darwin and modern science-第30节

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；　as　for　example　those　that　treat　of　entomology　or　horticulture；　or　in　the　writings　of　the　already　isolated　systematists　（This　isolation　of　the　systematists　is　the　one　most　melancholy　sequela　of　Darwinism。　　It　seems　an　irony　that　we　should　read　in　the　peroration　to　the　〃Origin〃　that　when　the　Darwinian　view　is　accepted　〃Systematists　will　be　able　to　pursue　their　labours　as　at　present；　but　they　will　not　be　incessantly　haunted　by　the　shadowy　doubt　whether　this　or　that　form　be　a　true　species。　　This；　I　feel　sure；　and　I　speak　after　experience；　will　be　no　slight　relief。　　The　endless　disputes　whether　or　not　some　fifty　species　of　British　brambles　are　good　species　will　cease。〃　　〃Origin〃；　6th　edition　（1882）；　page　425。　　True　they　have　ceased　to　attract　the　attention　of　those　who　lead　opinion；　but　anyone　who　will　turn　to　the　literature　of　systematics　will　find　that　they　have　not　ceased　in　any　other　sense。　　Should　there　not　be　something　disquieting　in　the　fact　that　among　the　workers　who　come　most　into　contact　with　specific　differences；　are　to　be　found　the　only　men　who　have　failed　to　be　persuaded　of　the　unreality　of　those　differences？）；　observations　with　this　special　bearing　were　from　time　to　time　related；　but　the　class　of　fact　on　which　Darwin　built　his　conceptions　of　Heredity　and　Variation　was　not　seen　in　the　highways　of　biology。　　It　formed　no　part　of　the　official　curriculum　of　biological　students；　and　found　no　place　among　the　subjects　which　their　teachers　were　investigating。

During　this　period　nevertheless　one　distinct　advance　was　made；　that　with　which　Weismann's　name　is　prominently　connected。　　In　Darwin's　genetic　scheme　the　hereditary　transmission　of　parental　experience　and　its　consequences　played　a　considerable　role。　　Exactly　how　great　that　role　was　supposed　to　be；　he　with　his　habitual　caution　refrained　from　specifying；　for　the　sufficient　reason　that　he　did　not　know。　　Nevertheless　much　of　the　process　of　Evolution；　especially　that　by　which　organs　have　become　degenerate　and　rudimentary；　was　certainly　attributed　by　Darwin　to　such　inheritance；　though　since　belief　in　the　inheritance　of　acquired　characters　fell　into　disrepute；　the　fact　has　been　a　good　deal　overlooked。　　The　〃Origin〃　without　〃use　and　disuse〃　would　be　a　materially　different　book。　　A　certain　vacillation　is　discernible　in　Darwin's　utterances　on　this　question；　and　the　fact　gave　to　the　astute　Butler　an　opportunity　for　his　most　telling　attack。　　The　discussion　which　best　illustrates　the　genetic　views　of　the　period　arose　in　regard　to　the　production　of　the　rudimentary　condition　of　the　wings　of　many　beetles　in　the　Madeira　group　of　islands；　and　by　comparing　passages　from　the　〃Origin〃　（6th　edition　pages　109　and　401。　　See　Butler；　〃Essays　on　Life；　Art；　and　Science〃；　page　265；　reprinted　1908；　and　〃Evolution；　Old　and　New〃；　chapter　XXII。　（2nd　edition）；　1882。）　Butler　convicts　Darwin　of　saying　first　that　this　condition　was　in　the　main　the　result　of　Selection；　with　disuse　aiding；　and　in　another　place　that　the　main　cause　of　degeneration　was　disuse；　but　that　Selection　had　aided。　　To　Darwin　however　I　think　the　point　would　have　seemed　one　of　dialectics　merely。　　To　him　the　one　paramount　purpose　was　to　show　that　somehow　an　Evolution　by　means　of　Variation　and　Heredity　might　have　brought　about　the　facts　observed；　and　whether　they　had　come　to　pass　in　the　one　way　or　the　other　was　a　matter　of　subordinate　concern。


To　us　moderns　the　question　at　issue　has　a　diminished　significance。　　For　over　all　such　debates　a　change　has　been　brought　by　Weismann's　challenge　for　evidence　that　use　and　disuse　have　any　transmitted　effects　at　all。　　Hitherto　the　transmission　of　many　acquired　characteristics　had　seemed　to　most　naturalists　so　obvious　as　not　to　call　for　demonstration。　　（W。　Lawrence　was　one　of　the　few　who　consistently　maintained　the　contrary　opinion。　　Prichard；　who　previously　had　expressed　himself　in　the　same　sense；　does　not；　I　believe　repeat　these　views　in　his　later　writings；　and　there　are　signs　that　he　came　to　believe　in　the　transmission　of　acquired　habits。　　See　Lawrence；　〃Lect。　Physiol。〃　1823；　pages　436…437；　447　Prichard；　Edin。　Inaug。　Disp。　1808　（not　seen　by　me）；　quoted　ibid。　and　〃Nat。　Hist。　Man〃；　1843；　pages　34　f。）　　Weismann's　demand　for　facts　in　support　of　the　main　proposition　revealed　at　once　that　none　having　real　cogency　could　be　produced。　　The　time…honoured　examples　were　easily　shown　to　be　capable　of　different　explanations。　　A　few　certainly　remain　which　cannot　be　so　summarily　dismissed；　butthough　it　is　manifestly　impossible　here　to　do　justice　to　such　a　subjectI　think　no　one　will　dispute　that　these　residual　and　doubtful　phenomena；　whatever　be　their　true　nature；　are　not　of　a　kind　to　help　us　much　in　the　interpretation　of　any　of　those　complex　cases　of　adaptation　which　on　the　hypothesis　of　unguided　Natural　Selection　are　especially　difficult　to　understand。　　Use　and　disuse　were　invoked　expressly　to　help　us　over　these　hard　places；　but　whatever　changes　can　be　induced　in　offspring　by　direct　treatment　of　the　parents；　they　are　not　of　a　kind　to　encourage　hope　of　real　assistance　from　that　quarter。　　It　is　not　to　be　denied　that　through　the　collapse　of　this　second　line　of　argument　the　Selection　hypothesis　has　had　to　take　an　increased　and　perilous　burden。　　Various　ways　of　meeting　the　difficulty　have　been　proposed；　but　these　mostly　resolve　themselves　into　improbable　attempts　to　expand　or　magnify　the　powers　of　Natural　Selection。

Weismann's　interpellation；　though　negative　in　purpose；　has　had　a　lasting　and　beneficial　effect；　for　through　his　thorough　demolition　of　the　old　loose　and　distracting　notions　of　inherited　experience；　the　ground　has　been　cleared　for　the　construction　of　a　true　knowledge　of　heredity　based　on　experimental　fact。

In　another　way　he　made　a　contribution　of　a　more　positive　character；　for　his　elaborate　speculations　as　to　the　genetic　meaning　of　cytological　appearances　have　led　to　a　minute　investigation　of　the　visible　phenomena　occurring　in　those　divisions　by　which　germ…cells　arise。　　Though　the　particular　views　he　advocated　have　very　largely　proved　incompatible　with　the　observed　facts　of　heredity；　yet　we　must　acknowledge　that　it　was　chiefly　through　the　stimulus　of　Weismann's　ideas　that　those　advances　in　cytology　were　made；　and　though　the　doctrine　of　the　continuity　of　germ…plasm　cannot　be　maintained　in　the　form　originally　propounded；　it　is　in　the　main　true　and　illuminating。　　（It　is　interesting　to　see　how　nearly　Butler　was　led　by　natural　penetration；　and　from　absolutely　opposite　conclusions；　back　to　this　underlying　truth：　　〃So　that　each　ovum　when　impregnate　should　be　considered　not　as　descended　from　its　ancestors；　but　as　being　a　continuation　of　the　personality　of　every　ovum　in　the　chain　of　its　ancestry；　which　every　ovum　IT　ACTUALLY　IS　quite　as　truly　as　the　octogenarian　IS　the　same　identity　with　the　ovum　from　which　he　has　been　developed。　　This　process　cannot　stop　short　of　the　primordial　cell；　which　again　will　probably　turn　out　to　be　but　a　brief　resting…place。　　We　therefore　prove　each　one　of　us　to　BE　ACTUALLY　the　primordial　cell　which　never　died　nor　dies；　but　has　differentiated　itself　into　the　life　of　the　world；　all　living　beings　whatever；　being　one　with　it　and　members　one　of　another；〃　〃Life　and　Habit〃；　1878；　page　86。）　　Nevertheless　in　the　present　state　of　knowledge　we　are　still　as　a　rule　quite　unable　to　connect　cytological　appearances　with　any　genetic　consequence　and　save　in　one　respect　（obviously　of　extreme　importanceto　be　spoken　of　later）　the　two　sets　of　phenomena　might；　for　all　we　can　see；　be　entirely　distinct。

I　cannot　avoid　attaching　importance　to　this　want　of　connection　between　the　nuclear　phenomena　and　the　features　of　bodily　organisation。　　All　attempts　to　investigate　Heredity　by　cytological　means　lie　under　the　disadvantage　that　it　is　the　nuclear　changes　which　can　alone　be　effectively　observed。　　Important　as　they　must　surely　be；　I　have　never　been　persuaded　that　the　rest　of　the　cell　counts　for　nothing。　　What　we　know　of　the　behaviour　and　variability　of　chromosomes　seems　in　my　opinion　quite　incompatible　with　the　belief　that　they　alone　govern　form；　and　are　the　sole　agents　responsible　in　heredity。　　（This　view　is　no　doubt　contrary　to　the　received　opinion。　　I　am　however　interested　to　see　it　lately　maintained　by　Driesch　（〃Science　and　Philosophy　of　the　Organism〃；　London；　1907；　page　233）；　and　from　the　recent　observations　of　Godlewski　it　has　received　distinct　experimental　support。）

If；　then；　progress　was　to　be　made　in　Genetics；　work　of　a　different　kind　was　required。　　To　learn　the　laws　of　Heredity　and　Variation　there　is　no　other　way　than　that　which　Darwin　himself　followed；　the　direct　examination　of　the　phenomena。　　A　beginning　could　be　made　by　collecting　fortuitous　observations　of　this　class；　which　have　often　thrown　a　suggestive　light；　but　such　evidence　can　be　at　best　but　superficial　and　some　more　penetrating　instrument　of　research　is　required。　　This　can　only　be　provided　by　actual　experiments　in　breeding。

The　truth　of　these　general　considerations　was　becoming　gradually　clear　to　many　of　us　when　in　1900　Mendel's　work　was　rediscovered。　　Segregation；　a　phenomenon　of　the　utmost　novelty；　was　thus　revealed。　　From　that　moment　not　only　in　the　problem　of　the　origin　of　species；　but　in　all　the　great　problems　of　biology　a　new　era　began。