darwin and modern science-第25章

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n　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。　　So　unexpected　was　the　discovery　that　many　naturalists　were　convinced　it　was　untrue；　and　at　once　proclaimed　Mendel's　conclusions　as　either　altogether　mistaken；　or　if　true；　of　very　limited　application。　　Many　fantastic　notions　about　the　workings　of　Heredity　had　been　asserted　as　general　principles　before：　　this　was　probably　only　another　fancy　of　the　same　class。

Nevertheless　those　who　had　a　preliminary　acquaintance　with　the　facts　of　Variation　were　not　wholly　unprepared　for　some　such　revelation。　　The　essential　deduction　from　the　discovery　of　segregation　was　that　the　characters　of　living　things　are　dependent　on　the　presence　of　definite　elements　or　factors；　which　are　treated　as　units　in　the　processes　of　Heredity。　　These　factors　can　thus　be　recombined　in　various　ways。　　They　act　sometimes　separately；　and　sometimes　they　interact　in　conjunction　with　each　other；　producing　their　various　effects。　　All　this　indicates　a　definiteness　and　specific　order　in　heredity；　and　therefore　in　variation。　　This　order　cannot　by　the　nature　of　the　case　be　dependent　on　Natural　Selection　for　its　existence；　but　must　be　a　consequence　of　the　fundamental　chemical　and　physical　nature　of　living　things。　　The　study　of　Variation　had　from　the　first　shown　that　an　orderliness　of　this　kind　was　present。　　The　bodies　and　the　properties　of　living　things　are　cosmic；　not　chaotic。　　No　matter　how　low　in　the　scale　we　go；　never　do　we　find　the　slightest　hint　of　a　diminution　in　that　all…pervading　orderliness；　nor　can　we　conceive　an　organism　existing　for　a　moment　in　any　other　state。　　Moreover　not　only　does　this　order　prevail　in　normal　forms；　but　again　and　again　it　is　to　be　seen　in　newly…sprung　varieties；　which　by　general　consent　cannot　have　been　subjected　to　a　prolonged　Selection。　　The　discovery　of　Mendelian　elements　admirably　coincided　with　and　at　once　gave　a　rationale　of　these　facts。　　Genetic　Variation　is　then　primarily　the　consequence　of　additions　to；　or　omissions　from；　the　stock　of　elements　which　the　species　contains。　　The　further　investigation　of　the　species…problem　must　thus　proceed　by　the　analytical　method　which　breeding　experiments　provide。

In　the　nine　years　which　have　elapsed　since　Mendel's　clue　became　generally　known；　progress　has　been　rapid。　　We　now　understand　the　process　by　which　a　polymorphic　race　maintains　its　polymorphism。　　When　a　family　consists　of　dissimilar　members；　given　the　numerical　proportions　in　which　these　members　are　occurring；　we　can　represent　their　composition　symbolically　and　state　what　types　can　be　transmitted　by　the　various　members。　　The　difficulty　of　the　〃swamping　effects　of　intercrossing〃　is　practically　at　an　end。　　Even　the　famous　puzzle　of　sex…limited　inheritance　is　solved；　at　all　events　in　its　more　regular　manifestations；　and　we　know　now　how　it　is　brought　about　that　the　normal　sisters　of　a　colour…blind　man　can　transmit　the　colour…blindness　while　his　normal　brothers　cannot　transmit　it。

We　are　still　only　on　the　fringe　of　the　inquiry。　　It　can　be　seen　extending　and　ramifying　in　many　directions。　　To　enumerate　these　here　would　be　impossible。　　A　whole　new　range　of　possibilities　is　being　brought　into　view　by　study　of　the　interrelations　between　the　simple　factors。　　By　following　up　the　evidence　as　to　segregation；　indications　have　been　obtained　which　can　only　be　interpreted　as　meaning　that　when　many　factors　are　being　simultaneously　redistributed　among　the　germ…cells；　certain　of　them　exert　what　must　be　described　as　a　repulsion　upon　other　factors。　　We　cannot　surmise　whither　this　discovery　may　lead。

In　the　new　light　all　the　old　problems　wear　a　fresh　aspect。　　Upon　the　question　of　the　nature　of　Sex；　for　example；　the　bearing　of　Mendelian　evidence　is　close。　　Elsewhere　I　have　shown　that　from　several　sets　of　parallel　experiments　the　conclusion　is　almost　forced　upon　us　that；　in　the　types　investigated；　of　the　two　sexes　the　female　is　to　be　regarded　as　heterozygous　in　sex；　containing　one　unpaired　dominant　element；　while　the　male　is　similarly　homozygous　in　the　absence　of　that　element。　　（In　other　words；　the　ova　are　each　EITHER　female；　OR　male　（i。e。　non…female）；　but　the　sperms　are　all　non…female。）　　It　is　not　a　little　remarkable　that　on　this　pointwhich　is　the　only　one　where　observations　of　the　nuclear　processes　of　gameto…genesis　have　yet　been　brought　into　relation　with　the　visible　characteristics　of　the　organisms　themselvesthere　should　be　diametrical　opposition　between　the　results　of　breeding　experiments　and　those　derived　from　cytology。

Those　who　have　followed　the　researches　of　the　American　school　will　be　aware　that；　after　it　had　been　found　in　certain　insects　that　the　spermatozoa　were　of　two　kinds　according　as　they　contained　or　did　not　contain　the　accessory　chromosome；　E。B。　Wilson　succeeded　in　proving　that　the　sperms　possessing　this　accessory　body　were　destined　to　form　FEMALES　on　fertilisation；　while　sperms　without　it　form　males；　the　eggs　being　apparently　indifferent。　　Perhaps　the　most　striking　of　all　this　series　of　observations　is　that　lately　made　by　T。H。　Morgan　（Morgan；　〃Proc。　Soc。　Exp。　Biol。　Med。〃　V。　1908；　and　von　Baehr；　〃Zool。　Anz。〃　XXXII。　page　507；　1908。）；　s