darwin and modern science-第55节

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ther　by　the　theory　of　types；　which　owed　its　origin　to　the　great　anatomists　of　the　preceding　generation；　to　Cuvier；　L。　Agassiz；　J。　Muller；　and　R。　Owen；　they　were　now　combined　together　into　one　organic　whole　by　the　theory　of　descent　and　by　the　hypothesis　of　recapitulation　which　was　deduced　from　that　theory。　　The　view　（First　clearly　enunciated　by　Fritz　Muller　in　his　well…known　work；　〃Fur　Darwin〃；　Leipzig；　1864；　（English　Edition；　〃Facts　for　Darwin〃；　1869）。）　that　a　knowledge　of　embryonic　and　larval　histories　would　lay　bare　the　secrets　of　race…history　and　enable　the　course　of　evolution　to　be　traced；　and　so　lead　to　the　discovery　of　the　natural　system　of　classification；　gave　a　powerful　stimulus　to　morphological　study　in　general　and　to　embryological　investigation　in　particular。　　In　Darwin's　words：　　〃Embryology　rises　greatly　in　interest；　when　we　look　at　the　embryo　as　a　picture；　more　or　less　obscured；　of　the　progenitor；　either　in　its　adult　or　larval　state；　of　all　the　members　of　the　same　great　class。〃　　（〃Origin〃　（6th　edition）；　page　396。）　　In　the　period　under　consideration　the　output　of　embryological　work　has　been　enormous。　　No　group　of　the　animal　kingdom　has　escaped　exhaustive　examination　and　no　effort　has　been　spared　to　obtain　the　embryos　of　isolated　and　out　of　the　way　forms；　the　development　of　which　might　have　an　important　bearing　upon　questions　of　phylogeny　and　classification。　　Marine　zoological　stations　have　been　established；　expeditions　have　been　sent　to　distant　countries；　and　the　methods　of　investigation　have　been　greatly　improved。　　The　result　of　this　activity　has　been　that　the　main　features　of　the　developmental　history　of　all　the　most　important　animals　are　now　known　and　the　curiosity　as　to　developmental　processes；　so　greatly　excited　by　the　promulgation　of　the　Darwinian　theory；　has　to　a　considerable　extent　been　satisfied。

To　what　extent　have　the　results　of　this　vast　activity　fulfilled　the　expectations　of　the　workers　who　have　achieved　them？　　The　Darwin　centenary　is　a　fitting　moment　at　which　to　take　stock　of　our　position。　　In　this　inquiry　we　shall　leave　out　of　consideration　the　immense　and　intensely　interesting　additions　to　our　knowledge　of　Natural　History。　　These　may　be　said　to　constitute　a　capital　fund　upon　which　philosophers；　poets　and　men　of　science　will　draw　for　many　generations。　　The　interest　of　Natural　History　existed　long　before　Darwinian　evolution　was　thought　of　and　will　endure　without　any　reference　to　philosophic　speculations。　　She　is　a　mistress　in　whose　face　are　beauties　and　in　whose　arms　are　delights　elsewhere　unattainable。　　She　is　and　always　has　been　pursued　for　her　own　sake　without　any　reference　to　philosophy；　science；　or　utility。

Darwin's　own　views　of　the　bearing　of　the　facts　of　embryology　upon　questions　of　wide　scientific　interest　are　perfectly　clear。　　He　writes　（〃Origin〃　（6th　edition）；　page　395。）：

〃On　the　other　hand　it　is　highly　probable　that　with　many　animals　the　embryonic　or　larval　stages　show　us；　more　or　less　completely；　the　condition　of　the　progenitor　of　the　whole　group　in　its　adult　state。　　In　the　great　class　of　the　Crustacea；　forms　wonderfully　distinct　from　each　other；　namely；　suctorial　parasites；　cirripedes；　entomostraca；　and　even　the　malacostraca；　appear　at　first　as　larvae　under　the　nauplius…form；　and　as　these　larvae　live　and　feed　in　the　open　sea；　and　are　not　adapted　for　any　peculiar　habits　of　life；　and　from　other　reasons　assigned　by　Fritz　Muller；　it　is　probable　that　at　some　very　remote　period　an　independent　adult　animal；　resembling　the　Nauplius；　existed；　and　subsequently　produced；　along　several　divergent　lines　of　descent；　the　above…named　great　Crustacean　groups。　　So　again　it　is　probable；　from　what　we　know　of　the　embryos　of　mammals；　birds；　fishes；　and　reptiles；　that　these　animals　are　the　modified　descendants　of　some　ancient　progenitor；　which　was　furnished　in　its　adult　state　with　branchiae；　a　swim…　bladder；　four　fin…like　limbs；　and　a　long　tail；　all　fitted　for　an　aquatic　life。

〃As　all　the　organic　beings；　extinct　and　recent；　which　have　ever　lived；　can　be　arranged　within　a　few　great　classes；　and　as　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　ver