darwin and modern science-第58节

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heritance　is　the　persistence　of　the　structure　of　the　machine；　characters　are　the　products　of　the　working　of　the　machine；　variation　in　genetic　characters　is　due　to　the　alteration　（mutation）　in　the　arrangement　of　the　machinery；　while　variation　in　acquired　characters　（Lamarckian）　is　due　to　differences　in　the　mode　of　working　the　machinery。　　The　machinery　when　it　starts　（in　the　new　zygote）　has　the　power　of　grinding　out　certain　results；　which　we　call　the　characters　of　the　organism。　　These　appear　at　successive　intervals　of　time；　and　the　orderly　manifestation　of　them　is　what　we　call　the　life…history　of　the　organism。　　This　brings　us　back　to　the　question　with　which　we　started　this　discussion；　viz。　what　is　the　relation　of　these　variations　in　structure；　which　successively　appear　in　an　organism　and　constitute　its　life…history；　to　the　mutational　variations　which　appear　in　different　organisms　of　the　same　brood　or　species。　　The　question　is　brought　home　to　us　when　we　ask　what　is　a　bud…sport；　such　as　a　nectarine　appearing　on　a　peach…tree？　　From　one　point　of　view；　it　is　simply　a　mutation　appearing　in　asexual　reproduction；　from　another　it　is　one　of　these　successional　characters　（〃growth　variations〃）　which　constitute　the　life…history　of　the　zygote；　for　it　appears　in　the　same　zygote　which　first　produces　a　peach。　　Here　our　analogy　of　a　machine　which　only　works　in　one　way　seems　to　fail　us；　for　these　bud…sports　do　not　appear　in　all　parts　of　the　organism；　only　in　certain　buds　or　parts　of　it；　so　that　one　part　of　the　zygotic　machine　would　appear　to　work　differently　to　another。　　To　discuss　this　question　further　would　take　us　too　far　from　our　subject。　　Suffice　it　to　say　that　we　cannot　answer　it；　any　more　than　we　can　this　further　question　of　burning　interest　at　the　present　day；　viz。　to　what　extent　and　in　what　manner　is　the　machine　itself　altered　by　the　particular　way　in　which　it　is　worked。　　In　connection　with　this　question　we　can　only　submit　one　consideration：　　the　zygotic　machine　can；　by　its　nature；　only　work　once；　so　that　any　alteration　in　it　can　only　be　ascertained　by　studying　the　replicas　of　it　which　are　produced　in　the　reproductive　organs。

It　is　a　peculiarity　that　the　result　which　we　call　the　ripening　of　the　generative　organs　nearly　always　appears　among　the　final　products　of　the　action　of　the　zygotic　machine。　　It　is　remarkable　that　this　should　be　the　case。　　What　is　the　reason　of　it？　　The　late　appearance　of　functional　reproductive　organs　is　almost　a　universal　law；　and　the　explanation　of　it　is　suggested　by　expressing　the　law　in　another　way；　viz。　that　the　machine　is　almost　always　so　constituted　that　it　ceases　to　work　efficiently　soon　after　the　reproductive　organs　have　sufficiently　discharged　their　function。　　Why　this　should　occur　we　cannot　explain：　　it　is　an　ultimate　fact　of　nature；　and　cannot　be　included　in　any　wider　category。　　The　period　during　which　the　reproductive　organs　can　act　may　be　short　as　in　ephemerids　or　long　as　in　man　and　trees；　and　there　is　no　reason　to　suppose　that　their　action　damages　the　vital　machinery；　though　sometimes；　as　in　the　case　of　annual　plants　（Metschnikoff）；　it　may　incidentally　do　so；　but；　long　or　short；　the　cessation　of　their　actions　is　always　a　prelude　to　the　end。　　When　they　and　their　action　are　impaired；　the　organism　ceases　to　react　with　precision　to　the　environment；　and　the　organism　as　a　whole　undergoes　retrogressive　changes。

It　has　been　pointed　out　above　that　there　is　reason　to　believe　that　at　the　dawn　of　life　the　life…cycle　was；　EITHER　IN　ESSE　OR　IN　POSSE；　at　least　as　long　as　it　is　at　the　present　time。　　The　qualification　implied　by　the　words　in　italics　is　necessary；　for　it　is　clearly　possible　that　the　external　conditions　then　existing　were　not　suitable　for　the　production　of　all　the　stages　of　the　potential　life…history；　and　that　what　we　call　organic　evolution　has　consisted　in　a　gradual　evolution　of　new　environments　to　which　the　organism's　innate　capacity　of　change　has　enabled　it　to　adapt　itself。　　We　have　warrant　for　this　possibility　in　the　case　of　the　Axolotl　and　in　other　similar　cases　of　neoteny。　　And　these　cases　further　bring　home　to　us　the　fact；　to　which　I　have　already　referred；　that　the　full　development　of　the　functional　reproductive　organs　is　nearly　always　associated　with　the　final　stages　of　the　life…history。

On　this　view　of　the　succession　of　characters　in　the　life…history　of　organisms；　how　shall　we　explain　the　undoubted　fact　that　the　development　of　buds　hardly　ever　presents　any　phenomena　corresponding　to　the　embryonic　and　larval　changes？　　The　reason　is　clearly　this；　that　budding　usually　occurs　after　the　embryonic　stage　is　past；　when　the　characters　of　embryonic　life　have　been　worked　out　by　the　machine。　　When　it　takes　place　at　an　early　stage　in　embryonic　life；　as　it　does　in　cases　of　so…called　embryonic　fission；　the　product　shows；　either　partly　or　entirely；　phenomena　similar　to　those　of　embryonic　development。　　The　only　case　known　to　me　in　which　budding　by　the　adult　is　accompanied　by　morphological　features　similar　to　those　displayed　by　embryos　is　furnished　by　the　budding　of　the　medusiform　spore…sacs　of　hydrozoon　polyps。　　But　this　case　is　exceptional；　for　here　we　have　to　do　with　an　attempt；　which　fails；　to　form　a　free…swimming　organism；　the　medusa；　and　the　vestiges　which　appear　in　the　buds　are　the　umbrella…cavity；　marginal　tentacles；　circular　canal；　etc。；　of　the　medusa　arrested　in　development。

But　the　question　still　remains；　are　there　no　cases　in　which；　as　implied　by　the　recapitulation　theory；　variations　in　any　organ　are　confined　to　the　period　in　which　the　organ　is　functional　and　do　not　affect　it　in　the　embryonic　stages？　　The　teeth　of　the　whalebone　whales　may　be　cited　as　a　case　in　which　this　is　said　to　occur；　but　here　the　teeth　are　only　imperfectly　developed　in　the　embryo　and　are　soon　absorbed。　　They　have　been　affected　by　the　change　which　has　produced　their　disappearance　in　the　adult；　but　not　to　complete　extinction。　　Nor　are　they　now　likely　to　be　extinguished；　for　having　become　exclusively　embryonic　they　are　largely　protected　from　the　action　of　natural　selection。　　This　consideration　brings　up　a　most　important　aspect　of　the　question；　so　far　as　disappearing　organs　are　concerned。　　Every　organ　is　laid　down　at　a　certain　period　in　the　embryo　and　undergoes　a　certain　course　of　growth　until　it　obtains　full　functional　development。　　When　for　any　cause　reduction　begins；　it　is　affected　at　all　stages　of　its　growth；　unless　it　has　functional　importance　in　the　larva；　and　in　some　cases　its　life　is　shortened　at　one　or　both　ends。　　In　cases；　as　in　that　of　the　whale's　teeth；　in　which　it　entirely　disappears　in　the　adult；　the　latter　part　of　its　life　is　cut　off；　in　others；　the　beginning　of　its　life　may　be　deferred。　　This　happens；　for　instance；　with　the　spiracle　of　many　Elasmobranchs；　which　makes　its　appearance　after　the　hyobranchial　cleft；　not　before　it　as　it　should　do；　being　anterior　to　it　in　position；　and　as　it　does　in　the　Amniota　in　which　it　shows　no　reduction　in　size　as　compared　with　the　other　pharyngeal　clefts。　　In　those　Elasmobranchs　in　which　it　is　absent　in　the　adult　but　present　in　the　embryo　（e。g。　Carcharias）　its　life　is　shortened　at　both　ends。　　Many　more　instances　of　organs；　of　which　the　beginning　and　end　have　been　cut　off；　might　be　mentioned；　e。g。　the　muscle…plate　coelom　of　Aves；　the　primitive　streak　and　the　neurenteric　canal　of　amniote　blastoderms。　　In　yet　other　cases　in　which　the　reduced　organ　is　almost　on　the　verge　of　disappearance；　it　may　appear　for　a　moment　and　disappear　more　than　once　in　the　course　of　development。　　As　an　instance　of　this　striking　phenomenon　I　may　mention　the　neurenteric　canal　of　avine　embryos；　and　the　anterior　neuropore　of　Ascidians。　　Lastly　the　reduced　organ　may　disappear　in　the　developing　stages　before　it　does　so　in　the　adult。　　As　an　instance　of　this　may　be　mentioned　the　mandibular　palp　of　those　Crustacea　with　zoaea　larvae。　　This　structure　disappears　in　the　larva　only　to　reappear　in　a　reduced　form　in　later　stages。　　In　all　these　cases　we　are　dealing　with　an　organ　which；　we　imagine；　attained　a　fuller　functional　development　at　some　previous　stage　in　race…history；　but　in　most　of　them　we　have　no　proof　that　it　did　so。　　It　may　be；　and　the　possibility　must　not　be　lost　sight　of；　that　these　organs　never　were　anything　else　than　functionless　and　that　though　they　have　been　got　rid　of　in　the　adult　by　elimination　in　the　course　of　time；　they　have　been　able　to　persist　in　embryonic　stages　which　are　protected　from　the　full　action　of　natural　selection。　　There　is　no　reason　to　suppose　that　living　matter　at　its　first　appearance　differed　from　non…living　matter　in　possessing　only　properties　conducive　to　its　well…being　and　prolonged　existence。　　No　one　thinks　that　the　properties　of　the　various　forms　of　inorganic　matter　are　all　strictly　related　to　external　conditions。　Of　what　use　to　the　diamond　is　its　high　specific　gravity　and　high　refrangibility；　and　to　gold　of　its　yellow　colour　and　great　weight？　　These　substances　continue　to　exist　in　virtue　of　other　properties　than　these。　　It　is　impossible　to　suppose　that　the　properties　of　living　matter　at　its　first　appearance　were　all　useful　to　it；　for　even　now　after　aeons　of　elimination　we　find　that　it　possesses　many　useless　organs　and　that　many　of　its　relations　to　the　external　world　are　capable　of　con