darwin and modern science-第129节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


mpound　species；　frequently　consisting　of　a　very　large　number　of　smaller　or　elementary　species　formerly　included　under　the　comprehensive　term　varieties。　　Hybridisation　has　in　most　cases　affected　our　garden　and　cultivated　plants　so　that　they　do　not　represent　pure　species　but　a　mixture　of　species。

But　this　consideration　has　no　essential　bearing　on　Darwin's　point　of　view；　according　to　which　the　nature　of　the　sexual　cells　is　influenced　by　external　conditions。　　Even　individuals　growing　close　to　one　another　are　only　apparently　exposed　to　identical　conditions。　　Their　sexual　cells　may　therefore　be　differently　influenced　and　thus　give　favourable　results　on　crossing；　as　〃the　benefits　which　so　generally　follow　from　a　cross　between　two　plants　apparently　depend　on　the　two　differing　somewhat　in　constitution　or　character。〃　　As　a　matter　of　fact　we　are　familiar　with　a　large　number　of　cases　in　which　the　condition　of　the　reproductive　organs　is　influenced　by　external　conditions。　　Darwin　has　himself　demonstrated　this　for　self…sterile　plants；　that　is　plants　in　which　self…fertilisation　produces　no　result。　　This　self…sterility　is　affected　by　climatic　conditions：　　thus　in　Brazil　Eschscholzia　californica　is　absolutely　sterile　to　the　pollen　of　its　own　flowers；　the　descendants　of　Brazilian　plants　in　Darwin's　cultures　were　partially　self…fertile　in　one　generation　and　in　a　second　generation　still　more　so。　　If　one　has　any　doubt　in　this　case　whether　it　is　a　question　of　the　condition　of　the　style　and　stigma；　which　possibly　prevents　the　entrance　of　the　pollen…tube　or　even　its　development；　rather　than　that　of　the　actual　sexual　cells；　in　other　cases　there　is　no　doubt　that　an　influence　is　exerted　on　the　latter。

Janczewski　（Janczewski；　〃Sur　les　antheres　steriles　des　Groseilliers〃；　〃Bull。　de　l'acad。　des　sciences　de　Cracovie〃；　June；　1908。）　has　recently　shown　that　species　of　Ribes　cultivated　under　unnatural　conditions　frequently　produce　a　mixed　（i。e。　partly　useless）　or　completely　sterile　pollen；　precisely　as　happens　with　hybrids。　　There　are；　therefore；　substantial　reasons　for　the　conclusion　that　conditions　of　life　exert　an　influence　on　the　sexual　cells。　　〃Thus　the　proposition　that　the　benefit　from　cross…fertilisation　depends　on　the　plants　which　are　crossed　having　been　subjected　during　previous　generations　to　somewhat　different　conditions；　or　to　their　having　varied　from　some　unknown　cause　as　if　they　had　been　thus　subjected；　is　securely　fortified　on　all　sides。〃　　（〃Cross　and　Self　fertilisation〃　（1st　edition）；　page　444。）

We　thus　obtain　an　insight　into　the　significance　of　sexuality。　　If　an　occasional　and　slight　alteration　in　the　conditions　under　which　plants　and　animals　live　is　beneficial　（Reasons　for　this　are　given　by　Darwin　in　〃Variation　under　Domestication〃　（2nd　edition）；　Vol。　II。　page　127。）；　crossing　between　organisms　which　have　been　exposed　to　different　conditions　becomes　still　more　advantageous。　　The　entire　constitution　is　in　this　way　influenced　from　the　beginning；　at　a　time　when　the　whole　organisation　is　in　a　highly　plastic　state。　　The　total　life…energy；　so　to　speak；　is　increased；　a　gain　which　is　not　produced　by　asexual　reproduction　or　by　the　union　of　sexual　cells　of　plants　which　have　lived　under　the　same　or　only　slightly　different　conditions。　　All　the　wonderful　arrangements　for　cross…　fertilisation　now　appear　to　be　useful　adaptations。　　Darwin　was；　however；　far　from　giving　undue　prominence　to　this　point　of　view；　though　this　has　been　to　some　extent　done　by　others。　　He　particularly　emphasised　the　following　consideration：〃But　we　should　always　keep　in　mind　that　two　somewhat　opposed　ends　have　to　be　gained；　the　first　and　more　important　one　being　the　production　of　seeds　by　any　means；　and　the　second；　cross…　fertilisation。〃　　（〃Cross　and　Self　fertilisation〃　（1st　edition）；　page　371。）　　Just　as　in　some　orchids　and　cleistogamic　flowers　self…pollination　regularly　occurs；　so　it　may　also　occur　in　other　cases。　　Darwin　showed　that　Pisum　sativum　and　Lathyrus　odoratus　belong　to　plants　in　which　self…pollination　is　regularly　effected；　and　that　this　accounts　for　the　constancy　of　certain　sorts　of　these　plants；　while　a　variety　of　form　is　produced　by　crossing。　　Indeed　among　his　culture　plants　were　some　which　derived　no　benefit　from　crossing。　　Thus　in　the　sixth　self…fertilised　generation　of　his　Ipomoea　cultures　the　〃Hero〃　made　its　appearance；　a　form　slightly　exceeding　its　crossed　companion　in　height；　this　was　in　the　highest　degree　self…fertile　and　handed　on　its　characteristics　to　both　children　and　grandchildren。　　Similar　forms　were　found　in　Mimulus　luteus　and　Nicotiana　（In　Pisum　sativum　also　the　crossing　of　two　individuals　of　the　same　variety　produced　no　advantage；　Darwin　attributed　this　to　the　fact　that　the　plants　had　for　several　generations　been　self…fertilised　and　in　each　generation　cultivated　under　almost　the　same　conditions。　　Tschermak　（〃Ueber　kunstliche　Kreuzung　an　Pisum　sativum〃）　afterwards　recorded　the　same　result；　but　he　found　on　crossing　different　varieties　that　usually　there　was　no　superiority　as　regards　height　over　the　products　of　self…fertilisation；　while　Darwin　found　a　greater　height　represented　by　the　ratios　100：75　and　100：60。）；　types　which；　after　self…fertilisation；　have　an　enhanced　power　of　seed…production　and　of　attaining　a　greater　height　than　the　plants　of　the　corresponding　generation　which　are　crossed　together　and　self…fertilised　and　grown　under　the　same　conditions。　　〃Some　observations　made　on　other　plants　lead　me　to　suspect　that　self…fertilisation　is　in　some　respects　beneficial；　although　the　benefit　thus　derived　is　as　a　rule　very　small　compared　with　that　from　a　cross　with　a　distinct　plant。〃　　（〃Cross　and　Self　fertilisation〃；　page　350。）　　We　are　as　ignorant　of　the　reason　why　plants　behave　differently　when　crossed　and　self…fertilised　as　we　are　in　regard　to　the　nature　of　the　differentiation　of　the　sexual　cells；　which　determines　whether　a　union　of　the　sexual　cells　will　prove　favourable　or　unfavourable。

It　is　impossible　to　discuss　the　different　results　of　cross…fertilisation；　one　point　must；　however；　be　emphasised；　because　Darwin　attached　considerable　importance　to　it。　　It　is　inevitable　that　pollen　of　different　kinds　must　reach　the　stigma。　　It　was　known　that　pollen　of　the　same　〃species〃　is　dominant　over　the　pollen　of　another　species；　that；　in　other　words；　it　is　prepotent。　　Even　if　the　pollen　of　the　same　species　reaches　the　stigma　rather　later　than　that　of　another　species；　the　latter　does　not　effect　fertilisation。

Darwin　showed　that　the　fertilising　power　of　the　pollen　of　another　variety　or　of　another　individual　is　greater　than　that　of　the　plant's　own　pollen。　　（〃Cross　and　Self　fertilisation〃；　page　391。）　　This　has　been　demonstrated　in　the　case　of　Mimulus　luteus　（for　the　fixed　white…flowering　variety）　and　Iberis　umbellata　with　pollen　of　another　variety；　and　observations　on　cultivated　plants；　such　as　cabbage；　horseradish；　etc。　gave　similar　results。　　It　is；　however；　especially　remarkable　that　pollen　of　another　individual　of　the　same　variety　may　be　prepotent　over　the　plant's　own　pollen。　　This　results　from　the　superiority　of　plants　crossed　in　this　manner　over　self…　fertilised　plants。　　〃Scarcely　any　result　from　my　experiments　has　surprised　me　so　much　as　this　of　the　prepotency　of　pollen　from　a　distinct　individual　over　each　plant's　own　pollen；　as　proved　by　the　greater　constitutional　vigour　of　the　crossed　seedlings。〃　　（Ibid。　page　397。）　　Similarly；　in　self…　fertile　plants　the　flowers　of　which　have　not　been　deprived　of　the　male　organs；　pollen　brought　to　the　stigma　by　the　wind　or　by　insects　from　another　plant　effects　fertilisation；　even　if　the　plant's　own　pollen　has　reached　the　stigma　somewhat　earlier。

Have　the　results　of　his　experimental　investigations　modified　the　point　of　view　from　which　Darwin　entered　on　his　researches；　or　not？　　In　the　first　place　the　question　is；　whether　or　not　the　opinion　expressed　in　the　Orchid　book　that　there　is　〃Something　injurious〃　connected　with　self…fertilisation；　has　been　confirmed。　　We　can；　at　all　events；　affirm　that　Darwin　adhered　in　essentials　to　his　original　position；　but　self…fertilisation　afterwards　assumed　a　greater　importance　than　it　formerly　possessed。　　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