darwin and modern science-第56章

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The　available　data；　though　still　very　fragmentary；　certainly　suggest　that　both　Equisetales　and　Sphenophyllales　may　have　sprung　from　a　common　stock　having　certain　fern…like　characters。　　On　the　other　hand　the　Sphenophylls；　and　especially　the　peculiar　genus　Cheirostrobus；　have　in　their　anatomy　a　good　deal　in　common　with　the　Lycopods；　and　of　late　years　they　have　been　regarded　as　the　derivatives　of　a　stock　common　to　that　class　and　the　Equisetales。　　At　any　rate　the　characters　of　the　Sphenophyllales　and　of　the　new　group　Pseudoborniales　suggest　the　existence；　at　a　very　early　period；　of　a　synthetic　race　of　plants；　combining　the　characters　of　various　phyla　of　the　Vascular　Cryptogams。　　It　may　further　be　mentioned　that　the　Psilotaceae；　an　isolated　epiphytic　family　hitherto　referred　to　the　Lycopods；　have　been　regarded　by　several　recent　authors　as　the　last　survivors　of　the　Sphenophyllales；　which　they　resemble　both　in　their　anatomy　and　in　the　position　of　their　sporangia。

The　Lycopods；　so　far　as　their　early　history　is　known；　are　remarkable　rather　for　their　high　development　in　Palaeozoic　times　than　for　any　indications　of　a　more　primitive　ancestry。　　In　the　recent　Flora；　two　of　the　four　living　genera　（Excluding　Psilotaceae。）　（Selaginella　and　Isoetes）　have　spores　of　two　kinds；　while　the　other　two　（Lycopodium　and　Phylloglossum）　are　homosporous。　　Curiously　enough；　no　certain　instance　of　a　homosporous　Palaeozoic　Lycopod　has　yet　been　discovered；　though　well…preserved　fructifications　are　numerous。　　Wherever　the　facts　have　been　definitely　ascertained；　we　find　two　kinds　of　spore；　differentiated　quite　as　sharply　as　in　any　living　members　of　the　group。　　Some　of　the　Palaeozoic　Lycopods；　in　fact；　went　further；　and　produced　bodies　of　the　nature　of　seeds；　some　of　which　were　actually　regarded；　for　many　years；　as　the　seeds　of　Gymnosperms。　　This　specially　advanced　form　of　fructification　goes　back　at　least　as　far　as　the　Lower　Carboniferous；　while　the　oldest　known　genus　of　Lycopods；　Bothrodendron；　which　is　found　in　the　Devonian；　though　not　seed…bearing；　was　typically　heterosporous；　if　we　may　judge　from　the　Coal…measure　species。　　No　doubt　homosporous　Lycopods　existed；　but　the　great　prevalence　of　the　higher　mode　of　reproduction　in　days　which　to　us　appear　ancient；　shows　how　long　a　course　of　evolution　must　have　already　been　passed　through　before　the　oldest　known　members　of　the　group　came　into　being。　　The　other　characters　of　the　Palaeozoic　Lycopods　tell　the　same　tale；　most　of　them　attained　the　stature　of　trees；　with　a　corresponding　elaboration　of　anatomical　structure；　and　even　the　herbaceous　forms　show　no　special　simplicity。　　It　appears　from　recent　work　that　herbaceous　Lycopods；　indistinguishable　from　our　recent　Selaginellas；　already　existed　in　the　time　of　the　Coal…measures；　while　one　herbaceous　form　（Miadesmia）　is　known　to　have　borne　seeds。

The　utmost　that　can　be　said　for　primitiveness　of　character　in　Palaeozoic　Lycopods　is　that　the　anatomy　of　the　stem；　in　its　primary　ground…plan；　as　distinguished　from　its　secondary　growth；　was　simpler　than　that　of　most　Lycopodiums　and　Selaginellas　at　the　present　day。　　There　are　also　some　peculiarities　in　the　underground　organs　（Stigmaria）　which　suggest　the　possibility　of　a　somewhat　imperfect　differentiation　between　root　and　stem；　but　precisely　parallel　difficulties　are　met　with　in　the　case　of　the　living　Selaginellas；　and　in　some　degree　in　species　of　Lycopodium。

In　spite　of　their　high　development　in　past　ages　the　Lycopods；　recent　and　fossil；　constitute；　on　the　whole；　a　homogeneous　group；　and　there　is　little　at　present　to　connect　them　with　other　phyla。　　Anatomically　some　relation　to　the　Sphenophylls　is　indicated；　and　perhaps　the　recent　Psilotaceae　give　some　support　to　this　connection；　for　while　their　nearest　alliance　appears　to　be　with　the　Sphenophylls；　they　approach　the　Lycopods　in　anatomy；　habit；　and　mode　of　branching。

The　typically　microphyllous　character　of　the　Lycopods；　and　the　simple　relation　between　sporangium　and　sporophyll　which　obtains　throughout　the　class；　have　led　various　botanists　to　regard　them　as　the　most　primitive　phylum　of　the　Vascular　Cryptogams。　　There　is　nothing　in　the　fossil　record　to　disprove　this　view；　but　neither　is　there　anything　to　support　it；　for　this　class　so　far　as　we　know　is　no　more　ancient　than　the　megaphyllous　Cryptogams；　and　its　earliest　representatives　show　no　special　simplicity。　　If　the　indications　of　affinity　with　Sphenophylls　are　of　any　value　the　Lycopods　are　open　to　suspicion　of　reduction　from　a　megaphyllous　ancestry；　but　there　is　no　direct　palaeontological　evidence　for　such　a　history。

The　general　conclusions　to　which　we　are　led　by　a　consideration　of　the　fossil　record　of　the　Vascular　Cryptogams　are　still　very　hypothetical；　but　may　be　provisionally　stated　as　follows：

The　Ferns　go　back　to　the　earliest　known　period。　　In　Mesozoic　times　practically　all　the　existing　families　had　appeared；　in　the　Palaeozoic　the　class　was　less　extensive　than　formerly　believed；　a　majority　of　the　supposed　Ferns　of　that　age　having　proved　to　be　seed…bearing　plants。　　The　oldest　authentic　representatives　of　the　Ferns　were　megaphyllous　plants；　broadly　speaking；　of　the　same　type　as　those　of　later　epochs；　though　differing　much　in　detail。　　As　far　back　as　the　record　extends　they　show　no　sign　of　becoming　merged　with　other　phyla　in　any　synthetic　group。

The　Equisetales　likewise　have　a　long　history；　and　manifestly　attained　their　greatest　development　in　Palaeozoic　times。　　Their　oldest　forms　show　an　approach　to　the　extinct　class　Sphenophyllales；　which　connects　them　to　some　extent；　by　anatomical　characters；　with　the　Lycopods。　　At　the　same　time　the　oldest　Equisetales　show　a　somewhat　megaphyllous　character；　which　was　more　marked　in　the　Devonian　Pseudoborniales。　　Some　remote　affinity　with　the　Ferns　（which　has　also　been　upheld　on　other　grounds）　may　thus　be　indicated。　It　is　possible　that　in　the　Sphenophyllales　we　may　have　the　much…modified　representatives　of　a　very　ancient　synthetic　group。

The　Lycopods　likewise　attained　their　maximum　in　the　Palaeozoic；　and　show；　on　the　whole；　a　greater　elaboration　of　structure　in　their　early　forms　than　at　any　later　period；　while　at　the　same　time　maintaining　a　considerable　degree　of　uniformity　in　morphological　characters　throughout　their　history。　The　Sphenophyllales　are　the　only　other　class　with　which　they　show　any　relation；　if　such　a　connection　existed；　the　common　point　of　origin　must　lie　exceedingly　far　back。

The　fossil　record；　as　at　present　known；　cannot；　in　the　nature　of　things；　throw　any　direct　light　on　what　is　perhaps　the　most　disputed　question　in　the　morphology　of　plantsthe　origin　of　the　alternating　generations　of　the　higher　Cryptogams　and　the　Spermophyta。　　At　the　earliest　period　to　which　terrestrial　plants　have　been　traced　back　all　the　groups　of　Vascular　Cryptogams　were　in　a　highly　advanced　stage　of　evolution；　while　innumerable　Seed…plantspresumably　the　descendants　of　Cryptogamic　ancestorswere　already　flourishing。　　On　the　other　hand　we　know　practically　nothing　of　Palaeozoic　Bryophyta；　and　the　evidence　even　for　their　existence　at　that　period　cannot　be　termed　conclusive。　　While　there　are　thus　no　palaeontological　grounds　for　the　hypothesis　that　the　Vascular　plants　came　of　a　Bryophytic　stock；　the　question　of　their　actual　origin　remains　unsolved。

III。　　NATURAL　SELECTION。

Hitherto　we　have　considered　the　palaeontological　record　of　plants　in　relation　to　Evolution。　　The　question　remains；　whether　the　record　throws　any　light　on　the　theory　of　which　Darwin　and　Wallace　were　the　authorsthat　of　Natural　Selection。　　The　subject　is　clearly　one　which　must　be　investigated　by　other　methods　than　those　of　the　palaeontologist；　still　there　are　certain　important　points　involved；　on　which　the　palaeontological　record　appears　to　bear。

One　of　these　points　is　the　supposed　distinction　between　morphological　and　adaptive　characters；　on　which　Nageli；　in　particular；　laid　so　much　stress。　　The　question　is　a　difficult　one；　it　was　discussed　by　Darwin　（〃Origin　of　Species〃　（6th　edition）；　pages　170…176。）；　who；　while　showing　that　the　apparent　distinction　is　in　part　to　be　explained　by　our　imperfect　knowledge　of　function；　recognised　the　existence　of　important　morphological　characters　which　are　not　adaptations。　　The　following　passage　expresses　his　conclusion。　〃Thus；　as　I　am　inclined　to　believe；　morphological　differences；　which　we　consider　as　importantsuch　as　the　arrangement　of　the　leaves；　the　divisions　of　the　flower　or　of　the　ovarium；　the　position　of　the　ovules；　etc。first　appeared　in　many　cases　as　fluctuating　variations；　which　sooner　or　later　became　constant　through　the　nature　of　the　organism　and　of　the　surrounding　conditions；　as　well　as　through　the　inter…crossing　of　distinct　individuals；　but　not　through　natural　selection；　for　as　these　morphological　characters　do　not　affect　the　welfare　of　the　species；　any　slight　deviations　in　them　could　not　have　been　governed　or　accumulated　through　this　latter　agency。〃　　（Ibid。　page　176。）

This　is　a　sufficiently　liberal　concession；　Nageli；　however；　went　much　further　when　he　said：　　〃I　do　not　know　among　plants　a　morphological　modification　which　can　be　explained　on　utilitarian　principles。〃　　（See　〃More　Letters〃；　Vol。　II。　page　375　（footnote）。）　　If　this　were　true　the　field　of　Natural　Selection　would　be　so　seriously　restricted；　as　to　leave　the　theory　only　a　very　limited　importance。

It　can　be　shown；　as　the　writer　believes；　that　many　typical　〃morphological　characters；〃　on　which　the　distinction　between　great　classes　of　plants　is　based；　were　adaptive　in　origin；　and　even　that　their　constancy　is　due　to　their　functional　importance。　　Only　one　or　two　cases　will　be　mentioned；　where　the　fossil　evidence　affects　the　question。

The　pollen…tube　is　one　of　the　most　important　morphological　characters　of　the　Spermophyta　as　now　existingin　fact　the　name　Siphonogama　is　used　by　Engler　in　his　classification；　as　expressing　a　peculiarly　constant　character　of　the　Seed…plants。　　Yet　the　pollen…tube　is　a　manifest　adaptation；　following　on　the　adoption　of　the　seed…habit；　and　serving　first　to　bring　the　spermatozoids　with　greater　precision　to　their　goal；　and　ultimately　to　relieve　them　of　the　necessity　for　independent　movement。　　The　pollen…tube　is　constant　because　it　has　proved　to　be　indispensable。

In　the　Palaeozoic　Seed…plants　there　are　a　number　of　instances　in　which　the　pollen…grains；　contained　in　the　pollen…chamber　of　a　seed；　are　so　beautifully　preserved　that　the　presence　of　a　group　of　cells　within　the　grain　can　be　demonstrated；　sometimes　we　can　even　see　how　the　cell…walls　broke　down　to　emit　the　sperms；　and　quite　lately　it　is　said　that　the　sperms　themselves　have　been　recognised。　　（F。W。　Oliver；　〃On　Physostoma　elegans；　an　archaic　type　of　seed　from　the　Palaeozoic　Rocks〃；　〃Annals　of　Botany〃；　January；　1909。　　See　also　the　earlier　papers　there　cited。）　　In　no　case；　however；　is　there　as　yet　any　satisfactory　evidence　for　the　formation　of　a　pollen…tube；　it　is　probable　that　in　these　early　Seed…plants　the　pollen…　grains　remained　at　about　the　evolutionary　level　of　the　microspores　in　Pilularia　or　Selaginella；　and　discharged　their　spermatozoids　directly；　leaving　them　to　find　their　own　way　to　the　female　cells。　　It　thus　appears　that　there　were　once　Spermophyta　without　pollen…tubes。　　The　pollen…tube　method　ultimately　prevailed；　becoming　a　constant　〃m