darwin and modern science-第52章

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orts　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；　and；　so　far　as　the　higher　plants　are　concerned；　we　are　now　able　to　form　a　far　better　conception　than　before　of　the　probable　course　of　evolution。　　This　is　a　matter　of　phylogeny；　and　the　facts　will　be　considered　under　that　head；　our　immediate　point　is　that　the　new　knowledge　of　the　relations　between　the　classes　of　plants　in　question　materially　strengthens　the　case　for　the　theory　of　descent。　　The　discoveries　of　the　last　few　years　throw　light　especially　on　the　relation　of　the　Angiosperms　to　the　Gymnosperms；　on　that　of　the　Seed…plants　generally　to　the　Ferns；　and　on　the　interrelations　between　the　various　classes　of　the　higher　Cryptogams。

That　the　fossil　record　has　not　done　still　more　for　Evolution　is　due　to　the　fact　that　it　begins　too　latea　point　on　which　Darwin　laid　stress　（〃Origin　of　Species〃　（6th　edition）；　page　286。）　and　which　has　more　recently　been　elaborated　by　Poulton。　　（〃Essays　on　Evolution〃；　pages　46　et　seq。；　Oxford；　1908。）　　An　immense　proportion　of　the　whole　evolutionary　history　lies　behind　the　lowest　fossiliferous　rocks；　and　the　case　is　worse　for　plants　than　for　animals；　as　the　record　for　the　former　begins；　for　all　practical　purposes；　much　higher　up　in　the　rocks。

It　may　be　well　here　to　call　attention　to　a　question；　often　overlooked；　which　has　lately　been　revived　by　Reinke。　　（Reinke；　loc。　cit。　page　13。）　　As　all　admit；　we　know　nothing　of　the　origin　of　life；　consequently；　for　all　we　can　tell；　it　is　as　probable　that　life　began；　on　this　planet；　with　many　living　things；　as　with　one。　　If　the　first　organic　beings　were　many；　they　may　have　been　heterogeneous；　or　at　least　exposed　to　different　conditions；　from　their　origin；　in　either　case　there　would　have　been　a　number　of　distinct　series　from　the　beginning；　and　if　so　we　should　not　be　justified　in　assuming　that　all　organisms　are　related　to　one　another。　　There　may　conceivably　be　several　of　the　original　lines　of　descent　still　surviving；　or　represented　among　extinct　formsto　reverse　the　remark　of　a　distinguished　botanist；　there　may　be　several　Vegetable　Kingdoms！　　However　improbable　this　may　sound；　the　possibility　is　one　to　be　borne　in　mind。

That　all　VASCULAR　plants　really　belong　to　one　stock　seems　certain；　and　here　the　palaeontological　record　has　materially　strengthened　the　case　for　a　monophyletic　history。　　The　Bryophyta　are　not　likely　to　be　absolutely　distinct；　for　their　sexual　organs；　and　the　stomata　of　the　Mosses　strongly　suggest　community　of　descent　with　the　higher　plants；　if　this　be　so　it　no　doubt　establishes　a　certain　presumption　in　favour　of　a　common　origin　for　plants　generally；　for　the　gap　between　〃Mosses　and　Ferns〃　has　been　regarded　as　the　widest　in　the　Vegetable　Kingdom。　　The　direct　evidence　of　consanguinity　is　however　much　weaker　when　we　come　to　the　Algae；　and　it　is　conceivable　（even　if　improbable）　that　the　higher　plants　may　have　had　a　distinct　ancestry　（now　wholly　lost）　from　the　beginning。　　The　question　had　been　raised　in　Darwin's　time；　and　he　referred　to　it　in　these　words：　　〃No　doubt　it　is　possible；　as　Mr　G。H。　Lewes　has　urged；　that　at　the　first　commencement　of　life　many　different　forms　were　evolved；　but　if　so；　we　may　conclude　that　only　a　very　few　have　left　modified　descendants。〃　　（〃Origin　of　Species〃；　page　425。）　　This　question；　though　it　deserves　attention；　does　not　immediately　affect　the　subject　of　the　palaeontological　record　of　plants；　for　there　can　be　no　reasonable　doubt　as　to　the　interrelationship　of　those　groups　on　which　the　record　at　present　throws　light。

The　past　history　of　plants　by　no　means　shows　a　regular　progression　from　the　simple　to　the　complex；　but　often　the　contrary。　　This　apparent　anomaly　is　due　to　two　causes。

1。　　The　palaeobotanical　record　is　essentially　the　story　of　the　successive　ascendancy　of　a　series　of　dominant　families；　each　of　which　attained　its　maximum；　in　organisation　as　well　as　in　extent；　and　then　sank　into　comparative　obscurity；　giving　place　to　other　families；　which　under　new　conditions　were　better　able　to　take　a　leading　place。　　As　each　family　ran　its　downward　course；　either　its　members　underwent　an　actual　reduction　in　structure　as　they　became　relegated　to　herbaceous　or　perhaps　aquatic　life　（this　may　have　happened　with　the　Horsetails　and　with　Isoetes　if　derived　from　Lepidodendreae）；　or　the　higher　branches　of　the　family　were　crowded　out　altogether　and　only　the　〃poor　relations〃　were　able　to　maintain　their　position　by　evading　the　competition　of　the　ascendant　races；　this　is　also　illustrated　by　the　history　of　the　Lycopod　phylum。　　In　either　case　there　would　result　a　lowering　of　the　type　of　organisation　within　the　group。

2。　　The　course　of　real　progress　is　often　from　the　complex　to　the　simple。　　If；　as　we　shall　find　some　grounds　for　believing；　the　Angiosperms　came　from　a　type　with　a　flower　resembling　in　its　complexity　that　of　Mesozoic　〃Cycads；〃　almost　the　whole　evolution　of　the　flower　in　the　highest　plants　has　been　a　process　of　reduction。　　The　stamen；　in　particular；　has　undoubtedly　become　extremely　simplified　during　evolution；　in　the　most　primitive　known　seed…plants　it　was　a　highly　compound　leaf　or　pinna；　its　reduction　has　gone　on　in　the