darwin and modern science-第24节

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。　　Next　comes　the　question　of　polymorphic　species：　　their　occurrence　seems　to　have　been　a　source　of　much　doubt　and　difficulty　in　Darwin's　mind；　although　at　present　it　forms　one　of　the　main　supports　of　the　prevailing　explanation　of　the　origin　of　new　species。　　Darwin　simply　states　that　this　kind　of　variability　seems　to　be　of　a　peculiar　nature；　since　polymorphic　species　are　now　in　a　stable　condition　their　occurrence　gives　no　clue　as　to　the　mode　of　origin　of　new　species。　　Polymorphic　species　are　the　expression　of　the　result　of　previous　variability　acting　on　a　large　scale；　but　they　now　simply　consist　of　more　or　less　numerous　elementary　species；　which；　as　far　as　we　know；　do　not　at　present　exhibit　a　larger　degree　of　variability　than　any　other　more　uniform　species。　　The　vernal　whitlow…grass　（Draba　verna）　and　the　wild　pansy　are　the　best　known　examples；　both　have　spread　over　almost　the　whole　of　Europe　and　are　split　up　into　hundreds　of　elementary　forms。　　These　sub…species　show　no　signs　of　any　extraordinary　degree　of　variability；　when　cultivated　under　conditions　necessary　for　the　exclusion　of　inter…crossing。　　Hooker　has　shown；　in　the　case　of　some　ferns　distributed　over　still　wider　areas；　that　the　extinction　of　some　of　the　intermediate　forms　in　such　groups　would　suffice　to　justify　the　elevation　of　the　remaining　types　to　the　rank　of　distinct　species。　　Polymorphic　species　may　now　be　regarded　as　the　link　which　unites　ordinary　variability　with　the　historical　production　of　species。　　But　it　does　not　appear　that　they　had　this　significance　for　Darwin；　and；　in　fact；　they　exhibit　no　phenomena　which　could　explain　the　processes　by　which　one　species　has　been　derived　from　another。　　By　thus　narrowing　the　limits　of　the　species…producing　variability　Darwin　was　led　to　regard　small　deviations　as　the　source　from　which　natural　selection　derives　material　upon　which　to　act。　　But　even　these　are　not　all　of　the　same　type；　and　Darwin　was　well　aware　of　the　fact。

It　should　here　be　pointed　out　that　in　order　to　be　selected；　a　change　must　first　have　been　produced。　　This　proposition；　which　now　seems　self…evident；　has；　however；　been　a　source　of　much　difference　of　opinion　among　Darwin's　followers。　　The　opinion　that　natural　selection　produces　changes　in　useful　directions　has　prevailed　for　a　long　time。　　In　other　words；　it　was　assumed　that　natural　selection；　by　the　simple　means　of　singling　out；　could　induce　small　and　useful　changes　to　increase　and　to　reach　any　desired　degree　of　deviation　from　the　original　type。　　In　my　opinion　this　view　was　never　actually　held　by　Darwin。　　It　is　in　contradiction　with　the　acknowledged　aim　of　all　his　work；the　explanation　of　the　origin　of　species　by　means　of　natural　forces　and　phenomena　only。　　Natural　selection　acts　as　a　sieve；　it　does　not　single　out　the　best　variations；　but　it　simply　destroys　the　larger　number　of　those　which　are；　from　some　cause　or　another；　unfit　for　their　present　environment。　　In　this　way　it　keeps　the　strains　up　to　the　required　standard；　and；　in　special　circumstances；　may　even　improve　them。

Returning　to　the　variations　which　afford　the　material　for　the　sieving…　action　of　natural　selection；　we　may　distinguish　two　main　kinds。　　It　is　true　that　the　distinction　between　these　was　not　clear　at　the　time　of　Darwin；　and　that　he　was　unable　to　draw　a　sharp　line　between　them。　　Nevertheless；　in　many　cases；　he　was　able　to　separate　them；　and　he　often　discussed　the　question　which　of　the　two　would　be　the　real　source　of　the　differentiation　of　species。　　Certain　variations　constantly　occur；　especially　such　as　are　connected　with　size；　weight；　colour；　etc。　　They　are　usually　too　small　for　natural　selection　to　act　upon；　having　hardly　any　influence　in　the　struggle　for　life：　　others　are　more　rare；　occurring　only　from　time　to　time；　perhaps　once　or　twice　in　a　century；　perhaps　even　only　once　in　a　thousand　years。　　Moreover；　these　are　of　another　type；　not　simply　affecting　size；　number　or　weight；　but　bringing　about　something　new；　which　may　be　useful　or　not。　　Whenever　the　variation　is　useful　natural　selection　will　take　hold　of　it　and　preserve　it；　in　other　cases　the　variation　may　either　persist　or　disappear。

In　his　criticism　of　miscellaneous　objections　brought　forward　against　the　theory　of　natural　selection　after　the　publication　of　the　first　edition　of　〃The　Origin　of　Species〃；　Darwin　stated　his　view　on　this　point　very　clearly：〃The　doctrine　of　natural　selection　or　the　survival　of　the　fittest；　which　implies　that　when　variations　or　individual　differences　of　a　beneficial　nature　happen　to　arise；　these　will　be　preserved。〃　　（〃Origin　of　Species〃　（6th　edition）；　page　169；　1882。）　　In　this　sentence　the　words　〃HAPPEN　TO　ARISE〃　appear　to　me　of　prominent　significance。　　They　are　evidently　due　to　the　same　general　conception　which　prevailed　in　Darwin's　Pangenesis　hypothesis。　　（Cf。　de　Vries；　〃Intracellulare　Pangenesis〃；　page　73；　Jena；　1889；　and　〃Die　Mutationstheorie〃；　I。　page　63。　　Leipzig；　1901。）

A　distinction　is　indicated　between　ordinary　fluctuations　which　are　always　present；　and　such　variations　as　〃happen　to　arise〃　from　time　to　time。　　（（I　think　it　right　to　point　out　that　the　interpretation　of　this　passage　from　the　〃Origin〃　by　Professor　de　Vries　is　not　accepted　as　correct　either　by　Mr　Francis　Darwin　or　by　myself。　　We　do　not　believe　that　Darwin　intended　to　draw　any　distinction　between　TWO　TYPES　of　variation；　the　words　〃when　variations　or　individual　differences　of　a　beneficial　nature　happen　to　arise〃　are　not　in　our　opinion　meant　to　imply　a　distinction　between　ordinary　fluctuations　and　variations　which　〃happen　to　arise；〃　but　we　believe　that　〃or〃　is　here　used　in　the　sense　of　ALIAS。　　With　the　permission　of　Professor　de　Vries；　the　following　extract　is　quoted　from　a　letter　in　which　he　replied　to　the　objection　raised　to　his　reading　of　the　passage　in　question：

〃As　to　your　remarks　on　the　passage　on　page　6；　I　agree　that　it　is　now　impossible　to　see　clearly　how　far　Darwin　went　in　his　distinction　of　the　different　kinds　of　variability。　　Distinctions　were　only　dimly　guessed　at　by　him。　　But　in　our　endeavour　to　arrive　at　a　true　conception　of　his　view　I　think　that　the　chapter　on　Pangenesis　should　be　our　leading　guide；　and　that　we　should　try　to　interpret　the　more　difficult　passages　by　that　chapter。　　A　careful　and　often　repeated　study　of　the　Pangenesis　hypothesis　has　convinced　me　that　Darwin；　when　he　wrote　that　chapter；　was　well　aware　that　ordinary　variability　has　nothing　to　do　with　evolution；　but　that　other　kinds　of　variation　were　necessary。　　In　some　chapters　he　comes　nearer　to　a　clear　distinction　than　in　others。　　To　my　mind　the　expression　'happen　to　arise'　is　the　sharpest　indication　of　his　inclining　in　this　direction。　　I　am　quite　convinced　that　numerous　expressions　in　his　book　become　much　clearer　when　looked　at　in　this　way。〃

The　statement　in　this　passage　that　〃Darwin　was　well　aware　that　ordinary　variability　has　nothing　to　do　with　evolution；　but　that　other　kinds　of　variation　were　necessary〃　is　contradicted　by　many　passages　in　the　〃Origin〃。　A。C。S。））　　The　latter　afford　the　material　for　natural　selection　to　act　upon　on　the　broad　lines　of　organic　development；　but　the　first　do　not。　　Fortuitous　variations　are　the　species…producing　kind；　which　the　theory　requires；　continuous　fluctuations　constitute；　in　this　respect；　a　useless　type。

Of　late；　the　study　of　variability　has　returned　to　the　recognition　of　this　distinction。　　Darwin's　variations；　which　from　time　to　time　happen　to　arise；　are　MUTATIONS；　the　opposite　type　being　commonly　designed　fluctuations。　　A　large　mass　of　facts；　collected　during　the　last　few　decades；　has　confirmed　this　view；　which　in　Darwin's　time　could　only　be　expressed　with　much　reserve；　and　everyone　knows　that　Darwin　was　always　very　careful　in　statements　of　this　kind。

From　the　same　chapter　I　may　here　cite　the　following　paragraph：　　〃Thus　as　I　am　inclined　to　believe；　morphological　differences；。。。such　as　the　arrangement　of　the　leaves；　the　divisions　of　the　flower　or　of　the　ovarium；　the　position　of　the　ovules；　etc。first　appeared　in　many　cases　as　fluctuating　variations；　which　sooner　or　later　became　constant　through　the　nature　of　the　organism　and　of　the　surrounding　conditions。。。but　NOT　THROUGH　NATURAL　SELECTION　（The　italics　are　mine　（H。　de　V。）。）；　for　as　these　morphological　characters　do　not　affect　the　welfare　of　the　species；　any　slight　deviation　in　them　could　not　have　been　governed　or　accumulated　through　this　latter　agency。〃　　（〃Origin　of　Species〃　（6th　edition）；　page　176。）　　We　thus　see　that　in　Darwin's　opinion；　all　small　variations　had　not　the　same　importance。　　In　favourable　circumstances　some　could　become　constant；　but　others　could　not。

Since　the　appearance　of　the　first　edition　of　〃The　Origin　of　Species〃　fluctuating　variability　has　been　thoroughly　studied　by　Quetelet。　　He　discovered　the　law；　which　governs　all　phenomena　of　organic　life　falling　under　this　head。　　It　is　a　very　simple　law；　and　states　that　individual　variations　follow　the　laws　of　probability。　　He　proved　it；　in　the　first　place；　for　the　size　of　the　human　body；　using　the　measurements　published　for　Belgian　recruits；　he　then　extended　it　to　various　other　measurements　of　parts　of　the　body；　and　finally　concluded　that　it　must　be　of　universal　validity　for　all　organic　beings。　　It　must　hold　true　for　all　characters　in　man；　physical　as　well