darwin and modern science-第32节

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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　which　develop　into　new　individuals；　the　sterility　of　these　daughter…individuals　is　sensibly　reduced　or　may　be　entirely　absent。　　The　fertility　once　re…established；　the　sterility　does　not　return　in　the　later　progeny；　a　fact　strongly　suggestive　of　segregation。　　Now　if　the　sterility　of　the　cross…bred　be　really　the　consequence　of　the　meeting　of　two　complementary　factors；　we　see　that　the　phenomenon　could　only　be　produced　among　the　divergent　offspring　of　one　species　by　the　acquisition　of　at　least　TWO　new　factors；　for　if　the　acquisition　of　a　single　factor　caused　sterility　the　line　would　then　end。　　Moreover　each　factor　must　be　separately　acquired　by　distinct　individuals；　for　if　both　were　present　together；　the　possessors　would　by　hypothesis　be　sterile。　　And　in　order　to　imitate　the　case　of　species　each　of　these　factors　must　be　acquired　by　distinct　breeds。　　The　factors　need　not；　and　probably　would　not；　produce　any　other　perceptible　effects；　they　might；　like　the　colour…factors　present　in　white　flowers；　make　no　difference　in　the　form　or　other　characters。　　Not　till　the　cross　was　actually　made　between　the　two　complementary　individuals　would　either　factor　come　into　play；　and　the　effects　even　then　might　be　unobserved　until　an　attempt　was　made　to　breed　from　the　cross…bred。

Next；　if　the　factors　responsible　for　sterility　were　acquired；　they　would　in　all　probability　be　peculiar　to　certain　individuals　and　would　not　readily　be　distributed　to　the　whole　breed。　　Any　member　of　the　breed　also　into　which　BOTH　the　factors　were　introduced　would　drop　out　of　the　pedigree　by　virtue　of　its　sterility。　　Hence　the　evidence　that　the　various　domesticated　breeds　say　of　dogs　or　fowls　can　when　mated　together　produce　fertile　offspring；　is　beside　the　mark。　　The　real　question　is；　Do　they　ever　produce　sterile　offspring？　　I　think　the　evidence　is　clearly　that　sometimes　they　do；　oftener　perhaps　than　is　commonly　supposed。　　These　suggestions　are　quite　amenable　to　e