darwin and modern science-第45节

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In　the　nineteenth　chapter　of　the　〃Generelle　Morphologie〃a　part　of　which　has　been　republished；　without　any　alteration；　after　a　lapse　of　forty　years　I　made　a　critical　study　of　Lamarck's　theory　of　descent　and　of　Darwin's　theory　of　selection；　and　endeavoured　to　bring　the　complex　phenomena　of　heredity　and　adaptation　under　definite　laws　for　the　first　time。　　Heredity　I　divided　into　conservative　and　progressive：　　adaptation　into　indirect　（or　potential）　and　direct　（or　actual）。　　I　then　found　it　possible　to　give　some　explanation　of　the　correlation　of　the　two　physiological　functions　in　the　struggle　for　life　（selection）；　and　to　indicate　the　important　laws　of　divergence　（or　differentiation）　and　complexity　（or　division　of　labour）；　which　are　the　direct　and　inevitable　outcome　of　selection。　　Finally；　I　marked　off　dysteleology　as　the　science　of　the　aimless　（vestigial；　abortive；　atrophied；　and　useless）　organs　and　parts　of　the　body。　　In　all　this　I　worked　from　a　strictly　monistic　standpoint；　and　sought　to　explain　all　biological　phenomena　on　the　mechanical　and　naturalistic　lines　that　had　long　been　recognised　in　the　study　of　inorganic　nature。　　Then　（1866）；　as　now；　being　convinced　of　the　unity　of　nature；　the　fundamental　identity　of　the　agencies　at　work　in　the　inorganic　and　the　organic　worlds；　I　discarded　vitalism；　teleology；　and　all　hypotheses　of　a　mystic　character。

It　was　clear　from　the　first　that　it　was　essential；　in　the　monistic　conception　of　evolution；　to　distinguish　between　the　laws　of　conservative　and　progressive　heredity。　　Conservative　heredity　maintains　from　generation　to　generation　the　enduring　characters　of　the　species。　　Each　organism　transmits　to　its　descendants　a　part　of　the　morphological　and　physiological　qualities　that　it　has　received　from　its　parents　and　ancestors。　　On　the　other　hand；　progressive　heredity　brings　new　characters　to　the　species　characters　that　were　not　found　in　preceding　generations。　　Each　organism　may　transmit　to　its　offspring　a　part　of　the　morphological　and　physiological　features　that　it　has　itself　acquired；　by　adaptation；　in　the　course　of　its　individual　career；　through　the　use　or　disuse　of　particular　organs；　the　influence　of　environment；　climate；　nutrition；　etc。　　At　that　time　I　gave　the　name　of　〃progressive　heredity〃　to　this　inheritance　of　acquired　characters；　as　a　short　and　convenient　expression；　but　have　since　changed　the　term　to　〃transformative　heredity〃　（as　distinguished　from　conservative）。　　This　term　is　preferable；　as　inherited　regressive　modifications　（degeneration；　retrograde　metamorphisis；　etc。）　come　under　the　same　head。

Transformative　heredityor　the　transmission　of　acquired　charactersis　one　of　the　most　important　principles　in　evolutionary　science。　　Unless　we　admit　it　most　of　the　facts　of　comparative　anatomy　and　physiology　are　inexplicable。　　That　was　the　conviction　of　Darwin　no　less　than　of　Lamarck；　of　Spencer　as　well　as　Virchow；　of　Huxley　as　well　as　Gegenbaur；　indeed　of　the　great　majority　of　speculative　biologists。　　This　fundamental　principle　was　for　the　first　time　called　in　question　and　assailed　in　1885　by　August　Weismann　of　Freiburg；　the　eminent　zoologist　to　whom　the　theory　of　evolution　owes　a　great　deal　of　valuable　support；　and　who　has　attained　distinction　by　his　extension　of　the　theory　of　selection。　　In　explanation　of　the　phenomena　of　heredity　he　introduced　a　new　theory；　the　〃theory　of　the　continuity　of　the　germ…plasm。〃　　According　to　him　the　living　substance　in　all　organisms　consists　of　two　quite　distinct　kinds　of　plasm；　somatic　and　germinal。　　The　permanent　germ…plasm；　or　the　active　substance　of　the　two　germ…cells　（egg…　cell　and　sperm…cell）；　passes　unchanged　through　a　series　of　generations；　and　is　not　affected　by　environmental　influences。　　The　environment　modifies　only　the　soma…plasm；　the　organs　and　tissues　of　the　body。　　The　modifications　that　these　parts　undergo　through　the　influence　of　the　environment　or　their　own　activity　（use　and　habit）；　do　not　affect　the　germ…plasm；　and　cannot　therefore　be　transmitted。

This　theory　of　the　continuity　of　the　germ…plasm　has　been　expounded　by　Weismann　during　the　last　twenty…four　years　in　a　number　of　able　volumes；　and　is　regarded　by　many　biologists；　such　as　Mr　Francis　Galton；　Sir　E。　Ray　Lankester；　and　Professor　J。　Arthur　Thomson　（who　has　recently　made　a　thoroughgoing　defence　of　it　in　his　important　work　〃Heredity〃　（London；　1908。））；　as　the　most　striking　advance　in　evolutionary　science。　　On　the　other　hand；　the　theory　has　been　rejected　by　Herbert　Spencer；　Sir　W。　Turner；　Gegenbaur；　Kolliker；　Hertwig；　and　many　others。　　For　my　part　I　have；　with　all　respect　for　the　distinguished　Darwinian；　contested　the　theory　from　the　first；　because　its　whole　foundation　seems　to　me　erroneous；　and　its　deductions　do　not　seem　to　be　in　accord　with　the　main　facts　of　comparative　morphology　and　physiology。　　Weismann's　theory　in　its　entirety　is　a　finely　conceived　molecular　hypothesis；　but　it　is　devoid　of　empirical　basis。　　The　notion　of　the　absolute　and　permanent　independence　of　the　germ…plasm；　as　distinguished　from　the　soma…plasm；　is　purely　speculative；　as　is　also　the　theory　of　germinal　selection。　　The　determinants；　ids；　and　idants；　are　purely　hypothetical　elements。　　The　experiments　that　have　been　devised　to　demonstrate　their　existence　really　prove　nothing。

It　seems　to　me　quite　improper　to　describe　this　hypothetical　structure　as　〃Neodarwinism。〃　　Darwin　was　just　as　convinced　as　Lamarck　of　the　transmission　of　acquired　characters　and　its　great　importance　in　the　scheme　of　evolution。　　I　had　the　good　fortune　to　visit　Darwin　at　Down　three　times　and　discuss　with　him　the　main　principles　of　his　system；　and　on　each　occasion　we　were　fully　agreed　as　to　the　incalculable　importance　of　what　I　call　transformative　inheritance。　　It　is　only　proper　to　point　out　that　Weismann's　theory　of　the　germ…plasm　is　in　express　contradiction　to　the　fundamental　principles　of　Darwin　and　Lamarck。　　Nor　is　it　more　acceptable　in　what　one　may　call　its　〃ultradarwinism〃the　idea　that　the　theory　of　selection　explains　everything　in　the　evolution　of　the　organic　world。　　This　belief　in　the　〃omnipotence　of　natural　selection〃　was　not　shared　by　Darwin　himself。　　Assuredly；　I　regard　it　as　of　the　utmost　value；　as　the　process　of　natural　selection　through　the　struggle　for　life　affords　an　explanation　of　the　mechanical　origin　of　the　adapted　organisation。　　It　solves　the　great　problem：　　how　could　the　finely　adapted　structure　of　the　animal　or　plant　body　be　formed　unless　it　was　built　on　a　preconceived　plan？　　It　thus　enables　us　to　dispense　with　the　teleology　of　the　metaphysician　and　the　dualist；　and　to　set　aside　the　old　mythological　and　poetic　legends　of　creation。　　The　idea　had　occurred　in　vague　form　to　the　great　Empedocles　2000　years　before　the　time　of　Darwin；　but　it　was　reserved　for　modern　research　to　give　it　ample　expression。　　Nevertheless；　natural　selection　does　not　of　itself　give　the　solution　of　all　our　evolutionary　problems。　　It　has　to　be　taken　in　conjunction　with　the　transformism　of　Lamarck；　with　which　it　is　in　complete　harmony。

The　monumental　greatness　of　Charles　Darwin；　who　surpasses　every　other　student　of　science　in　the　nineteenth　century　by　the　loftiness　of　his　monistic　conception　of　nature　and　the　progressive　influence　of　his　ideas；　is　perhaps　best　seen　in　the　fact　that　not　one　of　his　many　successors　has　succeeded　in　modifying　his　theory　of　descent　in　any　essential　point　or　in　discovering　an　entirely　new　standpoint　in　the　interpretation　of　the　organic　world。　　Neither　Nageli　nor　Weismann；　neither　De　Vries　nor　Roux；　has　done　this。　　Nageli；　in　his　〃Mechanisch…Physiologische　Theorie　der　Abstammungslehre〃　（Munich；　1884。）；　which　is　to　a　great　extent　in　agreement　with　Weismann；　constructed　a　theory　of　the　idioplasm；　that　represents　it　（like　the　germ…plasm）　as　developing　continuously　in　a　definite　direction　from　internal　causes。　　But　his　internal　〃principle　of　progress〃　is　at　the　bottom　just　as　teleological　as　the　vital　force　of　the　Vitalists；　and　the　micellar　structure　of　the　idioplasm　is　just　as　hypothetical　as　the　〃dominant〃　structure　of　the　germ…plasm。　　In　1889　Moritz　Wagner　sought　to　explain　the　origin　of　species　by　migration　and　isolation；　and　on　that　basis　constructed　a　special　〃migration…theory。〃　　This；　however；　is　not　out　of　harmony　with　the　theory　of　selection。　　It　merely　elevates　one　single　factor　in　the　theory　to　a　predominant　position。　　Isolation　is　only　a　special　case　of　selection；　as　I　had　pointed　out　in　the　fifteenth　chapter　of　my　〃Natural　history　of　creation〃。　　The　〃mutation…theory〃　of　De　Vries　（〃Die　Mutationstheorie〃；　Leipzig；　1903。）；　that　would　explain　the　origin　of　species　by　sudden　and　saltatory　variations　rather　than　by　gradual　modification；　is　regarded　by　many　botanists　as　a　great　step　in　advance；　but　it　is　generally　rejected　by　zoologists。　　It　affords　no　explanation　of　the　facts　of　adaptation；　and　has　no　causal　value。

Much　more　important　than　these　theories　is　that　of　Wilhelm　Roux　（〃Der　Kampf　der　Theile　im　Organismus〃；　Leipzig；　1881。）　of　〃the　struggle　of　parts　within　the　organism；　a　supplementation　of　the　theory　of　mechanical　adaptation。〃　　He　explains　the　functional　autoformation　of　the　purposive　structure　by　a　combination　of　Darwin's　principle　of　selection　with　Lamarck's　idea　of　transformative　heredity；　and　applies　the　two　in　conjun