darwin and modern science-第31节

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y　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。）；　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　gene