darwin and modern science-第9节

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　was　to　attain　to　maturity　and　reproduction　might　not　be　a　matter　of　chance；　but　might　be　determined　by　the　constitution　of　the　individuals　themselves；　according　as　they　were　more　or　less　fitted　for　survival。　　With　this　idea　the　foundation　of　the　theory　of　selection　was　laid。

In　ARTIFICIAL　SELECTION　the　breeder　chooses　out　for　pairing　only　such　individuals　as　possess　the　character　desired　by　him　in　a　somewhat　higher　degree　than　the　rest　of　the　race。　　Some　of　the　descendants　inherit　this　character；　often　in　a　still　higher　degree；　and　if　this　method　be　pursued　throughout　several　generations；　the　race　is　transformed　in　respect　of　that　particular　character。

NATURAL　SELECTION　depends　on　the　same　three　factors　as　ARTIFICIAL　SELECTION：　　on　VARIABILITY；　INHERITANCE；　and　SELECTION　FOR　BREEDING；　but　this　last　is　here　carried　out　not　by　a　breeder　but　by　what　Darwin　called　the　〃struggle　for　existence。〃　　This　last　factor　is　one　of　the　special　features　of　the　Darwinian　conception　of　nature。　　That　there　are　carnivorous　animals　which　take　heavy　toll　in　every　generation　of　the　progeny　of　the　animals　on　which　they　prey；　and　that　there　are　herbivores　which　decimate　the　plants　in　every　generation　had　long　been　known；　but　it　is　only　since　Darwin's　time　that　sufficient　attention　has　been　paid　to　the　facts　that；　in　addition　to　this　regular　destruction；　there　exists　between　the　members　of　a　species　a　keen　competition　for　space　and　food；　which　limits　multiplication；　and　that　numerous　individuals　of　each　species　perish　because　of　unfavourable　climatic　conditions。　　The　〃struggle　for　existence；〃　which　Darwin　regarded　as　taking　the　place　of　the　human　breeder　in　free　nature；　is　not　a　direct　struggle　between　carnivores　and　their　prey；　but　is　the　assumed　competition　for　survival　between　individuals　OF　THE　SAME　species；　of　which；　on　an　average；　only　those　survive　to　reproduce　which　have　the　greatest　power　of　resistance；　while　the　others；　less　favourably　constituted；　perish　early。　　This　struggle　is　so　keen；　that；　within　a　limited　area；　where　the　conditions　of　life　have　long　remained　unchanged；　of　every　species；　whatever　be　the　degree　of　fertility；　only　two；　ON　AN　AVERAGE；　of　the　descendants　of　each　pair　survive；　the　others　succumb　either　to　enemies；　or　to　disadvantages　of　climate；　or　to　accident。　　A　high　degree　of　fertility　is　thus　not　an　indication　of　the　special　success　of　a　species；　but　of　the　numerous　dangers　that　have　attended　its　evolution。　　Of　the　six　young　brought　forth　by　a　pair　of　elephants　in　the　course　of　their　lives　only　two　survive　in　a　given　area；　similarly；　of　the　millions　of　eggs　which　two　thread…worms　leave　behind　them　only　two　survive。　　It　is　thus　possible　to　estimate　the　dangers　which　threaten　a　species　by　its　ratio　of　elimination；　or；　since　this　cannot　be　done　directly；　by　its　fertility。

Although　a　great　number　of　the　descendants　of　each　generation　fall　victims　to　accident；　among　those　that　remain　it　is　still　the　greater　or　lesser　fitness　of　the　organism　that　determines　the　〃selection　for　breeding　purposes；〃　and　it　would　be　incomprehensible　if；　in　this　competition；　it　were　not　ultimately；　that　is；　on　an　average；　the　best　equipped　which　survive；　in　the　sense　of　living　long　enough　to　reproduce。

Thus　the　principle　of　natural　selection　is　THE　SELECTION　OF　THE　BEST　FOR　REPRODUCTION；　whether　the　〃best〃　refers　to　the　whole　constitution；　to　one　or　more　parts　of　the　organism；　or　to　one　or　more　stages　of　development。　　Every　organ；　every　part；　every　character　of　an　animal；　fertility　and　intelligence　included；　must　be　improved　in　this　manner；　and　be　gradually　brought　up　in　the　course　of　generations　to　its　highest　attainable　state　of　perfection。　　And　not　only　may　improvement　of　parts　be　brought　about　in　this　way；　but　new　parts　and　organs　may　arise；　since；　through　the　slow　and　minute　steps　of　individual　or　〃fluctuating〃　variations；　a　part　may　be　added　here　or　dropped　out　there；　and　thus　something　new　is　produced。

The　principle　of　selection　solved　the　riddle　as　to　how　what　was　purposive　could　conceivably　be　brought　about　without　the　intervention　of　a　directing　power；　the　riddle　which　animate　nature　presents　to　our　intelligence　at　every　turn；　and　in　face　of　which　the　mind　of　a　Kant　could　find　no　way　out；　for　he　regarded　a　solution　of　it　as　not　to　be　hoped　for。　　For；　even　if　we　were　to　assume　an　evolutionary　force　that　is　continually　transforming　the　most　primitive　and　the　simplest　forms　of　life　into　ever　higher　forms；　and　the　homogeneity　of　primitive　times　into　the　infinite　variety　of　the　present；　we　should　still　be　unable　to　infer　from　this　alone　how　each　of　the　numberless　forms　adapted　to　particular　conditions　of　life　should　have　appeared　PRECISELY　AT　THE　RIGHT　MOMENT　IN　THE　HISTORY　OF　THE　EARTH　to　which　their　adaptations　were　appropriate；　and　precisely　at　the　proper　place　in　which　all　the　conditions　of　life　to　which　they　were　adapted　occurred：　　the　humming…birds　at　the　same　time　as　the　flowers；　the　trichina　at　the　same　time　as　the　pig；　the　bark…coloured　moth　at　the　same　time　as　the　oak；　and　the　wasp…like　moth　at　the　same　time　as　the　wasp　which　protects　it。　　Without　processes　of　selection　we　should　be　obliged　to　assume　a　〃pre…established　harmony〃　after　the　famous　Leibnitzian　model；　by　means　of　which　the　clock　of　the　evolution　of　organisms　is　so　regulated　as　to　strike　in　exact　synchronism　with　that　of　the　history　of　the　earth！　　All　forms　of　life　are　strictly　adapted　to　the　conditions　of　their　life；　and　can　persist　under　these　conditions　alone。

There　must　therefore　be　an　intrinsic　connection　between　the　conditions　and　the　structural　adaptations　of　the　organism；　and；　SINCE　THE　CONDITIONS　OF　LIFE　CANNOT　BE　DETERMINED　BY　THE　ANIMAL　ITSELF；　THE　ADAPTATIONS　MUST　BE　CALLED　FORTH　BY　THE　CONDITIONS。

The　selection　theory　teaches　us　how　this　is　conceivable；　since　it　enables　us　to　understand　that　there　is　a　continual　production　of　what　is　non…　purposive　as　well　as　of　what　is　purposive；　but　the　purposive　alone　survives；　while　the　non…purposive　perishes　in　the　very　act　of　arising。　　This　is　the　old　wisdom　taught　long　ago　by　Empedocles。

II。　　THE　LAMARCKIAN　PRINCIPLE。

Lamarck；　as　is　well　known；　formulated　a　definite　theory　of　evolution　at　the　beginning　of　the　nineteenth　century；　exactly　fifty　years　before　the　Darwin…　Wallace　principle　of　selection　was　given　to　the　world。　　This　brilliant　investigator　also　endeavoured　to　support　his　theory　by　demonstrating　forces　which　might　have　brought　about　the　transformations　of　the　organic　world　in　the　course　of　the　ages。　　In　addition　to　other　factors；　he　laid　special　emphasis　on　the　increased　or　diminished　use　of　the　parts　of　the　body；　assuming　that　the　strengthening　or　weakening　which　takes　place　from　this　cause　during　the　individual　life；　could　be　handed　on　to　the　offspring；　and　thus　intensified　and　raised　to　the　rank　of　a　specific　character。　　Darwin　also　regarded　this　LAMARCKIAN　PRINCIPLE；　as　it　is　now　generally　called；　as　a　factor　in　evolution；　but　he　was　not　fully　convinced　of　the　transmissibility　of　acquired　characters。

As　I　have　here　to　deal　only　with　the　theory　of　selection；　I　need　not　discuss　the　Lamarckian　hypothesis；　but　I　must　express　my　opinion　that　there　is　room　for　much　doubt　as　to　the　cooperation　of　this　principle　in　evolution。　　Not　only　is　it　difficult　to　imagine　how　the　transmission　of　functional　modifications　could　take　place；　but；　up　to　the　present　time；　notwithstanding　the　endeavours　of　many　excellent　investigators；　not　a　single　actual　proof　of　such　inheritance　has　been　brought　forward。　　Semon's　experiments　on　plants　are；　according　to　the　botanist　Pfeffer；　not　to　be　relied　on；　and　even　the　recent；　beautiful　experiments　made　by　Dr　Kammerer　on　salamanders；　cannot；　as　I　hope　to　show　elsewhere；　be　regarded　as　proof；　if　only　because　they　do　not　deal　at　all　with　functional　modifications；　that　is；　with　modifications　brought　about　by　use；　and　it　is　to　these　ALONE　that　the　Lamarckian　principle　refers。

III。　　OBJECTIONS　TO　THE　THEORY　OF　SELECTION。

（a）　　Saltatory　evolution。

The　Darwinian　doctrine　of　evolution　depends　essentially　on　THE　CUMULATIVE　AUGMENTATION　of　minute　variations　in　the　direction　of　utility。　　But　can　such　minute　variations；　which　are　undoubtedly　continually　appearing　among　the　individuals　of　the　same　species；　possess　any　selection…value；　can　they　determine　which　individuals　are　to　survive；　and　which　are　to　succumb；　can　they　be　increased　by　natural　selection　till　they　attain　to　the　highest　development　of　a　purposive　variation？

To　many　this　seems　so　improbable　that　they　have　urged　a　theory　of　evolution　by　leaps　from　species　to　species。　　Kolliker；　in　1872；　compared　the　evolution　of　species　with　the　processes　which　we　can　observe　in　the　individual　life　in　cases　of　alternation　of　generations。　　But　a　polyp　only　gives　rise　to　a　medusa　because　it　has　itself　arisen　from　one；　and　there　can　be　no　question　of　a　medusa　ever　having　arisen　suddenly　and　de　novo　from　a　polyp…bud；　if　only　because　both　forms　are　adapted　in　their　structure　as　a　whole；　and　in　every　detail　to　the　conditions　of　their　life。　　A　sudden　origin；　in　a　natural　way；　of　numerous　adaptations　is　inconceivable。　　Even　the　degeneration　of　a　medusoid　from　a　free…swimming　animal　to　a　mere　brood…　sac　（gonophore）　is　not　sudden　and　salt