darwin and modern science- 第64部分

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　Conifers　and　modern　Cycads；　as　well　as　in　the　Angiosperms；　though　in　different　ways　and　to　a　varying　extent。

The　seed　offers　another　striking　example；　the　Palaeozoic　seeds　（if　we　leave　the　seed…like　organs　of　certain　Lycopods　out　of　consideration）　were　always；　so　far　as　we　know；　highly　complex　structures；　with　an　elaborate　vascular　system；　a　pollen…chamber；　and　often　a　much…differentiated　testa。　　In　the　present　day　such　seeds　exist　only　in　a　few　Gymnosperms　which　retain　their　ancient　charactersin　all　the　higher　Spermophytes　the　structure　is　very　much　simplified；　and　this　holds　good　even　in　the　Coniferae；　where　there　is　no　countervailing　complication　of　ovary　and　stigma。

Reduction；　in　fact；　is　not　always；　or　even　generally；　the　same　thing　as　degeneration。　　Simplification　of　parts　is　one　of　the　most　usual　means　of　advance　for　the　organism　as　a　whole。　　A　large　proportion　of　the　higher　plants　are　microphyllous　in　comparison　with　the　highly　megaphyllous　fern…　like　forms　from　which　they　appear　to　have　been　derived。

Darwin　treated　the　general　question　of　advance　in　organisation　with　much　caution；　saying：　　〃The　geological　record。。。does　not　extend　far　enough　back；　to　show　with　unmistakeable　clearness　that　within　the　known　history　of　the　world　organisation　has　largely　advanced。〃　　（〃Origin　of　Species〃；　page　308。）　　Further　on　（Ibid。　page　309。）　he　gives　two　standards　by　which　advance　may　be　measured：　　〃We　ought　not　solely　to　compare　the　highest　members　of　a　class　at　any　two　periods。。。but　we　ought　to　compare　all　the　members；　high　and　low；　at　the　two　periods。〃　　Judged　by　either　standard　the　Horsetails　and　Club　Mosses　of　the　Carboniferous　were　higher　than　those　of　our　own　day；　and　the　same　is　true　of　the　Mesozoic　Cycads。　　There　is　a　general　advance　in　the　succession　of　classes；　but　not　within　each　class。

Darwin's　argument　that　〃the　inhabitants　of　the　world　at　each　successive　period　in　its　history　have　beaten　their　predecessors　in　the　race　for　life；　and　are；　in　so　far；　higher　in　the　scale〃　（〃Origin　of　Species〃；　page　315。）　is　unanswerable；　but　we　must　remember　that　〃higher　in　the　scale〃　only　means　〃better　adapted　to　the　existing　conditions。〃　　Darwin　points　out　（Ibid。　page　279。）　that　species　have　remained　unchanged　for　long　periods；　probably　longer　than　the　periods　of　modification；　and　only　underwent　change　when　the　conditions　of　their　life　were　altered。　　Higher　organisation；　judged　by　the　test　of　success；　is　thus　purely　relative　to　the　changing　conditions；　a　fact　of　which　we　have　a　striking　illustration　in　the　sudden　incoming　of　the　Angiosperms　with　all　their　wonderful　floral　adaptations　to　fertilisation　by　the　higher　families　of　Insects。

II。　　PHYLOGENY。

The　question　of　phylogeny　is　really　inseparable　from　that　of　the　truth　of　the　doctrine　of　evolution；　for　we　cannot　have　historical　evidence　that　evolution　has　actually　taken　place　without　at　the　same　time　having　evidence　of　the　course　it　has　followed。

As　already　pointed　out；　the　progress　hitherto　made　has　been　rather　in　the　way　of　joining　up　the　great　classes　of　plants　than　in　tracing　the　descent　of　particular　species　or　genera　of　the　recent　flora。　　There　appears　to　be　a　difference　in　this　respect　from　the　Animal　record；　which　tells　us　so　much　about　the　descent　of　living　species；　such　as　the　elephant　or　the　horse。　　The　reason　for　this　difference　is　no　doubt　to　be　found　in　the　fact　that　the　later　part　of　the　palaeontological　record　is　the　most　satisfactory　in　the　case　of　animals　and　the　least　so　in　the　case　of　plants。　　The　Tertiary　plant…remains；　in　the　great　majority　of　instances；　are　impressions　of　leaves；　the　conclusions　to　be　drawn　from　which　are　highly　precarious；　until　the　whole　subject　of　Angiospermous　palaeobotany　has　been　reinvestigated；　it　would　be　rash　to　venture　on　any　statements　as　to　the　descent　of　the　families　of　Dicotyledons　or　Monocotyledons。

Our　attention　will　be　concentrated　on　the　following　questions；　all　relating　to　the　phylogeny　of　main　groups　of　plants：　　i。　The　Origin　of　the　Angiosperms。　　ii。　The　Origin　of　the　Seed…plants。　　iii。