darwin and modern science-第47章

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　　This；　indeed；　may　have　occurred；　as　is　suggested　below；　but　the　evidence　we　have　seems　to　indicate　that　evolutionary　modification　has　proceeded　by　ALTERING　and　not　by　SUPERSEDING：　　that　is　to　say　that　each　stage　in　the　life…history；　as　we　see　it　to…day；　has　proceeded　from　a　corresponding　stage　in　a　former　era　by　the　modification　of　that　stage　and　not　by　the　creation　of　a　new　one。　　Let　me；　at　the　risk　of　repetition；　explain　my　meaning　more　fully　by　taking　a　concrete　illustration。　　The　mandibulo…hyoid　cleft　（spiracle）　of　the　elasmobranch　fishes；　the　lateral　digits　of　the　pig's　foot；　the　hind…limbs　of　whales；　the　enlarged　digit　of　the　ostrich's　foot　are　supposed　to　be　organs　which　have　been　recently　modified。　　This　modification　is　not　confined　to　the　final　adult　stage　of　the　life…history　but　characterises　them　throughout　the　whole　of　their　development。　　A　stage　with　a　reduced　spiracle　does　not　proceed　in　development　from　a　preceding　stage　in　which　the　spiracle　shows　no　reduction：　　it　is　reduced　at　its　first　appearance。　　The　same　statement　may　be　made　of　organs　which　have　entirely　disappeared　in　the　adult；　such　as　bird's　teeth　and　snake's　fore…limbs：　　the　adult　stage　in　which　they　have　disappeared　is　not　preceded　by　embryonic　stages　in　which　the　teeth　and　limbs　or　rudiments　of　them　are　present。　　In　fact　the　evidence　indicates　that　adult　variations　of　any　part　are　accompanied　by　precedent　variations　in　the　same　direction　in　the　embryo。　　The　evidence　seems　to　show；　not　that　a　stage　is　added　on　at　the　end　of　the　life…history；　but　only　that　some　of　the　stages　in　the　life…history　are　modified。　　Indeed；　on　the　wider　view　of　development　taken　in　this　essay；　a　view　which　makes　it　coincident　with　life；　one　would　not　expect　often　to　find；　even　if　new　stages　are　added　in　the　course　of　evolution；　that　they　are　added　at　the　end　of　the　series　when　the　organism　has　passed　through　its　reproductive　period。　　It　is　possible　of　course　that　new　stages　have　been　intercalated　in　the　course　of　the　life…　history；　though　it　is　difficult　to　see　how　this　has　occurred。　　It　is　much　more　likely；　if　we　may　judge　from　available　evidence；　that　every　stage　has　had　its　counterpart　in　the　ancestral　form　from　which　it　has　been　derived　by　descent　with　modification。　　Just　as　the　adult　phase　of　the　living　form　differs；　owing　to　evolutionary　modification；　from　the　adult　phase　of　the　ancestor　from　which　it　has　proceeded；　so　each　larval　phase　will　differ　for　the　same　reason　from　the　corresponding　larval　phase　in　the　life…history　of　the　ancestor。　　Inasmuch　as　the　organism　is　variable　at　every　stage　of　its　independent　existence　and　is　exposed　to　the　action　of　natural　selection　there　is　no　reason　why　it　should　escape　modification　at　any　stage。

If　there　is　any　truth　in　these　considerations　it　would　seem　to　follow　that　at　the　dawn　of　life　the　life…cycle　must　have　been；　either　in　posse　or　in　esse；　at　least　as　long　as　it　is　at　the　present　time；　and　that　the　peculiarity　of　passing　through　a　series　of　stages　in　which　new　characters　are　successively　evolved　is　a　primordial　quality　of　living　matter。

Before　leaving　this　part　of　the　subject；　it　is　necessary　to　touch　upon　another　aspect　of　it。　　What　are　these　variations　in　structure　which　succeed　one　another　in　the　life…history　of　an　organism？　　I　am　conscious　that　I　am　here　on　the　threshold　of　a　chamber　which　contains　the　clue　to　some　of　our　difficulties；　and　that　I　cannot　enter　it。　　Looked　at　from　one　point　of　view　they　belong　to　the　class　of　genetic　variations；　which　depend　upon　the　structure　or　constitution　of　the　protoplasm；　but　instead　of　appearing　in　different　zygotes　（A　zygote　is　a　fertilised　ovum；　i。e。　a　new　organism　resulting　from　the　fusion　of　an　ovum　and　a　spermatozoon。）；　they　are　present　in　the　same　zygote　though　at　different　times　in　its　life…history。　　They　are　of　the　same　order　as　the　mutational　variations　of　the　modern　biologist　upon　which　the　appearance　of　a　new　character　depends。　　What　is　a　genetic　or　mutational　variation？　　It　is　a　genetic　character　which　was　not　present　in　either　of　the　parents。　　But　these　〃growth　variations〃　were　present　in　the　parents；　and　in　this　they　differ　from　mutational　variations。　　But　what　are　genetic　characters？　　They　are　characters　which　must　appear　if　any　development　occurs。　　They　are　usually　contrasted　with　〃acquired　characters；〃　using　the　expression　〃acquired　character〃　in　the　Lamarckian　sense。　　But　strictly　speaking　they　ARE　acquired　characters；　for　the　zygote　at　first　has　none　of　the　characters　which　it　subsequently　acquires；　but　only　the　power　of　acquiring　them　in　response　to　the　action　of　the　environment。　　But　the　characters　so　acquired　are　not　what　we　technically　understand　and　what　Lamarck　meant　by　〃acquired　characters。〃　　They　are　genetic　characters；　as　defined　above。　　What　then　are　Lamarck's　〃acquired　characters〃？　　They　are　variations　in　genetic　characters　caused　in　a　particular　way。　　There　are；　in　fact；　two　kinds　of　variation　in　genetic　characters　depending　on　the　mode　of　causation。　　Firstly；　there　are　those　variations　consequent　upon　a　variation　in　the　constitution　of　the　protoplasm　of　a　particular　zygote；　and　independent　of　the　environment　in　which　the　organism　develops；　save　in　so　far　as　this　simply　calls　them　forth：　　these　are　the　so…called　genetic　or　mutational　variations。　　Secondly；　there　are　those　variations　which　occur　in　zygotes　of　similar　germinal　constitution　and　which　are　caused　solely　by　differences　in　the　environment　to　which　the　individuals　are　respectively　exposed：　　these　are　the　〃acquired　characters〃　of　Lamarck　and　of　authors　generally。　　In　consequence　of　this　double　sense　in　which　the　term　〃acquired　characters〃　may　be　used；　great　confusion　may　and　does　occur。　　If　the　protoplasm　be　compared　to　a　machine；　and　the　external　conditions　to　the　hand　that　works　the　machine；　then　it　may　be　said　that；　as　the　machine　can　only　work　in　one　way；　it　can　only　produce　one　kind　of　result　（genetic　character）；　but　the　particular　form　or　quality　（Lamarckian　〃acquired　character〃）　of　the　result　will　depend　upon　the　hand　that　works　the　machine　（environment）；　just　as　the　quality　of　the　sound　produced　by　a　fiddle　depends　entirely　upon　the　hand　which　plays　upon　it。　　It　would　be　improper　to　apply　the　term　〃mutation〃　to　those　genetic　characters　which　are　not　new　characters　or　new　variants　of　old　characters；　but　such　genetic　characters　are　of　the　same　nature　as　those　characters　to　which　the　term　mutation　has　been　applied。　　It　may　be　noticed　in　passing　that　it　is　very　questionable　if　the　modern　biologist　has　acted　in　the　real　interests　of　science　in　applying　the　term　mutation　in　the　sense　in　which　he　has　applied　it。　　The　genetic　characters　of　organisms　come　from　one　of　two　sources：　　either　they　are　old　characters　and　are　due　to　the　action　of　what　we　call　inheritance　or　they　are　new　and　are　due　to　what　we　call　variation。　　If　the　term　mutation　is　applied　to　the　actual　alteration　of　the　machinery　of　the　protoplasm；　no　objection　can　be　felt　to　its　use；　but　if　it　be　applied；　as　it　is；　to　the　product　of　the　action　of　the　altered　machine；　viz。　to　the　new　genetic　character；　it　leads　to　confusion。　　Inheritance　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　o