darwin and modern science-第69节

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re　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　〃morphological　character；〃　for　no　other　reason　than　because；　under　the　new　conditions；　it　provided　a　more　perfect　mechanism　for　the　accomplishment　of　the　act　of　fertilisation。　　We　have　still；　in　the　Cycads　and　Ginkgo；　the　transitional　case；　where　the　tube　remains　short；　serves　mainly　as　an　anchor　and　water…reservoir；　but　yet　is　able；　by　its　slight　growth；　to　give　the　spermatozoids　a　〃lift〃　in　the　right　direction。　　In　other　Seed…plants　the　sperms　are　mere　passengers；　carried　all　the　way　by　the　pollen…tube；　this　fact　has　alone　rendered　the　Angiospermous　method　of　fertilisation　through　a　stigma　possible。

We　may　next　take　the　seed　itselfthe　very　type　of　a　morphological　character。　　Our　fossil　record　does　not　go　far　enough　back　to　tell　us　the　origin　of　the　seed　in　the　Cycadophyta　and　Pteridosperms　（the　main　line　of　its　development）　but　some　interesting　sidelights　may　be　obtained　from　the　Lycopod　phylum。　　In　two　Palaeozoic　genera；　as　we　have　seen；　seed…like　organs　are　known　to　have　been　developed；　resembling　true　seeds　in　the　presence　of　an　integument　and　of　a　single　functional　embryo…sac；　as　well　as　in　some　other　points。　　We　will　call　these　organs　〃seeds〃　for　the　sake　of　shortness。　　In　one　genus　（Lepidocarpon）　the　seeds　were　borne　on　a　cone　indistinguishable　from　that　of　the　ordinary　cryptogamic　Lepidodendreae；　the　typical　Lycopods　of　the　period；　while　the　seed　itself　retained　much　of　the　detailed　structure　of　the　sporangium　of　that　family。　　In　the　second　genus；　Miadesmia；　the　seed…bearing　plant　was　herbaceous；　and　much　like　a　recent　Selaginella。　　（See　Margaret　Benson；　〃Miadesmia　membranacea；　a　new　Palaeozoic　Lycopod　with　a　seed…like　structure〃；　〃Phil。　Trans。　Royal　Soc。　Vol。　199；　B。　1908。）　　The　seeds　of　the　two　genera　are　differently　constructed；　and　evidently　had　an　independent　origin。　　Here；　then；　we　have　seeds　arising　casually；　as　it　were；　at　different　points　among　plants　which　otherwise　retain　all　the　characters　of　their　cryptogamic　fellows；　the　seed　is　not　yet　a　morphological　character　of　importance。　　To　suppose　that　in　these　isolated　cases　the　seed　sprang　into　being　in　obedience　to　a　Law　of　Advance　（〃Vervollkommungsprincip〃）；　from　which　other　contemporary　Lycopods　were　exempt；　involves　us　in　unnecessary　mysticism。　　On　the　other　hand　it　is　not　difficult　to　see　how　these　seeds　may　have　arisen；　as　adaptive　structures；　under　the　influence　of　Natural　Selection。　　The　seed…like　structure　afforded　protection　to　the　prothallus；　and　may　have　enabled　the　embryo　to　be　launched　on　the　world　in　greater　security。　　There　was　further；　as　we　may　suppose；　a　gain　in　certainty　of　fertilisation。　　As　the　writer　has　pointed　out　elsewhere；　the　chances　against　the　necessary　association　of　the　small　male　with　the　large　female　spores　must　have　been　enormously　great　when　the　cones　were　borne　high　up　on　tall　trees。　　The　same　difficulty　may　have　existed　in　the　case　of　the　herbaceous　Miadesmia；　if；　as　Miss　Benson　conjectures；　it　was　an　epiphyte。　　One　way　of　solving　the　problem　was　for　pollination　to　take　place　while　the　megaspore　was　still　on　the　parent　plant；　and　this　is　just　what　the　formation　of　an　ovule　or　seed　was　likely　to　secure。

The　seeds　of　the　Pteridosperms；　unlike　those　of　the　Lycopod　stock；　have　not　yet　been　found　in　statu　nascendiin　all　known　cases　they　were　already　highly　developed　organs　and　far　removed　from　the　cryptogamic　sporangium。　　But　in　two　respects　we　find　that　these　seeds；　or　some　of　them；　had　not　yet　realised　their　possibilities。　　In　the　seed　of　Lyginodendron　and　other　cases　the　micropyle；　or　orifice　of　the　integument；　was　not　the　passage　through　which　the　pollen　entered；　the　open　neck　of　the　pollen…chamber　protruded　through　the　micropyle　and　itself　received　the　pollen。　　We　have　met　with　an　analogous　case；　at　a　more　advanced　stage　of　evolution；　in　the　Bennettiteae；　where　the　wall　of　the　gynaecium；　though　otherwise　closed；　did　not　provide　a　stigma　to　catch　the　pollen；　but　allowed　the　micropyles　of　the　ovules　to　protrude　and　receive　the　pollen　in　the　old　gymnospermous　fashion。　　The　integument　in　the　one　case　and　the　pistil　in　the　other　had　not　yet　assumed　all　the　functions　to　which　the　organ　ultimately　became　adapted。　　Again；　no　Palaeozoic　seed　has　yet　been　found　to　contain　an　embryo；　though　the　preservation　is　often　good　enough　for　it　to　have　been　recognised　if　present。　　It　is　probable　that　the　nursing　of　the　embryo　had　not　yet　come　to　be　one　of　the　functions　of　the　seed；　and　that　the　whole　embryonic　development　was　relegated　to　the　germination　stage。

In　these　two　points；　the　reception　of　the　pollen　by　the　micropyle　and　the　nursing　of　the　embryo；　it　appears　that　many　Palaeozoic　seeds　were　imperfect；　as　compared　with　the　typical　seeds　of　later　times。　　As　evolution　went　on；　one　function　was　superadded　on　another；　and　it　appears　impossible　to　resist　the　conclusion　that　the　whole　differentiation　of　the　seed　was　a　process　of　adaptation；　and　consequently　governed　by　Natural　Selection；　just　as　much　as　the　specialisation　of　the　rostellum　in　an　Orchid；　or　of　the　pappus　in　a　Composite。

Did　space　allow；　other　examples　might　be　added。