darwin and modern science-第57节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


undamental　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　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　wor