darwin and modern science-第51章

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nts；　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　ammonoid　complexity　of　sutures；　and　in　the　Permian　their　number　and　variety　cause　them　to　form　a　striking　element　of　the　marine　faunas。　　It　is　in　the　Mesozoic　era；　however；　that　these　shells　attain　their　full　development；　increasing　enormously　in　the　Triassic；　they　culminate　in　the　Jurassic　in　the　number　of　families；　genera　and　species；　in　the　complexity　of　the　sutures；　and　in　the　variety　of　shell…ornamentation。　　A　slow　decline　begins　in　the　Cretaceous；　ending　in　the　complete　extinction　of　the　whole　group　at　the　end　of　that　period。　　As　a　final　phase　in　the　history　of　the　ammonites；　there　appear　many　so…called　〃abnormal〃　genera；　in　which　the　shell　is　irregularly　coiled；　or　more　or　less　uncoiled；　in　some　forms　becoming　actually　straight。　　It　is　interesting　to　observe　that　some　of　these　genera　are　not　natural　groups；　but　are　〃polyphyletic；〃　i。e。　are　each　derived　from　several　distinct　ancestral　genera；　which　have　undergone　a　similar　kind　of　degeneration。

In　the　huge　assembly　of　ammonites　it　is　not　yet　possible　to　arrange　all　the　forms　in　a　truly　natural　classification；　which　shall　express　the　various　interrelations　of　the　genera；　yet　several　beautiful　series　have　already　been　determined。　　In　these　series　the　individual　development　of　the　later　general　shows　transitory　stages　which　are　permanent　in　antecedent　genera。　　To　give　a　mere　catalogue　of　names　without　figures　would　not　make　these　series　more　intelligible。

The　Brachiopoda；　or　〃lamp…shells；〃　are　a　phylum　of　which　comparatively　few　survive　to　the　present　day；　their　shells　have　a　superficial　likeness　to　those　of　the　bivalved　Mollusca；　but　are　not　homologous　with　the　latter；　and　the　phylum　is　really　very　distinct　from　the　molluscs。　　While　greatly　reduced　now；　these　animals　were　incredibly　abundant　throughout　the　Palaeozoic　era；　great　masses　of　limestone　being　often　composed　almost　exclusively　of　their　shells；　and　their　variety　is　in　keeping　with　their　individual　abundance。　　As　in　the　case　of　the　ammonites；　the　problem　is　to　arrange　this　great　multitude　of　forms　in　an　orderly　array　that　shall　express　the　ramifications　of　the　group　according　to　a　genetic　system。　　For　many　brachiopods；　both　recent　and　fossil；　the　individual　development；　or　ontogeny；　has　been　worked　out　and　has　proved　to　be　of　great　assistance　in　the　problems　of　classification　and　phylogeny。　　Already　very　encouraging　progress　has　been　made　in　the　solution　of　these　problems。　　All　brachiopods　form　first　a　tiny；　embryonic　shell；　called　the　protegulum；　which　is　believed　to　represent　the　ancestral　form　of　the　whole　group；　and　in　the　more　advanced　genera　the　developmental　stages　clearly　indicate　the　ancestral　genera　of　the　series；　the　succession　of　adult　forms　in　time　corresponding　to　the　order　of　the　ontogenetic　stages。　　The　transformation　of　the　delicate　calcareous　supports　of　the　arms；　often　exquisitely　preserved；　are　extremely　interesting。　　Many　of　the　Palaeozoic　genera　had　these　supports　coiled　like　a　pair　of　spiral　springs；　and　it　has　been　shown　that　these　genera　were　derived　from　types　in　which　the　supports　were　simply　shelly　loops。

The　long　extinct　class　of　crustacea　known　as　the　Trilobites　are　li