darwin and modern science-第46章

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ch　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；　whether　of　reduction　or　increase；　of　organs　are　manifest　during　the　whole　of　the　development　of　the　organ　and　do　not　merely　affect　the　adult。　　And　on　reflection　we　see　that　it　could　hardly　be　otherwise。　　All　such　evidence　is　distinctly　at　variance　with　the　theory　of　recapitulation；　at　least　as　applied　to　embryos。　　In　the　case　of　larvae　of　course　the　case　will　be　different；　for　in　them　the　organs　are　functional；　and　reduction　in　the　adult　will　not　be　accompanied　by　reduction　in　the　larva　unless　a　change　in　the　conditions　of　life　of　the　larva　enables　it　to　occur。

If　after　50　years　of　research　and　close　examination　of　the　facts　of　embryology　the　recapitulation　theory　is　still　without　satisfactory　proof；　it　seems　desirable　to　take　a　wider　sweep　and　to　inquire　whether　the　facts　of　embryology　cannot　be　included　in　a　larger　category。

As　has　been　pointed　out　by　Huxley；　development　and　life　are　co…extensive；　and　it　is　impossible　to　point　to　any　period　in　the　life　of　an　organism　when　the　developmental　changes　cease。　　It　is　true　that　these　changes　take　place　more　rapidly　at　the　commencement　of　life；　but　they　are　never　wholly　absent；　and　those　which　occur　in　the　later　or　so…called　adult　stages　of　life　do　not　differ　in　their　essence；　however　much　they　may　differ　in　their　degree；　from　those　which　occur　during　the　embryonic　and　larval　periods。　　This　consideration　at　once　brings　the　changes　of　the　embryonic　period　into　the　same　category　as　those　of　the　adult　and　suggests　that　an　explanation　which　will　account　for　the　one　will　account　for　the　other。　　What　then　is　the　problem　we　are　dealing　with？　　Surely　it　is　this：　　Why　does　an　organism　as　soon　as　it　is　established　at　the　fertilisation　of　the　ovum　enter　upon　a　cycle　of　transformations　which　never　cease　until　death　puts　an　end　to　them？　In　other　words　what　is　the　meaning　of　that　cycle　of　changes　which　all　organisms　present　in　a　greater　or　less　degree　and　which　constitute　the　very　essence　of　life？　　It　is　impossible　to　give　an　answer　to　this　question　so　long　as　we　remain　within　the　precincts　of　Biologyand　it　is　not　my　present　purpose　to　penetrate　beyond　those　precincts　into　the　realms　of　philosophy。　We　have　to　do　with　an　ultimate　biological　fact；　with　a　fundamental　property　of　living　matter；　which　governs　and　includes　all　its　other　properties。　　How　may　this　property　be　stated？　　Thus：　　it　is　a　property　of　living　matter　to　react　in　a　remarkable　way　to　external　forces　without　undergoing　destruction。　　The　life…cycle；　of　which　the　embryonic　and　larval　periods　are　a　part；　consists　of　the　orderly　interaction　between　the　organism　and　its　environment。　　The　action　of　the　environment　produces　certain　morphological　changes　in　the　organism。　　These　changes　enable　the　organism　to　come　into　relation　with　new　external　forces；　to　move　into　what　is　practically　a　new　environment；　which　in　its　turn　produces　further　structural　changes　in　the　organism。　　These　in　their　turn　enable；　indeed　necessitate；　the　organism　to　move　again　into　a　new　environment；　and　so　the　process　continues　until　the　structural　changes　are　of　such　a　nature　that　the　organism　is　unable　to　adapt　itself　to　the　environment　in　which　it　finds　itself。　　The　essential　condition　of　success　in　this　process　is　that　the　organism　should　always　shift　into　the　environment　to　which　its　new　structure　is　suitedany　failure　in　this　leading　to　the　impairment　of　the　organism。　　In　most　cases　the　shifting　of　the　environment　is　a　very　gradual　process　（whether　consisting　in　the　very　slight　and　gradual　alteration　in　the　relation　of　the　embryo　as　a　whole　to　the　egg…shell　or　uterine　wall；　or　in　the　relations　of　its　parts　to　each　other；　or　in　the　successive　phases　of　adult　life）；　and　the　morphological　changes　in　connection　with　each　step　of　it　are　but　slight。　　But　in　some　cases　jumps　are　made　such　as　we　find　in　the　phenomena　known　as　hatching；　birth；　and　metamorphosis。

This　property　of　reacting　to　the　environment　without　undergoing　destruction　is；　as　has　been　stated；　a　fundamental　property　of　organisms。　　It　is　impossible　to　conceive　of　any　matter；　to　which　the　term　living　could　be　applied；　being　without　it。　　And　with　this　property　of　reacting　to　the　environment　goes　the　further　property　of　undergoing　a　change　which　alters　the　relation　of　the　organism　to　the　old　environment　and　places　it　in　a　new　environment。　　If　this　reasoning　is　correct；　it　necessarily　follows　that　this　property　must　have　been　possessed　by　living　matter　at　its　first　appearance　on　the　earth。　　In　other　words　living　matter　must　always　have　presented　a　life…cycle；　and　the　question　arises　what　kind　of　modification　has　that　cycle　undergone？　　Has　it　increased　or　diminished　in　duration　and　complexity　since　organisms　first　appeared　on　the　earth？　　The　current　view　is　that　the　cycle　was　at　first　very　short　and　that　it　has　increased　in　length　by　the　evolutionary　creation　of　new　adult　phases；　that　these　new　phases　are　in　addition　to　those　already　existing　and　that　each　of　them　as　it　appears　takes　over　from　the　preceding　adult　phase　the　functional　condition　of　the　reproductive　organs。　　According　to　the　same　view　the　old　adult　phases　are　not　obliterated　but　persist　in　a　more　or　less　modified　form　as　larval　stages。　　It　is　further　supposed　that　as　the　life…history　lengthens　at　one　end　by　the　addition　of　new　adult　phases；　it　is　shortened　at　the　other　by　the　abbreviation　of　embryonic　development　and　by　the　absorption　of　some　of　the　early　larval　stages　into　the　embryonic　period；　but　on　the　whole　the　lengthening　process　has　exceeded　that　of　shortening；　so　that　the　whole　life…history　has；　with　the　progress　of　evolution；　become　longer　and　more　complicated。

Now　there　can　be　no　doubt　that　the　life…history　of　organisms　has　been　shortened　in　the　way　above　suggested；　for　cases　are　known　in　which　this　can　practically　be　seen　to　occur　at　the　present　day。　　But　the　process　of　lengthening　by　the　creation　of　new　stages　at　the　other　end　of　the　life…　cycle　is　more　difficult　to　conceive　and　moreover　there　is　no　evidence　for　its　having　occurred。　　This；　indeed；　may　have　occurred；　as　is　suggested　below；　but　the　evidence　we　have　seems　to　indicate