darwin and modern science-第76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！


　with　the　help　of　facts　supplied　by　comparative　morphology；　the　history　of　development；　geographical　distribution；　and　palaeontology。　　（See　Lotsy；　〃Vorlesungen〃　（Jena；　I。　1906；　II。　1908）；　for　summary　of　the　facts。）　　A　glance　at　the　controversy　which　is　going　on　today　in　regard　to　different　hypotheses　shows　that　the　same　material　may　lead　different　investigators　to　form　entirely　different　opinions。　　Our　ultimate　aim　is　to　find　a　solution　of　the　problem　as　to　the　cause　of　the　origin　of　species。　　Indeed　such　attempts　are　now　being　made：　　they　are　justified　by　the　fact　that　under　cultivation　new　and　permanent　strains　are　produced；　the　fundamental　importance　of　this　was　first　grasped　by　Darwin。　　New　points　of　view　in　regard　to　these　lines　of　inquiry　have　been　adopted　by　H。　de　Vries　who　has　succeeded　in　obtaining　from　Oenothera　Lamarckiana　a　number　of　constant　〃elementary〃　species。　　Even　if　it　is　demonstrated　that　he　was　simply　dealing　with　the　complex　splitting　up　of　a　hybrid　（Bateson；　〃Reports　to　the　Evolution　Committee　of　the　Royal　Society〃；　London；　1902；　cf。　also　Lotsy；　〃Vorlesungen〃；　Vol。　I。　page　234。）；　the　facts　adduced　in　no　sense　lose　their　very　great　value。

We　must　look　at　the　problem　in　its　simplest　form；　we　find　it　in　every　case　where　a　new　race　differs　essentially　from　the　original　type　in　a　single　character　only；　for　example；　in　the　colour　of　the　flowers　or　in　the　petalody　of　the　stamens　（doubling　of　flowers）。　　In　this　connection　we　must　keep　in　view　the　fact　that　every　visible　character　in　a　plant　is　the　resultant　of　the　cooperation　of　specific　structure；　with　its　various　potentialities；　and　the　influence　of　the　environment。　　We　know；　that　in　a　pure　species　all　characters　vary；　that　a　blue…flowering　Campanula　or　a　red　Sempervivum　can　be　converted　by　experiment　into　white…flowering　forms；　that　a　transformation　of　stamens　into　petals　may　be　caused　by　fungi　or　by　the　influence　of　changed　conditions　of　nutrition；　or　that　plants　in　dry　and　poor　soil　become　dwarfed。　　But　so　far　as　the　experiments　justify　a　conclusion；　it　would　appear　that　such　alterations　are　not　inherited　by　the　offspring。　　Like　all　other　variations　they　appear　only　so　long　as　special　conditions　prevail　in　the　surroundings。

It　has　been　shown　that　the　case　is　quite　different　as　regards　the　white…　flowering；　double　or　dwarf　races；　because　these　retain　their　characters　when　cultivated　under　practically　identical　conditions；　and　side　by　side　with　the　blue；　single…flowering　or　tall　races。　　The　problem　may　therefore　be　stated　thus：　　how　can　a　character；　which　appears　in　the　one　case　only　under　the　strictly　limited　conditions　of　the　experiment；　in　other　cases　become　apparent　under　the　very　much　wider　conditions　of　ordinary　cultivation？　　If　a　character　appears；　in　these　circumstances；　in　the　case　of　all　individuals；　we　then　speak　of　constant　races。　　In　such　simple　cases　the　essential　point　is　not　the　creation　of　a　new　character　but　rather　an　ALTERATION　OF　THIS　CHARACTER　IN　ACCORDANCE　WITH　THE　ENVIRONMENT。　　In　the　examples　mentioned　the　modified　character　in　the　simple　varieties　（or　a　number　of　characters　in　elementary　species）　appears　more　or　less　suddenly　and　is　constant　in　the　above　sense。　　The　result　is　what　de　Vries　has　termed　a　Mutation。　　In　this　connection　we　must　bear　in　mind　the　fact　that　no　difference；　recognisable　externally；　need　exist　between　individual　variation　and　mutation。　　Even　the　most　minute　quantitative　difference　between　two　plants　may　be　of　specific　value　if　it　is　preserved　under　similar　external　conditions　during　many　successive　generations。　　We　do　not　know　how　this　happens。　　We　may　state　the　problem　in　other　terms；　by　saying　that　the　specific　structure　must　be　altered。　　It　is　possible；　to　some　extent；　to　explain　this　sudden　alteration；　if　we　regard　it　as　a　chemical　alteration　of　structure　either　in　the　specific　qualities　of　the　proteids　or　of　the　unknown　carriers　of　life。　　In　the　case　of　many　organic　compounds　their　morphological　characters　（the　physical　condition；　crystalline　form；　etc。）　are　at　once　changed　by　alteration　of　atomic　relations　or　by　incorporation　of　new　radicals。　　（For　instance　ethylchloride　（C2H5Cl）　is　a　gas　at　21　deg　C。；　ethylenechloride　（C2H4Cl2）　a　fluid　boiling　at　84　deg　C。；　beta　trichlorethane　（C2H3Cl3）　a　fluid　boiling　at　113　deg　C。；　perchlorethane　（C2Cl6）　a　crystalline　substance。　　Klebs；　（〃Willkurliche　Entwickelungsanderungen〃　page　158。）　　Much　more　important；　however；　would　be　an　answer　to　the　question；　whether　an　individual　variation　can　be　converted　experimentally　into　an　inherited　charactera　mutation　in　de　Vries's　sense。

In　all　circumstances　we　may　recognise　as　a　guiding　principle　the　assumption　adopted　by　Lamarck；　Darwin；　and　many　others；　that　the　inheritance　of　any　one　character；　or　in　more　general　terms；　the　transformation　of　one　species　into　another；　is；　in　the　last　instance；　to　be　referred　to　a　change　in　the　environment。　　From　a　causal…mechanical　point　of　view　it　is　not　a　priori　conceivable　that　one　species　can　ever　become　changed　into　another　so　long　as　external　conditions　remain　constant。　　The　inner　structure　of　a　species　must　be　essentially　altered　by　external　influences。　　Two　methods　of　experimental　research　may　be　adopted；　the　effect　of　crossing　distinct　species　and；　secondly；　the　effect　of　definite　factors　of　the　environment。

The　subject　of　hybridisation　is　dealt　with　in　another　part　of　this　essay。　　It　is　enough　to　refer　here　to　the　most　important　fact；　that　as　the　result　of　combinations　of　characters　of　different　species　new　and　constant　forms　are　produced。　　Further；　Tschermack；　Bateson　and　others　have　demonstrated　the　possibility　that　hitherto　unknown　inheritable　characters　may　be　produced　by　hybridisation。

The　other　method　of　producing　constant　races　by　the　influence　of　special　external　conditions　has　often　been　employed。　　The　sporeless　races　of　Bacteria　and　Yeasts　（Cf。　Detto；　〃Die　Theorie　der　direkten　Anpassung。。。〃；　pages　98　et　seq。；　Jena；　1904；　see　also　Lotsy；　〃Vorlesungen〃；　II。　pages　636　et　seq。；　where　other　similar　cases　are　described。）　are　well　known；　in　which　an　internal　alteration　of　the　cells　is　induced　by　the　influence　of　poison　or　higher　temperature；　so　that　the　power　of　producing　spores　even　under　normal　conditions　appears　to　be　lost。　　A　similar　state　of　things　is　found　in　some　races　which　under　certain　definite　conditions　lose　their　colour　or　their　virulence。　　Among　the　phanerogams　the　investigations　of　Schubler　on　cereals　afford　parallel　cases；　in　which　the　influence　of　a　northern　climate　produces　individuals　which　ripen　their　seeds　early；　these　seeds　produce　plants　which　seed　early　in　southern　countries。　　Analogous　results　were　obtained　by　Cieslar　in　his　experiments；　seeds　of　conifers　from　the　Alps　when　planted　in　the　plains　produced　plants　of　slow　growth　and　small　diameter。

All　these　observations　are　of　considerable　interest　theoretically；　they　show　that　the　action　of　environment　certainly　induces　such　internal　changes；　and　that　these　are　transmitted　to　the　next　generation。　　But　as　regards　the　main　question；　whether　constant　races　may　be　obtained　by　this　means；　the　experiments　cannot　as　yet　supply　a　definite　answer。　　In　phanerogams；　the　influence　very　soon　dies　out　in　succeeding　generations；　in　the　case　of　bacteria；　in　which　it　is　only　a　question　of　the　loss　of　a　character　it　is　relatively　easy　for　this　to　reappear。　　It　is　not　impossible；　that　in　all　such　cases　there　is　a　material　hanging…on　of　certain　internal　conditions；　in　consequence　of　which　the　modification　of　the　character　persists　for　a　time　in　the　descendants；　although　the　original　external　conditions　are　no　longer　present。

Thus　a　slow　dying…out　of　the　effect　of　a　stimulus　was　seen　in　my　experiments　on　Veronica　chamaedrys。　　（Klebs；　〃Kunstliche　Metamorphosen〃；　Stuttgart；　1906；　page　132。）　　During　the　cultivation　of　an　artificially　modified　inflorescence　I　obtained　a　race　showing　modifications　in　different　directions；　among　which　twisting　was　especially　conspicuous。　　This　plant；　however；　does　not　behave　as　the　twisted　race　of　Dipsacus　isolated　by　de　Vries　（de　Vries；　〃Mutationstheorie〃；　Vol。　II。　Leipzig；　1903；　page　573。）；　which　produced　each　year　a　definite　percentage　of　twisted　individuals。　　In　the　vegetative　reproduction　of　this　Veronica　the　torsion　appeared　in　the　first；　also　in　the　second　and　third　year；　but　with　diminishing　intensity。　　In　spite　of　good　cultivation　this　character　has　apparently　now　disappeared；　it　disappeared　still　more　quickly　in　seedlings。　　In　another　character　of　the　same　Veronica　chamaedrys　the　influence　of　the　environment　was　stronger。　　The　transformation　of　the　inflorescences　to　foliage…shoots　formed　the　starting…point；　it　occurred　only　under　narrowly　defined　conditions；　namely　on　cultivation　as　a　cutting　in　moist　air　and　on　removal　of　all　other　leaf…　buds。　　In　the　majority　（7/10）　of　the　plants　obtained　from　the　transformed　shoots；　the　modification　appeared　in　the　following　year　without　any　interference。　　Of　the　three　plants　which　were　under　observation　several　years　the　first　lost　the　character　in　a　short　time；　while　the　two　others　still　retain　it；　after　vegetative　propagation；　in　varying　degrees。　　The　same　character　occurs　also　in　some　of　the　seedlings；　but　anything　approaching　a　constant　race　has