation　under　Domestication〃；　1875　II。　pages　426；　427。）　　But　the　building…stones　themselves；　the　variations；　have　their　basis　in　the　influences　which　cause　variation　in　those　vital　units　which　are　handed　on　from　one　generation　to　another；　whether；　taken　together　they　form　the　WHOLE　organism；　as　in　Bacteria　and　other　low　forms　of　life；　or　only　a　germ…substance；　as　in　unicellular　and　multicellular　organisms。　　（The　Author　and　Editor　are　indebted　to　Professor　Poulton　for　kindly　assisting　in　the　revision　of　the　proof　of　this　Essay。）


IV。　　VARIATION。　

By　HUGO　DE　VRIES；　Professor　of　Botany　in　the　University　of　Amsterdam。

I。　　DIFFERENT　KINDS　OF　VARIABILITY。

Before　Darwin；　little　was　known　concerning　the　phenomena　of　variability。　　The　fact；　that　hardly　two　leaves　on　a　tree　were　exactly　the　same；　could　not　escape　observation：　　small　deviations　of　the　same　kind　were　met　with　everywhere；　among　individuals　as　well　as　among　the　organs　of　the　same　plant。　　Larger　aberrations；　spoken　of　as　monstrosities；　were　for　a　long　time　regarded　as　lying　outside　the　range　of　ordinary　phenomena。　　A　special　branch　of　inquiry；　that　of　Teratology；　was　devoted　to　them；　but　it　constituted　a　science　by　itself；　sometimes　connected　with　morphology；　but　having　scarcely　any　bearing　on　the　processes　of　evolution　and　heredity。

Darwin　was　the　first　to　take　a　broad　survey　of　the　whole　range　of　variations　in　the　animal　and　vegetable　kingdoms。　　His　theory　of　Natural　Selection　is　based　on　the　fact　of　variability。　　In　order　that　this　foundation　should　be　as　strong　as　possible　he　collected　all　the　facts；　scattered　in　the　literature　of　his　time；　and　tried　to　arrange　them　in　a　scientific　way。　　He　succeeded　in　showing　that　variations　may　be　grouped　along　a　line　of　almost　continuous　gradations；　beginning　with　simple　differences　in　size　and　ending　with　monstrosities。　　He　was　struck　by　the　fact　that；　as　a　rule；　the　smaller　the　deviations；　the　more　frequently　they　appear；　very　abrupt　breaks　in　characters　being　of　rare　occurrence。

Among　these　numerous　degrees　of　variability　Darwin　was　always　on　the　look　out　for　those　which　might；　with　the　greatest　probability；　be　considered　as　affording　material　for　natural　selection　to　act　upon　in　the　development　of　new　species。　　Neither　of　the　extremes　complied　with　his　conceptions。　　He　often　pointed　out；　that　there　are　a　good　many　small　fluctuations；　which　in　this　respect　must　be　absolutely　useless。　　On　the　other　hand；　he　strongly　combated　the　belief；　that　great　changes　would　be　necessary　to　explain　the　origin　of　species。　　Some　authors　had　propounded　the　idea　that　highly　adapted　organs；　e。g。　the　wings　of　a　bird；　could　not　have　been　developed　in　any　other　way　than　by　a　comparatively　sudden　modification　of　a　well　defined　and　import