darwin and modern science-第7章

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d　come　to　the　conclusion　that　the　evolutionary　passages　are　entirely　neutralised　by　others　of　an　opposite　trend。　　There　is　the　same　difficulty　with　Buffon。

Hints　of　the　idea　of　Natural　Selection　have　been　detected　elsewhere。　　James　Watt　（See　Prof。　Patrick　Geddes's　article　〃Variation　and　Selection〃；　〃Encyclopaedia　Britannica　（9th　edition）　1888。）；　for　instance；　has　been　reported　as　one　of　the　anticipators　（1851）。　　But　we　need　not　prolong　the　inquiry　further；　since　Darwin　did　not　know　of　any　anticipations　until　after　he　had　published　the　immortal　work　of　1859；　and　since　none　of　those　who　got　hold　of　the　idea　made　any　use　of　it。　　What　Darwin　did　was　to　follow　the　clue　which　Malthus　gave　him；　to　realise；　first　by　genius　and　afterwards　by　patience；　how　the　complex　and　subtle　struggle　for　existence　works　out　a　natural　selection　of　those　organisms　which　vary　in　the　direction　of　fitter　adaptation　to　the　conditions　of　their　life。　　So　much　success　attended　his　application　of　the　Selection…formula　that　for　a　time　he　regarded　Natural　Selection　as　almost　the　sole　factor　in　evolution；　variations　being　pre…　supposed；　gradually；　however；　he　came　to　recognise　that　there　was　some　validity　in　the　factors　which　had　been　emphasized　by　Lamarck　and　by　Buffon；　and　in　his　well…known　summing　up　in　the　sixth　edition　of　the　〃Origin〃　he　says　of　the　transformation　of　species：　　〃This　has　been　effected　chiefly　through　the　natural　selection　of　numerous　successive；　slight；　favourable　variations；　aided　in　an　important　manner　by　the　inherited　effects　of　the　use　and　disuse　of　parts；　and　in　an　unimportant　manner；　that　is；　in　relation　to　adaptive　structures；　whether　past　or　present；　by　the　direct　action　of　external　conditions；　and　by　variations　which　seem　to　us　in　our　ignorance　to　arise　spontaneously。〃

To　sum　up：　　the　idea　of　organic　evolution；　older　than　Aristotle；　slowly　developed　from　the　stage　of　suggestion　to　the　stage　of　verification；　and　the　first　convincing　verification　was　Darwin's；　from　being　an　a　priori　anticipation　it　has　become　an　interpretation　of　nature；　and　Darwin　is　still　the　chief　interpreter；　from　being　a　modal　interpretation　it　has　advanced　to　the　rank　of　a　causal　theory；　the　most　convincing　part　of　which　men　will　never　cease　to　call　Darwinism。


III。　　THE　SELECTION　THEORY

By　August　Weismann。　Professor　of　Zoology　in　the　University　of　Freiburg　（Baden）。

I。　　THE　IDEA　OF　SELECTION。

Many　and　diverse　were　the　discoveries　made　by　Charles　Darwin　in　the　course　of　a　long　and　strenuous　life；　but　none　of　them　has　had　so　far…reaching　an　influence　on　the　science　and　thought　of　his　time　as　the　theory　of　selection。　　I　do　not　believe　that　the　theory　of　evolution　would　have　made　its　way　so　easily　and　so　quickly　after　Darwin　took　up　the　cudgels　in　favour　of　it；　if　he　had　not　been　able　to　support　it　by　a　principle　which　was　capable　of　solving；　in　a　simple　manner；　the　greatest　riddle　that　living　nature　presents　to　us；I　mean　the　purposiveness　of　every　living　form　relative　to　the　conditions　of　its　life　and　its　marvellously　exact　adaptation　to　these。

Everyone　knows　that　Darwin　was　not　alone　in　discovering　the　principle　of　selection；　and　that　the　same　idea　occurred　simultaneously　and　independently　to　Alfred　Russel　Wallace。　　At　the　memorable　meeting　of　the　Linnean　Society　on　1st　July；　1858；　two　papers　were　read　（communicated　by　Lyell　and　Hooker）　both　setting　forth　the　same　idea　of　selection。　　One　was　written　by　Charles　Darwin　in　Kent；　the　other　by　Alfred　Wallace　in　Ternate；　in　the　Malay　Archipelago。　　It　was　a　splendid　proof　of　the　magnanimity　of　these　two　investigators；　that　they　thus；　in　all　friendliness　and　without　envy；　united　in　laying　their　ideas　before　a　scientific　tribunal：　　their　names　will　always　shine　side　by　side　as　two　of　the　brightest　stars　in　the　scientific　sky。

But　it　is　with　Charles　Darwin　that　I　am　here　chiefly　concerned；　since　this　paper　is　intended　to　aid　in　the　commemoration　of　the　hundredth　anniversary　of　his　birth。

The　idea　of　selection　set　forth　by　the　two　naturalists　was　at　the　time　absolutely　new；　but　it　was　also　so　simple　that　Huxley　could　say　of　it　later；　〃How　extremely　stupid　not　to　have　thought　of　that。〃　　As　Darwin　was　led　to　the　general　doctrine　of　descent；　not　through　the　labours　of　his　predecessors　in　the　early　years　of　the　century；　but　by　his　own　observations；　so　it　was　in　regard　to　the　principle　of　selection。　　He　was　struck　by　the　innumerable　cases　of　adaptation；　as；　for　instance；　that　of　the　woodpeckers　and　tree…frogs　to　climbing；　or　the　hooks　and　feather…like　appendages　of　seeds；　which　aid　in　the　distribution　of　plants；　and　he　said　to　himself　that　an　explanation　of　adaptations　was　the　first　thing　to　be　sought　for　in　attempting　to　formulate　a　theory　of　evolution。

But　since　adaptations　point　to　CHANGES　which　have　been　undergone　by　the　ancestral　forms　of　existing　species；　it　is　necessary；　first　of　all；　to　inquire　how　far　species　in　general　are　VARIABLE。　　Thus　Darwin's　attention　was　directed　in　the　first　place　to　the　phenomenon　of　variability；　and　the　use　man　has　made　of　this；　from　very　early　times；　in　the　breeding　of　his　domesticated　animals　and　cultivated　plants。　　He　inquired　carefully　how　breeders　set　to　work；　when　they　wished　to　modify　the　structure　and　appearance　of　a　species　to　their　own　ends；　and　it　was　soon　clear　to　him　that　SELECTION　FOR　BREEDING　PURPOSES　played　the　chief　part。

But　how　was　it　possible　that　such　processes　should　occur　in　free　nature？　　Who　is　here　the　breeder；　making　the　selection；　choosing　out　one　individual　to　bring　forth　offspring　and　rejecting　others？　　That　was　the　problem　that　for　a　long　time　remained　a　riddle　to　him。

Darwin　himself　relates　how　illumination　suddenly　came　to　him。　　He　had　been　reading；　for　his　own　pleasure；　Malthus'　book　on　Population；　and；　as　he　had　long　known　from　numerous　observations；　that　every　species　gives　rise　to　many　more　descendants　than　ever　attain　to　maturity；　and　that；　therefore；　the　greater　number　of　the　descendants　of　a　species　perish　without　reproducing；　the　idea　came　to　him　that　the　decision　as　to　which　member　of　a　species　was　to　perish；　and　which　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…establis