darwin and modern science-第50部分

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s　all　within　each　class　have；　according　to　our　theory；　been　connected　together　by　fine　gradations；　the　best；　and；　if　our　collections　were　nearly　perfect；　the　only　possible　arrangement；　would　be　genealogical；　descent　being　the　hidden　bond　of　connexion　which　naturalists　have　been　seeking　under　the　term　of　the　Natural　System。　　On　this　view　we　can　understand　how　it　is　that；　in　the　eyes　of　most　naturalists；　the　structure　of　the　embryo　is　even　more　important　for　classification　than　that　of　the　adult。　　In　two　or　more　groups　of　animals；　however　much　they　may　differ　from　each　other　in　structure　and　habits　in　their　adult　condition；　if　they　pass　through　closely　similar　embryonic　stages；　we　may　feel　assured　that　they　all　are　descended　from　one　parent…　form；　and　are　therefore　closely　related。　　Thus；　community　in　embryonic　structure　reveals　community　of　descent；　but　dissimilarity　in　embryonic　development　does　not　prove　discommunity　of　descent；　for　in　one　of　two　groups　the　developmental　stages　may　have　been　suppressed；　or　may　have　been　so　greatly　modified　through　adaptation　to　new　habits　of　life；　as　to　be　no　longer　recognisable。　　Even　in　groups；　in　which　the　adults　have　been　modified　to　an　extreme　degree；　community　of　origin　is　often　revealed　by　the　structure　of　the　larvae；　we　have　seen；　for　instance；　that　cirripedes；　though　externally　so　like　shell…fish；　are　at　once　known　by　their　larvae　to　belong　to　the　great　class　of　crustaceans。　　As　the　embryo　often　shows　us　more　or　less　plainly　the　structure　of　the　less　modified　and　ancient　progenitor　of　the　group；　we　can　see　why　ancient　and　extinct　forms　so　often　resemble　in　their　adult　state　the　embryos　of　existing　species　of　the　same　class。　　Agassiz　believes　this　to　be　a　universal　law　of　nature；　and　we　may　hope　hereafter　to　see　the　law　proved　true。　　It　can；　however；　be　proved　true　only　in　those　cases　in　which　the　ancient　state　of　the　progenitor　of　the　group　has　not　been　wholly　obliterated；　either　by　successive　variations　having　supervened　at　a　very　early　period　of　growth；　or　by　such　variations　having　been　inherited　at　an　earlier　stage　than　that　at　which　they　first　appeared。　　It　should　also　be　borne　in　mind；　that　the　law　may　be　true；　but　yet；　owing　to　the　geological　record　not　extending　far　enough　back　in　time；　may　remain　for　a　long　period；　or　for　ever；　incapable　of　demonstration。　　The　law　will　not　strictly　hold　good　in　those　cases　in　which　an　ancient　form　became　adapted　in　its　larval　state　to　some　special　line　of　life；　and　transmitted　the　same　larval　state　to　a　whole　group　of　descendants；　for　such　larvae　will　not　resemble　any　still　more　ancient　form　in　its　adult　state。〃

As　this　passage　shows；　Darwin　held　that　embryology　was　of　interest　because　of　the　light　it　seems　to　throw　upon　ancestral　history　（phylogeny）　and　because　of　the　help　it　would　give　in　enabling　us　to　arrive　at　a　natural　system　of　classification。　　With　regard　to　the　latter　point；　he　quotes　with　approval　the　opinion　that　〃the　structure　of　the　embryo　is　even　more　important　for　classification　than　that　of　the　adult。〃　　What　justification　is　there　for　this　view？　　The　phase　of　life　chosen　for　the　ordinary　anatomical　and　physiological　studies；　namely；　the　adult　phase；　is　merely　one　of　the　large　number　of　stages　of　structure　through　which　the　organism　passes。　　By　far　the　greater　number　of　these　are　included　in　what　is　specially　called　the　developmental　or　（if　we　include　larvae　with　embryos）　embryonic　period；　for　the　developmental　changes　are　more　numerous　and　take　place　with　greater　rapidity　at　the　beginning　of　life　than　in　its　later　periods。　　As　each　of　these　stages　is　equal　in　value；　for　our　present　purpose；　to　the　adult　phase；　it　clearly　follows　that　if　there　is　anything　in　the　view　that　the　anatomical　study　of　organisms　is　of　importance　in　determining　their　mutual　relations；　the　study　of　the　organism　in　its　various　embryonic　（and　larval）　stages　must　have　a　greater　importance　than　the　study　of　the　single　and　arbitrarily　selected　stage　of　life　called　the　adult。

But　a　deeper　reason　than　this　has　been　assigned　for　the　importance　of　embryology　in　classification。　　It　has　been　asserted；　and　is　implied　by　Darwin　in　the　passage　quoted；　that　the　ancestral　history　is　repeated　in　a　condensed　form　in　the　embryonic；　and　that　a　study　of　the　latter　enables　us　to　form　a　picture　of　the　stages　of　structure　through　which　the　organism　has　passed　in　its　evolution。　　It　enables　us　on　this　view　to　reconstruct　the　pedigrees　of　animals　and　so　to　form　a　genealogical　tree　which　shall　be　the　true　expression　of　their　natural　relations。

The　real　question　which　we　have　to　consider　is　to　what　extent　the　embryological　studies　of　the　last　50　years　have　confirmed　or　rendered　probable　this　〃theory　of　recapitulation。〃　　In　the　first　place　it　must　be　noted　that　the　recapitulation　theory　is　itself　a　deduction　from　the　theory　of　evolution。　　The　facts　of　embryology；　particularly　of　vertebrate　embryology；　and　of　larval　history　receive；　it　is　argued；　an　explanation　on　the　view　that　the　successive　stages　of　development　are；　on　the　whole；　records　of　adult　stages　of　structure　which　the　species　has　passed　through　in　its　evolution。　　Whether　this　statement　will　bear　a　critical　verbal　examination　I　will　not　now　pause　to　inquire；　for　it　is　more　important　to　determine　whether　any　independent　facts　can　be　alleged　in　favour　of　the　theory。　　If　it　could　be　shown；　as　was　stated　to　be　the　case　by　L。　Agassiz；　that　ancient　and　extinct　forms　of　life　present　features　of　structure　now　only　found　in　embryos；　we　should　have　a　body　of　facts　of　the　greatest　importance　in　the　present　discussion。　　But　as　Huxley　（See　Huxley's　〃Scientific　Memoirs〃；　London；　1898；　Vol。　I。　page　303：　　〃There　is　no　real　parallel　between　the　successive　forms　assumed　in　the　development　of　the　life　of　the　individual　at　present；　and　those　which　have　appeared　at　different　epochs　in　the　past。〃　　See　also　his　Address　to　the　Geological　Society　of　London　（1862）　'On　the　Palaeontological　Evidence　of　Evolution'；　ibid。　Vol。　II。　page　512。）　has　shown　and　as　the　whole　course　of　palaeontological　and　embryological　investigation　has　demonstrated；　no　such　statement　can　be　made。　　The　extinct　forms　of　life　are　very　similar　to　those　now　existing　and　there　is　nothing　specially　embryonic　about　them。　　So　that　the　facts；　as　we　know　them；　lend　no　support　to　theory。

But　there　is　another　class　of　facts　which　have　been　alleged　in　favour　of　the　theory；　viz。　the　facts　which　have　been　included　in　the　generalisation　known　as　the　Law　of　v。　Baer。　　The　law　asserts　that　embryos　of　different　species　of　animals　of　the　same　group　are　more　alike　than　the　adults　and　that；　the　younger　the　embryo；　the　greater　are　the　resemblances。　　If　this　law　could　be　established　it　would　undoubtedly　be　a　strong　argument　in　favour　of　the　〃recapitulation〃　explanation　of　the　facts　of　embryology。　　But　its　truth　has　been　seriously　disputed。　　If　it　were　true　we　should　expect　to　find　that　the　embryos　of　closely　similar　species　would　be　indistinguishable　from　one　another；　but　this　is　notoriously　not　the　case。　　It　is　more　difficult　to　meet　the　assertion　when　it　is　made　in　the　form　given　above；　for　here　we　are　dealing　with　matters　of　opinion。　　For　instance；　no　one　would　deny　that　the　embryo　of　a　dogfish　is　different　from　the　embryo　of　a　rabbit；　but　there　is　room　for　difference　of　opinion　when　it　is　asserted　that　the　difference　is　less　than　the　difference　between　an　adult　dogfish　and　an　adult　rabbit。　　It　would　be　perfectly　true　to　say　that　the　differences　between　the　embryos　concern　other　organs　more　than　do　the　differences　between　the　adults；　but　who　is　prepared　to　affirm　that　the　presence　of　a　cephalic　coelom　and　of　cranial　segments；　of　external　gills；　of　six　gill　slits；　of　the　kidney　tubes　opening　into　the　muscle…plate　coelom；　of　an　enormous　yolk…sac；　of　a　neurenteric　canal；　and　the　absence　of　any　trace　of　an　amnion；　of　an　allantois　and　of　a　primitive　streak　are　not　morphological　facts　of　as　high　an　import　as　those　implied　by　the　differences　between　the　adults？　　The　generalisation　undoubtedly　had　its　origin　in　the　fact　that　there　is　what　may　be　called　a　family　resemblance　between　embryos　and　larvae；　but　this　resemblance；　which　is　by　no　means　exact；　is　largely　superficial　and　does　not　extend　to　anatomical　detail。

It　is　useless　to　say；　as　Weismann　has　stated　（〃The　Evolution　Theory〃；　by　A。　Weismann；　English　Translation；　Vol。　II。　page　176；　London；　1904。）；　that　〃it　cannot　be　disputed　that　the　rudiments　＇vestiges　his　translator　means＇　of　gill…arches　and　gill…clefts；　which　are　peculiar　to　one　stage　of　human　ontogeny；　give　us　every　ground　for　concluding　that　we　possessed　fish…like　ancestors。〃　　The　question　at　issue　is：　　did　the　pharyngeal　arches　and　clefts　of　mammalian　embryos　ever　discharge　a　branchial　function　in　an　adult　ancestor　of　the　mammalia？　　We　cannot　therefore；　without　begging　the　question　at　issue　in　the　grossest　manner；　apply　to　them　the　terms　〃gill…　arches〃　and　〃gill…clefts〃。　　That　they　are　homologous　with　the　〃gill…arches〃　and　〃gill…clefts〃　of　fishes　is　true；　but　there　is　no　evidence　to　show　that　they　ever　discharged　a　branchial　function。　　Until　such　evidence　is　forthcoming；　it　is　beside　the　point　to　say　that　it　〃cannot　be　disputed〃　that　they　are　evidence　of　a　piscine　ancestry。

It　must；　therefore；　be　admitted　that　one　outcome　of　the　progress　of　embryological　and　palaeontological　research　for　the　last　50　years　is　negative。　　The　recapitulation　theory　originated　as　a　deduction　from　the　evolution　theory　and　as　a　deduction　it　still　remains。

Let　us　before　leaving　the　subject　apply　another　test。　　If　the　evolution　theory　and　the　recapitulation　theory　are　both　true；　how　is　it　that　living　birds　are　not　only　without　teeth　but　have　no　rudiments　of　teeth　at　any　stage　of　their　existence？　　How　is　it　that　the　missing　digits　in　birds　and　mammals；　the　missing　or　reduced　limb　of　snakes　and　whales；　the　reduced　mandibulo…hyoid　cleft　of　elasmobranch　fishes　are　not　present　or　relatively　more　highly　developed　in　the　embryo　than　in　the　adult？　　How　is　it　that　when　a　marked　variation；　such　as　an　extra　digit；　or　a　reduced　limb；　or　an　extra　segment；　makes　its　appearance；　it　is　not　confined　to　the　adult　but　can　be　seen　all　through　the　development？　　All　the　clear　evidence　we　can　get　tends　to　show　that　marked　variations；　whet