darwin and modern science-第146章

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sed。　　Darwin　emphasised　the　fact　that　〃the　difference　between　the　self…fertilised　and　crossed　plants　raised　by　me　cannot　be　attributed　to　the　superiority　of　the　crossed；　but　to　the　inferiority　of　the　self…fertilised　seedlings；　due　to　the　injurious　effects　of　self…fertilisation。〃　　（Ibid。　page　437。）　　But　he　had　no　doubt　that　in　favourable　circumstances　self…fertilised　plants　were　able　to　persist　for　several　generations　without　crossing。　　An　occasional　crossing　appears　to　be　useful　but　not　indispensable　in　all　cases；　its　sporadic　occurrence　in　plants　in　which　self…pollination　habitually　occurs　is　not　excluded。　　Self…fertilisation　is　for　the　most　part　relatively　and　not　absolutely　injurious　and　always　better　than　no　fertilisation。　　〃Nature　abhors　perpetual　self…fertilisation〃　（It　is　incorrect　to　say；　as　a　writer　has　lately　said；　that　the　aphorism　expressed　by　Darwin　in　1859　and　1862；　〃Nature　abhors　perpetual　self…fertilisation；〃　is　not　repeated　in　his　later　works。　　The　sentence　is　repeated　in　〃Cross　and　Self　fertilisation〃　（page　8）；　with　the　addition；　〃If　the　word　perpetual　had　been　omitted；　the　aphorism　would　have　been　false。　　As　it　stands；　I　believe　that　it　is　true；　though　perhaps　rather　too　strongly　expressed。〃）　is；　however；　a　pregnant　expression　of　the　fact　that　cross…fertilisation　is　exceedingly　widespread　and　has　been　shown　in　the　majority　of　cases　to　be　beneficial；　and　that　in　those　plants　in　which　we　find　self…pollination　regularly　occurring　cross…　pollination　may　occasionally　take　place。

An　attempt　has　been　made　to　express　in　brief　the　main　results　of　Darwin's　work　on　the　biology　of　flowers。　　We　have　seen　that　his　object　was　to　elucidate　important　general　questions；　particularly　the　question　of　the　significance　of　sexual　reproduction。

It　remains　to　consider　what　influence　his　work　has　had　on　botanical　science。　　That　this　influence　has　been　very　considerable；　is　shown　by　a　glance　at　the　literature　on　the　biology　of　flowers　published　since　Darwin　wrote。　　Before　the　book　on　orchids　was　published　there　was　nothing　but　the　old　and　almost　forgotten　works　of　Kolreuter　and　Sprengel　with　the　exception　of　a　few　scattered　references。　　Darwin's　investigations　gave　the　first　stimulus　to　the　development　of　an　extensive　literature　on　floral　biology。　　In　Knuth's　〃Handbuch　der　Blutenbiologie〃　（〃Handbook　of　Flower　Pollination〃；　Oxford；　1906）　as　many　as　3792　papers　on　this　subject　are　enumerated　as　having　been　published　before　January　1；　1904。　　These　describe　not　only　the　different　mechanisms　of　flowers；　but　deal　also　with　a　series　of　remarkable　adaptations　in　the　pollinating　insects。　　As　a　fertilising　rain　quickly　calls　into　existence　the　most　varied　assortment　of　plants　on　a　barren　steppe；　so　activity　now　reigns　in　a　field　which　men　formerly　left　deserted。　　This　development　of　the　biology　of　flowers　is　of　importance　not　only　on　theoretical　grounds　but　also　from　a　practical　point　of　view。　　The　rational　breeding　of　plants　is　possible　only　if　the　flower…biology　of　the　plants　in　question　（i。e。　the　question　of　the　possibility　of　self…pollination；　self…　sterility；　etc。）　is　accurately　known。　　And　it　is　also　essential　for　plant…　breeders　that　they　should　have　〃the　power　of　fixing　each　fleeting　variety　of　colour；　if　they　will　fertilise　the　flowers　of　the　desired　kind　with　their　own　pollen　for　half…a…dozen　generations；　and　grow　the　seedlings　under　the　same　conditions。〃　　（〃Cross　and　Self　fertilisation〃　（1st　edition）；　page　460。）

But　the　influence　of　Darwin　on　floral　biology　was　not　confined　to　the　development　of　this　branch　of　Botany。　　Darwin's　activity　in　this　domain　has　brought　about　（as　Asa　Gray　correctly　pointed　out）　the　revival　of　teleology　in　Botany　and　Zoology。　　Attempts　were　now　made　to　determine；　not　only　in　the　case　of　flowers　but　also　in　vegetative　organs；　in　what　relation　the　form　and　function　of　organs　stand　to　one　another　and　to　what　extent　their　morphological　characters　exhibit　adaptation　to　environment。　　A　branch　of　Botany；　which　has　since　been　called　Ecology　（not　a　very　happy　term）　has　been　stimulated　to　vigorous　growth　by　floral　biology。

While　the　influence　of　the　work　on　the　biology　of　flowers　was　extraordinarily　great；　it　could　not　fail　to　elicit　opinions　at　variance　with　Darwin's　conclusions。　　The　opposition　was　based　partly　on　reasons　valueless　as　counterarguments；　partly　on　problems　which　have　still　to　be　solved；　to　some　extent　also　on　that　tendency　against　teleological　conceptions　which　has　recently　become　current。　　This　opposing　trend　of　thought　is　due　to　the　fact　that　many　biologists　are　content　with　teleological　explanations；　unsupported　by　proof；　it　is　also　closely　connected　with　the　fact　that　many　authors　estimate　the　importance　of　natural　selection　less　highly　than　Darwin　did。　　We　may　describe　the　objections　which　are　based　on　the　widespread　occurrence　of　self…　fertilisation　and　geitonogamy　as　of　little　importance。　　Darwin　did　not　deny　the　occurrence　of　self…fertilisation；　even　for　a　long　series　of　generations；　his　law　states　only　that　〃Nature　abhors　PERPETUAL　self…　fertilisation。〃　　（It　is　impossible　（as　has　been　attempted）　to　express　Darwin's　point　of　view　in　a　single　sentence；　such　as　H。　Muller's　statement　of　the　〃Knight…Darwin　law。〃　　The　conditions　of　life　in　organisms　are　so　various　and　complex　that　laws；　such　as　are　formulated　in　physics　and　chemistry；　can　hardly　be　conceived。）　　An　exception　to　this　rule　would　therefore　occur　only　in　the　case　of　plants　in　which　the　possibility　of　cross…pollination　is　excluded。　　Some　of　the　plants　with　cleistogamous　flowers　might　afford　examples　of　such　cases。　　We　have　already　seen；　however；　that　such　a　case　has　not　as　yet　been　shown　to　occur。　　Burck　believed　that　he　had　found　an　instance　in　certain　tropical　plants　（Anonaceae；　Myrmecodia）　of　the　complete　exclusion　of　cross…fertilisation。　　The　flowers　of　these　plants；　in　which；　however；in　contrast　to　the　cleistogamous　flowersthe　corolla　is　well　developed；　remain　closed　and　fruit　is　produced。

Loew　（E。　Loew；　〃Bemerkungen　zu　Burck。。。〃；　〃Biolog。　Centralbl。〃　XXVI。　（1906）。）　has　shown　that　cases　occur　in　which　cross…fertilisation　may　be　effected　even　in　these　〃cleistopetalous〃　flowers：　　humming　birds　visit　the　permanently　closed　flowers　of　certain　species　of　Nidularium　and　transport　the　pollen。　　The　fact　that　the　formation　of　hybrids　may　occur　as　the　result　of　this　shows　that　pollination　may　be　accomplished。

The　existence　of　plants　for　which　self…pollination　is　of　greater　importance　than　it　is　for　others　is　by　no　means　contradictory　to　Darwin's　view。　　Self…　fertilisation　is；　for　example；　of　greater　importance　for　annuals　than　for　perennials　as　without　it　seeds　might　fail　to　be　produced。　　Even　in　the　case　of　annual　plants　with　small　inconspicuous　flowers　in　which　self…　fertilisation　usually　occurs；　such　as　Senecio　vulgaris；　Capsella　bursa…　pastoris　and　Stellaria　media；　A。　Bateson　（Anna　Bateson；　〃The　effects　of　cross…fertilisation　on　inconspicuous　flowers〃；　〃Annals　of　Botany〃；　Vol。　I。　　1888；　page　255。）　found　that　cross…fertilisation　gave　a　beneficial　result；　although　only　in　a　slight　degree。　　If　the　favourable　effects　of　sexual　reproduction；　according　to　Darwin's　view；　are　correlated　with　change　of　environment；　it　is　quite　possible　that　this　is　of　less　importance　in　plants　which　die　after　ripening　their　seeds　（〃hapaxanthic〃）　and　which　in　any　case　constantly　change　their　situation。　　Objections　which　are　based　on　the　proof　of　the　prevalence　of　self…fertilisation　are　not；　therefore；　pertinent。　　At　first　sight　another　point　of　view；　which　has　been　more　recently　urged；　appears　to　have　more　weight。

W。　Burck　（Burck；　〃Darwin's　Kreuzeungsgesetz。。。〃；　〃Biol。　Centralbl〃。　XXVIII。　1908；　page　177。）　has　expressed　the　opinion　that　the　beneficial　results　of　cross…fertilisation　demonstrated　by　Darwin　concern　only　hybrid　plants。　　These　alone　become　weaker　by　self…pollination；　while　pure　species　derive　no　advantage　from　crossing　and　no　disadvantage　from　self…fertilisation。　　It　is　certain　that　some　of　the　plants　used　by　Darwin　were　of　hybrid　origin。　　（It　is　questionable　if　this　was　always　the　case。）　　This　is　evident　from　his　statements；　which　are　models　of　clearness　and　precision；　he　says　that　his　Ipomoea　plants　〃were　probably　the　offspring　of　a　cross。