darwin and modern science-第23节

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IV。　　VARIATION。　

By　HUGO　DE　VRIES；　Professor　of　Botany　in　the　University　of　Amsterdam。

I。　　DIFFERENT　KINDS　OF　VARIABILITY。

Before　Darwin；　little　was　known　concerning　the　phenomena　of　variability。　　The　fact；　that　hardly　two　leaves　on　a　tree　were　exactly　the　same；　could　not　escape　observation：　　small　deviations　of　the　same　kind　were　met　with　everywhere；　among　individuals　as　well　as　among　the　organs　of　the　same　plant。　　Larger　aberrations；　spoken　of　as　monstrosities；　were　for　a　long　time　regarded　as　lying　outside　the　range　of　ordinary　phenomena。　　A　special　branch　of　inquiry；　that　of　Teratology；　was　devoted　to　them；　but　it　constituted　a　science　by　itself；　sometimes　connected　with　morphology；　but　having　scarcely　any　bearing　on　the　processes　of　evolution　and　heredity。

Darwin　was　the　first　to　take　a　broad　survey　of　the　whole　range　of　variations　in　the　animal　and　vegetable　kingdoms。　　His　theory　of　Natural　Selection　is　based　on　the　fact　of　variability。　　In　order　that　this　foundation　should　be　as　strong　as　possible　he　collected　all　the　facts；　scattered　in　the　literature　of　his　time；　and　tried　to　arrange　them　in　a　scientific　way。　　He　succeeded　in　showing　that　variations　may　be　grouped　along　a　line　of　almost　continuous　gradations；　beginning　with　simple　differences　in　size　and　ending　with　monstrosities。　　He　was　struck　by　the　fact　that；　as　a　rule；　the　smaller　the　deviations；　the　more　frequently　they　appear；　very　abrupt　breaks　in　characters　being　of　rare　occurrence。

Among　these　numerous　degrees　of　variability　Darwin　was　always　on　the　look　out　for　those　which　might；　with　the　greatest　probability；　be　considered　as　affording　material　for　natural　selection　to　act　upon　in　the　development　of　new　species。　　Neither　of　the　extremes　complied　with　his　conceptions。　　He　often　pointed　out；　that　there　are　a　good　many　small　fluctuations；　which　in　this　respect　must　be　absolutely　useless。　　On　the　other　hand；　he　strongly　combated　the　belief；　that　great　changes　would　be　necessary　to　explain　the　origin　of　species。　　Some　authors　had　propounded　the　idea　that　highly　adapted　organs；　e。g。　the　wings　of　a　bird；　could　not　have　been　developed　in　any　other　way　than　by　a　comparatively　sudden　modification　of　a　well　defined　and　important　kind。　　Such　a　conception　would　allow　of　great　breaks　or　discontinuity　in　the　evolution　of　highly　differentiated　animals　and　plants；　shortening　the　time　for　the　evolution　of　the　whole　organic　kingdom　and　getting　over　numerous　difficulties　inherent　in　the　theory　of　slow　and　gradual　progress。　　It　would；　moreover；　account　for　the　genetic　relation　of　the　larger　groups　of　both　animals　and　plants。　　It　would；　in　a　word；　undoubtedly　afford　an　easy　means　of　simplifying　the　problem　of　descent　with　modification。

Darwin；　however；　considered　such　hypotheses　as　hardly　belonging　to　the　domain　of　science；　they　belong；　he　said；　to　the　realm　of　miracles。　　That　species　have　a　capacity　for　change　is　admitted　by　all　evolutionists；　but　there　is　no　need　to　invoke　modifications　other　than　those　represented　by　ordinary　variability。　　It　is　well　known　that　in　artificial　selection　this　tendency　to　vary　has　given　rise　to　numerous　distinct　races；　and　there　is　no　reason　for　denying　that　it　can　do　the　same　in　nature；　by　the　aid　of　natural　selection。　　On　both　lines　an　advance　may　be　expected　with　equal　probability。

His　main　argument；　however；　is　that　the　most　striking　and　most　highly　adapted　modifications　may　be　acquired　by　successive　variations。　　Each　of　these　may　be　slight；　and　they　may　affect　different　organs；　gradually　adapting　them　to　the　same　purpose。　　The　direction　of　the　adaptations　will　be　determined　by　the　needs　in　the　struggle　for　life；　and　natural　selection　will　simply　exclude　all　such　changes　as　occur　on　opposite　or　deviating　lines。　　In　this　way；　it　is　not　variability　itself　which　is　called　upon　to　explain　beautiful　adaptations；　but　it　is　quite　sufficient　to　suppose　that　natural　selection　has　operated　during　long　periods　in　the　same　way。　　Eventually；　all　the　acquired　characters；　being　transmitted　together；　would　appear　to　us；　as　if　they　had　all　been　simultaneously　developed。

Correlations　must　play　a　large　part　in　such　special　evolutions：　　when　one　part　is　modified；　so　will　be　other　parts。　　The　distribution　of　nourishment　will　come　in　as　one　of　the　causes；　the　reactions　of　different　organs　to　the　same　external　influences　as　another。　　But　no　doubt　the　more　effective　cause　is　that　of　the　internal　correlations；　which；　however；　are　still　but　dimly　understood。　　Darwin　repeatedly　laid　great　stress　on　this　view；　although　a　definite　proof　of　its　correctness　could　not　be　given　in　his　time。　　Such　proof　requires　the　direct　observation　of　a　mutation；　and　it　should　be　stated　here　that　even　the　first　observations　made　in　this　direction　have　clearly　confirmed　Darwin's　ideas。　　The　new　evening　primroses　which　have　sprung　in　my　garden　from　the　old　form　of　Oenothera　Lamarckiana；　and　which　have　evidently　been　derived　from　it；　in　each　case；　by　a　single　mutation；　do　not　differ　from　their　parent　species　in　one　character　only；　but　in　almost　all　their　organs　and　qualities。　　Oenothera　gigas；　for　example；　has　stouter　stems　and　denser　foliage；　the　leaves　are　larger　and　broader；　its　thick　flower…buds　produce　gigantic　flowers；　but　only　small　fruits　with　large　seeds。　　Correlative　changes　of　this　kind　are　seen　in　all　my　new　forms；　and　they　lend　support　to　the　view　that　in　the　gradual　development　of　highly　adapted　structures；　analogous　correlations　may　have　played　a　large　part。　　They　easily　explain　large　deviations　from　an　original　type；　without　requiring　the　assumption　of　too　many　steps。

Monstrosities；　as　their　name　implies；　are　widely　different　in　character　from　natural　species；　they　cannot；　therefore；　be　adduced　as　evidence　in　the　investigation　of　the　origin　of　species。　　There　is　no　doubt　that　they　may　have　much　in　common　as　regards　their　manner　of　origin；　and　that　the　origin　of　species；　once　understood；　may　lead　to　a　better　understanding　of　the　monstrosities。　　But　the　reverse　is　not　true；　at　least　not　as　regards　the　main　lines　of　development。　　Here；　it　is　clear；　monstrosities　cannot　have　played　a　part　of　any　significance。

Reversions；　or　atavistic　changes；　would　seem　to　give　a　better　support　to　the　theory　of　descent　through　modifications。　　These　have　been　of　paramount　importance　on　many　lines　of　evolution　of　the　animal　as　well　as　of　the　vegetable　kingdom。　　It　is　often　assumed　that　monocotyledons　are　descended　from　some　lower　group　of　dicotyledons；　probably　allied　to　that　which　includes　the　buttercup　family。　　On　this　view　the　monocotyledons　must　be　assumed　to　have　lost　the　cambium　and　all　its　influence　on　secondary　growth；　the　differentiation　of　the　flower　into　calyx　and　corolla；　the　second　cotyledon　or　seed…leaf　and　several　other　characters。　　Losses　of　characters　such　as　these　may　have　been　the　result　of　abrupt　changes；　but　this　does　not　prove　that　the　characters　themselves　have　been　produced　with　equal　suddenness。　　On　the　contrary；　Darwin　shows　very　convincingly　that　a　modification　may　well　be　developed　by　a　series　of　steps；　and　afterwards　suddenly　disappear。　　Many　monstrosities；　such　as　those　represented　by　twisted　stems；　furnish　direct　proofs　in　support　of　this　view；　since　they　are　produced　by　the　loss　of　one　character　and　this　loss　implies　secondary　changes　in　a　large　number　of　other　organs　and　qualities。

Darwin　criticises　in　detail　the　hypothesis　of　great　and　abrupt　changes　and　comes　to　the　conclusion　that　it　does　not　give　even　a　shadow　of　an　explanation　of　the　origin　of　species。　　It　is　as　improbable　as　it　is　unnecessary。

Sports　and　spontaneous　variations　must　now　be　considered。　　It　is　well　known　that　they　have　produced　a　large　number　of　fine　horticultural　varieties。　　The　cut…leaved　maple　and　many　other　trees　and　shrubs　with　split　leaves　are　known　to　have　been　produced　at　a　single　step；　this　is　true　in　the　case　of　the　single…leaf　strawberry　plant　and　of　the　laciniate　variety　of　the　greater　celandine：　　many　white　flowers；　white　or　yellow　berries　and　numerous　other　forms　had　a　similar　origin。　　But　changes　such　as　these　do　not　come　under　the　head　of　adaptations；　as　they　consist　for　the　most　part　in　the　loss　of　some　quality　or　organ　belonging　to　the　species　from　which　they　were　derived。　　Darwin　thinks　it　impossible　to　attribute　to　this　cause　the　innumerable　structures；　which　are　so　well　adapted　to　the　habits　of　life　of　each　species。　　At　the　present　time　we　should　say　that　such　adaptations　require　progressive　modifications；　which　are　additions　to　the　stock　of　qualities　already　possessed　by　the　ancestors；　and　cannot；　therefore；　be　explained　on　the　ground　of　a　supposed　analogy　with　sports；　which　are　for　the　most　part　of　a　retrogressive　nature。

Excluding　all　these　more　or　less　sudden　changes；　there　remains　a　long　series　of　gradations　of　variability；　but　all　of　these　are　not　assumed　by　Darwin　to　be　equally　fit　for　the　production　of　new　species。　　In　the　first　place；　he　disregards　all　mere　temporary　variations；　such　as　size；　albinism；　etc。；　further；　he　points　out　that　very　many　species　have　almost　certainly　been　produced　by　steps；　not　greater；　and　probably　not　very　much　smaller；　than　those　separating　closely　related　varieties。　　For　varieties　are　only　small　species。　　Ne