darwin and modern science-第63节

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noid　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　likewise　very　favourable　subjects　for　phylogenetic　studies。　　So　far　as　the　known　record　can　inform　us；　the　trilobites　are　exclusively　Palaeozoic　in　distribution；　but　their　course　must　have　begun　long　before　that　era；　as　is　shown　by　the　number　of　distinct　types　among　the　genera　of　the　lower　Cambrian。　　The　group　reached　the　acme　of　abundance　and　relative　importance　in　the　Cambrian　and　Ordovician；　then　followed　a　long；　slow　decline；　ending　in　complete　and　final　disappearance　before　the　end　of　the　Permian。　　The　newly…hatched　and　tiny　trilobite　larva；　known　as　the　protaspis；　is　very　near　to　the　primitive　larval　form　of　all　the　crustacea。　　By　the　aid　of　the　correlated　ontogenetic　stages　and　the　succession　of　the　adult　forms　in　the　rocks；　many　phylogenetic　series　have　been　established　and　a　basis　for　the　natural　arrangement　of　the　whole　class　has　been　laid。

Very　instructive　series　may　also　be　observed　among　the　Echinoderms　and；　what　is　very　rare；　we　are　able　in　this　sub…kingdom　to　demonstrate　the　derivation　of　one　class　from　another。　　Indeed；　there　is　much　reason　to　believe　that　the　extinct　class　Cystidea　of　the　Cambrian　is　the　ancestral　group；　from　which　all　the　other　Echinoderms；　star…fishes；　brittle…stars；　sea…urchins；　feather…stars；　etc。；　are　descended。

The　foregoing　sketch　of　the　palaeontological　record　is；　of　necessity；　extremely　meagre；　and　does　not　represent　even　an　outline　of　the　evidence；　but　merely　a　few　illustrative　examples；　selected　almost　at　random　from　an　immense　body　of　material。　　However；　it　will　perhaps　suffice　to　show　that　the　geological　record　is　not　so　hopelessly　incomplete　as　Darwin　believed　it　to　be。　　Since　〃The　Origin　of　Species〃　was　written；　our　knowledge　of　that　record　has　been　enormously　extended　and　we　now　possess；　no　complete　volumes；　it　is　true；　but　some　remarkably　full　and　illuminating　chapters。　　The　main　significance　of　the　whole　lies　in　the　fact；　that　JUST　IN　PROPORTION　TO　THE　COMPLETENESS　OF　THE　RECORD　IS　THE　UNEQUIVOCAL　CHARACTER　OF　ITS　TESTIMONY　TO　THE　TRUTH　OF　THE　EVOLUTIONARY　THEORY。

The　test　of　a　true；　as　distinguished　from　a　false；　theory　is　the　manner　in　which　newly　discovered　and　unanticipated　facts　arrange　themselves　under　it。　　No　more　striking　illustration　of　this　can　be　found　than　in　the　contrasted　fates　of　Cuvier's　theory　and　of　that　of　Darwin。　　Even　before　Cuvier's　death　his　views　had　been　undermined　and　the　progress　of　discovery　soon　laid　them　in　irreparable　ruin；　while　the　activity　of　half…a…century　in　many　different　lines　of　inquiry　has　established　the　theory　of　evolution　upon　a　foundation　of　ever　growing　solidity。　　It　is　Darwin's　imperishable　glory　that　he　prescribed　the　lines　along　which　all　the　biological　sciences　were　to　advance　to　conquests　not　dreamed　of　when　he　wrote。


XII。　　THE　PALAEONTOLOGICAL　RECORD。

II。　　PLANTS。

By　D。H。　SCOTT；　F。R。S。　President　of　the　Linnean　Society。

There　are　several　points　of　view　from　which　the　subject　of　the　present　essay　may　be　regarded。　　We　may　consider　the　fossil　record　of　plants　in　its　bearing：　　I。　on　the　truth　of　the　doctrine　of　Evolution；　II。　on　Phylogeny；　or　the　course　of　Evolution；　III。　on　the　theory　of　Natural　Selection。　　The　remarks　which　follow；　illustrating　certain　aspects　only　of　an　extensive　subject；　may　conveniently　be　grouped　under　these　three　headings。

I。　　THE　TRUTH　OF　EVOLUTION。

When　〃The　Origin　of　Species〃　was　written；　it　was　necessary　to　show　that　the　Geological　Record　was　favourable　to；　or　at　least　consistent　with；　the　Theory　of　Descent。　　The　point　is　argued；　closely　and　fully；　in　Chapter　X。　　〃On　the　Imperfection　of　the　Geological　Record；〃　and　Chapter　XI。　　〃On　the　Geological　Succession　of　Organic　Beings〃；　there　is；　however；　little　about　plants　in　these　chapters。　　At　the　present　time　the　truth　of　Evolution　is　no　longer　seriously　disputed；　though　there　are　writers；　like　Reinke；　who　insist；　and　rightly　so；　that　the　doctrine　is　still　only　a　belief；　rather　than　an　established　fact　of　science。　　（J。　Reinke；　〃Kritische　Abstammungslehre〃；　〃Wiesner…Festschrift〃；　page　11；　Vienna；　1908。）　　Evidently；　then；　however　little　the　Theory　of　Descent　may　be　questioned　in　our　own　day；　it　is　desirable　to　assure　ourselves　how　the　case　stands；　and　in　particular　how　far　the　evidence　from　fossil　plants　has　grown　stronger　with　time。

As　regards　direct　evidence　for　the　derivation　of　one　species　from　another；　there　has　probably　been　little　advance　since　Darwin　wrote；　at　least　so　we　must　infer　from　the　emphasis　laid　on　the　discontinuity　of　successive　fossil　species　by　great　systematic　authorities　like　Grand'Eury　and　Zeiller　in　their　most　recent　writings。　　We　must　either　adopt　the　mutationist　views　of　those　authors　（referred　to　in　the　last　section　of　this　essay）　or　must　still　rely　on　Darwin's　explanation　of　the　absence　of　numerous　intermediate　varieties。　　The　attempts　which　have　been　made　to　trace；　in　the　Tertiary　rocks；　the　evolution　of　recent　species；　cannot；　owing　to　the　imperfect　character　of　the　evidence；　be　regarded　as　wholly　satisfactory。

When　we　come　to　groups　of　a　somewhat　higher　order　we　have　an　interesting　history　of　the　evolution　of　a　recent　family　in　the　work；　not　yet　completed；　of　Kidston　and　Gwynne…Vaughan　on　the　fossil　Osmundaceae。　　（〃Trans。　Royal　Soc。　Edinburgh〃；　Vol。　45；　Part　III。　1907；　Vol。　46；　Part　II。　1908；　Vol。　46；　Part　III。　1909。）　　The　authors　are　able；　mainly　on　anatomical　evidence；　to　trace　back　this　now　limited　group　of　Ferns；　through　the　Tertiary　and　Mesozoic　to　the　Permian；　and　to　show；　with　great　probability；　how　their　structure　has　been　derived　from　that　of　early　Palaeozoic　types。

The　history　of　the　Ginkgoaceae；　now　represented　only　by　the　isolated　maidenhair　tree；　scarcely　known　in　a　wild　state；　offers　another　striking　example　of　a　family　which　can　be　traced　with　certainty　to　the　older　Mesozoic　and　perhaps　further　back　still。　　（See　Seward　and　Gowan；　〃The　Maidenhair　Tree　（Gingko　biloba）〃；　〃Annals　of　Botany〃；　Vol。　XIV。　1900；　page　109；　also　A。　Sprecher　〃Le　Ginkgo　biloba〃；　L。；　Geneva；　1907。）

On　the　wider　question　of　the　derivation　of　the　great　groups　of　plants；　a　very　considerable　advance　has　been　made