darwin and modern science-第26章

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Morgan；　〃Proc。　Soc。　Exp。　Biol。　Med。〃　V。　1908；　and　von　Baehr；　〃Zool。　Anz。〃　XXXII。　page　507；　1908。）；　since　confirmed　by　von　Baehr；　that　in　a　Phylloxeran　two　kinds　of　spermatids　are　formed；　respectively　with　and　without　an　accessory　（in　this　case；　DOUBLE）　chromosome。　　Of　these；　only　those　possessing　the　accessory　body　become　functional　spermatozoa；　the　others　degenerating。　　We　have　thus　an　elucidation　of　the　puzzling　fact　that　in　these　forms　fertilisation　results　in　the　formation　of　FEMALES　only。　　How　the　males　are　formedfor　of　course　males　are　eventually　produced　by　the　parthenogenetic　femaleswe　do　not　know。

If　the　accessory　body　is　really　to　be　regarded　as　bearing　the　factor　for　femaleness；　then　in　Mendelian　terms　female　is　DD　and　male　is　DR。　　The　eggs　are　indifferent　and　the　spermatozoa　are　each　male；　OR　female。　　But　according　to　the　evidence　derived　from　a　study　of　the　sex…limited　descent　of　certain　features　in　other　animals　the　conclusion　seems　equally　clear　that　in　them　female　must　be　regarded　as　DR　and　male　as　RR。　　The　eggs　are　thus　each　either　male　or　female　and　the　spermatozoa　are　indifferent。　　How　this　contradictory　evidence　is　to　be　reconciled　we　do　not　yet　know。　　The　breeding　work　concerns　fowls；　canaries；　and　the　Currant　moth　（Abraxas　grossulariata）。　　The　accessory　chromosome　has　been　now　observed　in　most　of　the　great　divisions　of　insects　（As　Wilson　has　proved；　the　unpaired　body　is　not　a　universal　feature　even　in　those　orders　in　which　it　has　been　observed。　Nearly　allied　types　may　differ。　　In　some　it　is　altogether　unpaired。　　In　others　it　is　paired　with　a　body　of　much　smaller　size；　and　by　selection　of　various　types　all　gradations　can　be　demonstrated　ranging　to　the　condition　in　which　the　members　of　the　pair　are　indistinguishable　from　each　other。）；　except；　as　it　happens；　Lepidoptera。　　At　first　sight　it　seems　difficult　to　suppose　that　a　feature　apparently　so　fundamental　as　sex　should　be　differently　constituted　in　different　animals；　but　that　seems　at　present　the　least　improbable　inference。　　I　mention　these　two　groups　of　facts　as　illustrating　the　nature　and　methods　of　modern　genetic　work。　　We　must　proceed　by　minute　and　specific　analytical　investigation。　　Wherever　we　look　we　find　traces　of　the　operation　of　precise　and　specific　rules。

In　the　light　of　present　knowledge　it　is　evident　that　before　we　can　attack　the　Species…problem　with　any　hope　of　success　there　are　vast　arrears　to　be　made　up。　　He　would　be　a　bold　man　who　would　now　assert　that　there　was　no　sense　in　which　the　term　Species　might　not　have　a　strict　and　concrete　meaning　in　contradistinction　to　the　term　Variety。　　We　have　been　taught　to　regard　the　difference　between　species　and　variety　as　one　of　degree。　　I　think　it　unlikely　that　this　conclusion　will　bear　the　test　of　further　research。　　To　Darwin　the　question；　What　is　a　variation？　presented　no　difficulties。　　Any　difference　between　parent　and　offspring　was　a　variation。　Now　we　have　to　be　more　precise。　　First　we　must；　as　de　Vries　has　shown；　distinguish　real；　genetic；　variation　from　FLUCTUATIONAL　variations；　due　to　environmental　and　other　accidents；　which　cannot　be　transmitted。　　Having　excluded　these　sources　of　error　the　variations　observed　must　be　expressed　in　terms　of　the　factors　to　which　they　are　due　before　their　significance　can　be　understood。　　For　example；　numbers　of　the　variations　seen　under　domestication；　and　not　a　few　witnessed　in　nature；　are　simply　the　consequence　of　some　ingredient　being　in　an　unknown　way　omitted　from　the　composition　of　the　varying　individual。　　The　variation　may　on　the　contrary　be　due　to　the　addition　of　some　new　element；　but　to　prove　that　it　is　so　is　by　no　means　an　easy　matter。　　Casual　observation　is　useless；　for　though　these　latter　variations　will　always　be　dominants；　yet　many　dominant　characteristics　may　arise　from　another　cause；　namely　the　meeting　of　complementary　factors；　and　special　study　of　each　case　in　two　generations　at　least　is　needed　before　these　two　phenomena　can　be　distinguished。

When　such　considerations　are　fully　appreciated　it　will　be　realised　that　medleys　of　most　dissimilar　occurrences　are　all　confused　together　under　the　term　Variation。　　One　of　the　first　objects　of　genetic　analysis　is　to　disentangle　this　mass　of　confusion。

To　those　who　have　made　no　study　of　heredity　it　sometimes　appears　that　the　question　of　the　effect　of　conditions　in　causing　variation　is　one　which　we　should　immediately　investigate；　but　a　little　thought　will　show　that　before　any　critical　inquiry　into　such　possibilities　can　be　attempted；　a　knowledge　of　the　working　of　heredity　under　conditions　as　far　as　possible　uniform　must　be　obtained。　　At　the　time　when　Darwin　was　writing；　if　a　plant　brought　into　cultivation　gave　off　an　albino　variety；　such　an　event　was　without　hesitation　ascribed　to　the　change　of　life。　　Now　we　see　that　albino　GAMETES；　germs；　that　is　to　say；　which　are　destitute　of　the　pigment…forming　factor；　may　have　been　originally　produced　by　individuals　standing　an　indefinite　number　of　generations　back　in　the　ancestry　of　the　actual　albino；　and　it　is　indeed　almost　certain　that　the　variation　to　which　the　appearance　of　the　albino　is　due　cannot　have　taken　place　in　a　generation　later　than　that　of　the　grandparents。　　It　is　true　that　when　a　new　DOMINANT　appears　we　should　feel　greater　confidence　that　we　were　witnessing　the　original　variation；　but　such　events　are　of　extreme　rarity；　and　no　such　case　has　come　under　the　notice　of　an　experimenter　in　modern　times；　as　far　as　I　am　aware。　　That　they　must　have　appeared　is　clear　enough。　　Nothing　corresponding　to　the　Brown…　breasted　Game　fowl　is　known　wild；　yet　that　colour　is　a　most　definite　dominant；　and　at　some　moment　since　Gallus　bankiva　was　domesticated；　the　element　on　which　that　special　colour　depends　must　have　at　least　once　been　formed　in　the　germ…cell　of　a　fowl；　but　we　need　harder　evidence　than　any　which　has　yet　been　produced　before　we　can　declare　that　this　novelty　came　through　over…feeding；　or　change　of　climate；　or　any　other　disturbance　consequent　on　domestication。　　When　we　reflect　on　the　intricacies　of　genetic　problems　as　we　must　now　conceive　them　there　come　moments　when　we　feel　almost　thankful　that　the　Mendelian　principles　were　unknown　to　Darwin。　　The　time　called　for　a　bold　pronouncement；　and　he　made　it；　to　our　lasting　profit　and　delight。　　With　fuller　knowledge　we　pass　once　more　into　a　period　of　cautious　expectation　and　reserve。

In　every　arduous　enterprise　it　is　pleasanter　to　look　back　at　difficulties　overcome　than　forward　to　those　which　still　seem　insurmountable；　but　in　the　next　stage　there　is　nothing　to　be　gained　by　disguising　the　fact　that　the　attributes　of　living　things　are　not　what　we　used　to　suppose。　　If　they　are　more　complex　in　the　sense　that　the　properties　they　display　are　throughout　so　regular　（I　have　in　view；　for　example；　the　marvellous　and　specific　phenomena　of　regeneration；　and　those　discovered　by　the　students　of　〃Entwicklungsmechanik〃。　　The　circumstances　of　its　occurrence　here　preclude　any　suggestion　that　this　regularity　has　been　brought　about　by　the　workings　of　Selection。　　The　attempts　thus　to　represent　the　phenomena　have　resulted　in　mere　parodies　of　scientific　reasoning。）　that　the　Selection　of　minute　random　variations　is　an　unacceptable　account　of　the　origin　of　their　diversity；　yet　by　virtue　of　that　very　regularity　the　problem　is　limited　in　scope　and　thus　simplified。

To　begin　with；　we　must　relegate　Selection　to　its　proper　place。　　Selection　permits　the　viable　to　continue　and　decides　that　the　non…viable　shall　perish；　just　as　the　temperature　of　our　atmosphere　decides　that　no　liquid　carbon　shall　be　found　on　the　face　of　the　earth：　　but　we　do　not　suppose　that　the　form　of　the　diamond　has　been　gradually　achieved　by　a　process　of　Selection。　　So　again；　as　the　course　of　descent　branches　in　the　successive　generations；　Selection　determines　along　which　branch　Evolution　shall　proceed；　but　it　does　not　decide　what　novelties　that　branch　shall　bring　forth。　　〃La　Nature　contient　le　fonds　de　toutes　ces　varietes；　mais　le　hazard　ou　l'art　les　mettent　en　oeuvre；〃　as　Maupertuis　most　truly　said。

Not　till　knowledge　of　the　genetic　properties　of　organisms　has　attained　to　far　greater　completeness　can　evolutionary　speculations　have　more　than　a　suggestive　value。　　By　genetic　experiment；　cytology　and　physiological　chemistry　aiding；　we　may　hope　to　acquire　such　knowledge。　　In　1872　Nathusius　wrote　（〃Vortrage　uber　Viehzucht　und　Rassenerkenntniss〃；　page　120；　Berlin；　1872。）：　　〃Das　Gesetz　der　Vererbung　ist　noch　nicht　erkannt；　der　Apfel　ist　noch　nicht　vom　Baum　der　Erkenntniss　gefallen；　welcher；　der　Sage　nach；　Newton　auf　den　rechten　Weg　zur　Ergrundung　der　Gravitationsgesetze　fuhrte。〃　　We　cannot　pretend　that　the　words　are　not　still　true；　but　in　Mendelian　analysis　the　seeds　of　that　apple…tree　at　last　are　sown。

If　we　were　asked　what　discovery　would　do　most　to　forward　our　inquiry；　what　one　bit　of　knowledge　would　more　than　any　other　illuminate　the　problem；　I　think　we　may　give　the　answer　without　hesitation。　　The　greatest　advance　that　we　can　foresee　will　be　made　when　it　is　found　possible　to　connect　the　geometrical　phenomena　of　development　with　the　chemical。　　The　geometrical　symmetry　of　living　things　is　the　key　to　a　knowledge　of　their　regularity；　and　the　forces　which　cause　it。　　In　the　symmetry　of　the　dividing　cell　the　basis　of　that　resemblance　we　call　Heredity　is　contained。　　To　imitate　the　morphological　phenomena　of　life　we　have　to　devise　a　system　which　can　divide。　　It　must　be　able　to　divide；　and　to　segment　asgrosslya　vibrating　plate　or　rod　does；　or　as　an　icicle　can　do　as　it　becomes　ribbed　in　a　continuous　stream　of　water；　but　with　this　distinction；　that　the　distribution　of　chemical　differences　and　properties　must　simultaneously　be　decided　and　disposed　in　orderly　relation　to　the　pattern　of　the　segmentation。　　Even　if　a　model　which　would　do　this　could　be　constructed　it　might　prove　to　be　a　useful　beginning。

This　may　be　looking　too　far　ahead。　　If　we　had　to　choose　some　one　piece　of　more　proximate　knowledge　which　we　would　more　especially　like　to　acquire；　I　suppose　we　should　ask　for　the　secret　of　interracial　sterility。　　Nothing　has　yet　been　discovered　to　remove　the　grave　difficulty；　by　which　Huxley　in　particular　was　so　much　oppressed；　that　among　the　many　varieties　produced　under　domesticationwhich　we　all　regard　as　analogous　to　the　species　seen　in　natureno　clear　case　of　interracial　sterility　has　been　demonstrated。　　The　phenomenon　is　probably　the　only　one　to　which　the　domesticated　products　seem　to　afford　no　parallel。　　No　solution　of　the　difficulty　can　be　offered　which　has　positive　value；　but　it　is　perhaps　worth　considering　the　facts　in　the　light　of　modern　ideas。　　It　should　be　observed　that　we　are　not　discussing　incompatibility　of　two　species　to　produce　offspring　（a　totally　distinct　phenomenon）；　but　the　sterility　of　the　offspring　which　many　of　them　do　produce。

When　two　species；　both　perfectly　fertile　severally；　produce　on　crossing　a　sterile　progeny；　there　is　a　presumption　that　the　sterility　is　due　to　the　development　in　the　hybrid　of　some　substance　which　can　only　be　formed　by　the　meeting　of　two　complementary　factors。　　That　some　such　account　is　correct　in　essence　may　be　inferred　from　the　well…known　observation　that　if　the　hybrid　is　not　totally　sterile　but　only　partially　so；　and　thus　is　able　to　form　some　good　germ…cells