darwin and modern science-第62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！



Recent　discoveries　in　Egypt　have　thrown　much　light　upon　a　problem　which　long　baffled　the　palaeontologist；　namely；　the　origin　of　the　elephants。　　（C。W。　Andrews；　〃On　the　Evolution　of　the　Proboscidea〃；　〃Phil。　Trans。　Roy。　Soc。〃　London；　Vol。　196；　1904；　page　99。）　　Early　representatives　of　this　order；　Mastodons；　had　appeared　almost　simultaneously　（in　the　geological　sense　of　that　word）　in　the　upper　Miocene　of　Europe　and　North　America；　but　in　neither　continent　was　any　more　ancient　type　known　which　could　plausibly　be　regarded　as　ancestral　to　them。　　Evidently；　these　problematical　animals　had　reached　the　northern　continents　by　migrating　from　some　other　region；　but　no　one　could　say　where　that　region　lay。　　The　Eocene　and　Oligocene　beds　of　the　Fayoum　show　us　that　the　region　sought　for　is　Africa；　and　that　the　elephants　form　just　such　a　series　of　gradual　modifications　as　we　have　found　among　other　hoofed　animals。　　The　later　steps　of　the　transformation；　by　which　the　mastodons　lost　their　lower　tusks；　and　their　relatively　small　and　simple　grinding　teeth　acquired　the　great　size　and　highly　complex　structure　of　the　true　elephants；　may　be　followed　in　the　uppermost　Miocene　and　Pliocene　fossils　of　India　and　southern　Europe。

Egypt　has　also　of　late　furnished　some　very　welcome　material　which　contributes　to　the　solution　of　another　unsolved　problem　which　had　quite　eluded　research；　the　origin　of　the　whales。　　The　toothed…whales　may　be　traced　back　in　several　more　or　less　parallel　lines　as　far　as　the　lower　Miocene；　but　their　predecessors　in　the　Oligocene　are　still　so　incompletely　known　that　safe　conclusions　can　hardly　be　drawn　from　them。　　In　the　middle　Eocene　of　Egypt；　however；　has　been　found　a　small；　whale…like　animal　（Protocetus）；　which　shows　what　the　ancestral　toothed…whale　was　like；　and　at　the　same　time　seems　to　connect　these　thoroughly　marine　mammals　with　land…　animals。　　Though　already　entirely　adapted　to　an　aquatic　mode　of　life；　the　teeth；　skull　and　backbone　of　Protocetus　display　so　many　differences　from　those　of　the　later　whales　and　so　many　approximations　to　those　of　primitive；　carnivorous　land…mammals；　as；　in　a　large　degree；　to　bridge　over　the　gap　between　the　two　groups。　　Thus　one　of　the　most　puzzling　of　palaeontological　questions　is　in　a　fair　way　to　receive　a　satisfactory　answer。　　The　origin　of　the　whalebone…whales　and　their　relations　to　the　toothed…whales　cannot　yet　be　determined；　since　the　necessary　fossils　have　not　been　discovered。

Among　the　carnivorous　mammals；　phylogenetic　series　are　not　so　clear　and　distinct　as　among　the　hoofed　animals；　chiefly　because　the　carnivores　are　individually　much　less　abundant；　and　well…preserved　skeletons　are　among　the　prizes　of　the　collector。　　Nevertheless；　much　has　already　been　learned　concerning　the　mutual　relations　of　the　carnivorous　families；　and　several　phylogenetic　series；　notably　that　of　the　dogs；　are　quite　complete。　　It　has　been　made　extremely　probable　that　the　primitive　dogs　of　the　Eocene　represent　the　central　stock；　from　which　nearly　or　quite　all　the　other　families　branched　off；　though　the　origin　and　descent　of　the　cats　have　not　yet　been　determined。

It　should　be　clearly　understood　that　the　foregoing　account　of　mammalian　descent　is　merely　a　selection　of　a　few　representative　cases　and　might　be　almost　indefinitely　extended。　　Nothing　has　been　said；　for　example；　of　the　wonderful　museum　of　ancient　mammalian　life　which　is　entombed　in　the　rocks　of　South　America；　especially　of　Patagonia；　and　which　opens　a　world　so　entirely　different　from　that　of　the　northern　continents；　yet　exemplifying　the　same　laws　of　〃descent　with　modification。〃　　Very　beautiful　phylogenetic　series　have　already　been　established　among　these　most　interesting　and　marvellously　preserved　fossils；　but　lack　of　space　forbids　a　consideration　of　them。

The　origin　of　the　mammalia；　as　a　class；　offers　a　problem　of　which　palaeontology　can　as　yet　present　no　definitive　solution。　　Many　morphologists　regard　the　early　amphibia　as　the　ancestral　group　from　which　the　mammals　were　derived；　while　most　palaeontologists　believe　that　the　mammals　are　descended　from　the　reptiles。　　The　most　ancient　known　mammals；　those　from　the　upper　Triassic　of　Europe　and　North　America；　are　so　extremely　rare　and　so　very　imperfectly　known；　that　they　give　little　help　in　determining　the　descent　of　the　class；　but；　on　the　other　hand；　certain　reptilian　orders　of　the　Permian　period；　especially　well　represented　in　South　Africa；　display　so　many　and　such　close　approximations　to　mammalian　structure；　as　strongly　to　suggest　a　genetic　relationship。　　It　is　difficult　to　believe　that　all　those　likenesses　should　have　been　independently　acquired　and　are　without　phylogenetic　significance。

Birds　are　comparatively　rare　as　fossils　and　we　should　therefore　look　in　vain　among　them　for　any　such　long　and　closely　knit　series　as　the　mammals　display　in　abundance。　　Nevertheless；　a　few　extremely　fortunate　discoveries　have　made　it　practically　certain　that　birds　are　descended　from　reptiles；　of　which　they　represent　a　highly　specialised　branch。　　The　most　ancient　representative　of　this　class　is　the　extraordinary　genus　Archaeopteryx　from　the　upper　Jurassic　of　Bavaria；　which；　though　an　unmistakable　bird；　retains　so　many　reptilian　structures　and　characteristics　as　to　make　its　derivation　plain。　　Not　to　linger　over　anatomical　minutiae；　it　may　suffice　to　mention　the　absence　of　a　horny　beak；　which　is　replaced　by　numerous　true　teeth；　and　the　long　lizard…like　tail；　which　is　made　up　of　numerous　distinct　vertebrae；　each　with　a　pair　of　quill…like　feathers　attached　to　it。　　Birds　with　teeth　are　also　found　in　the　Cretaceous；　though　in　most　other　respects　the　birds　of　that　period　had　attained　a　substantially　modern　structure。　　Concerning　the　interrelations　of　the　various　orders　and　families　of　birds；　palaeontology　has　as　yet　little　to　tell　us。

The　life　of　the　Mesozoic　era　was　characterised　by　an　astonishing　number　and　variety　of　reptiles；　which　were　adapted　to　every　mode　of　life；　and　dominated　the　air；　the　sea　and　the　land；　and　many　of　which　were　of　colossal　proportions。　　Owing　to　the　conditions　of　preservation　which　obtained　during　the　Mesozoic　period；　the　history　of　the　reptiles　is　a　broken　and　interrupted　one；　so　that　we　can　make　out　many　short　series；　rather　than　any　one　of　considerable　length。　　While　the　relations　of　several　reptilian　orders　can　be　satisfactorily　determined；　others　still　baffle　us　entirely；　making　their　first　known　appearance　in　a　fully　differentiated　state。　　We　can　trace　the　descent　of　the　sea…dragons；　the　Ichthyosaurs　and　Plesiosaurs；　from　terrestrial　ancestors；　but　the　most　ancient　turtles　yet　discovered　show　us　no　closer　approximation　to　any　other　order　than　do　the　recent　turtles；　and　the　oldest　known　Pterosaurs；　the　flying　dragons　of　the　Jurassic；　are　already　fully　differentiated。　　There　is；　however；　no　ground　for　discouragement　in　this；　for　the　progress　of　discovery　has　been　so　rapid　of　late　years；　and　our　knowledge　of　Mesozoic　life　has　increased　with　such　leaps　and　bounds；　that　there　is　every　reason　to　expect　a　solution　of　many　of　the　outstanding　problems　in　the　near　future。

Passing　over　the　lower　vertebrates；　for　lack　of　space　to　give　them　any　adequate　consideration；　we　may　briefly　take　up　the　record　of　invertebrate　life。　　From　the　overwhelming　mass　of　material　it　is　difficult　to　make　a　representative　selection　and　even　more　difficult　to　state　the　facts　intelligibly　without　the　use　of　unduly　technical　language　and　without　the　aid　of　illustrations。

Several　groups　of　the　Mollusca；　or　shell…fish；　yield　very　full　and　convincing　evidence　of　their　descent　from　earlier　and　simpler　forms；　and　of　these　none　is　of　greater　interest　than　the　Ammonites；　an　extinct　order　of　the　cephalopoda。　　The　nearest　living　ally　of　the　ammonites　is　the　pearly　nautilus；　the　other　existing　cephalopods；　such　as　the　squids；　cuttle…fish；　octopus；　etc。；　are　much　more　distantly　related。　　Like　the　nautilus；　the　ammonites　all　possess　a　coiled　and　chambered　shell；　but　their　especial　characteristic　is　the　complexity　of　the　〃sutures。〃　　By　sutures　is　meant　the　edges　of　the　transverse　partitions；　or　septa；　where　these　join　the　shell…　wall；　and　their　complexity　in　the　fully　developed　genera　is　extraordinary；　forming　patterns　like　the　most　elaborate　oak…leaf　embroidery；　while　in　the　nautiloids　the　sutures　form　simple　curves。　　In　the　rocks　of　the　Mesozoic　era；　wherever　conditions　of　preservation　are　favourable；　these　beautiful　shells　are　stored　in　countless　multitudes；　of　an　incredible　variety　of　form；　size　and　ornamentation；　as　is　shown　by　the　fact　that　nearly　5000　species　have　already　been　described。　　The　ammonites　are　particularly　well　adapted　for　phylogenetic　studies；　because；　by　removing　the　successive　whorls　of　the　coiled　shell；　the　individual　development　may　be　followed　back　in　inverse　order；　to　the　microscopic　〃protoconch；〃　or　embryonic　shell；　which　lies　concealed　in　the　middle　of　the　coil。　　Thus　the　valuable　aid　of　embryology　is　obtained　in　determining　relationships。

The　descent　of　the　ammonites；　taken　as　a　group；　is　simple　and　clear；　they　arose　as　a　branch　of　the　nautiloids　in　the　lower　Devonian；　the　shells　known　as　goniatites　having　zigzag；　angulated　sutures。　　Late　in　the　succeeding　Carboniferous　period　appear　shells　with　a　truly　ammonoi