darwin and modern science-第86章

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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　not　been　produced。

Another　means　of　producing　new　races　has　been　attempted　by　Blaringhem。　　（Blaringhem；　〃Mutation　et　Traumatisme〃；　Paris；　1907。）　　On　removing　at　an　early　stage　the　main　shoots　of　different　plants　he　observed　various　abnormalities　in　the　newly　formed　basal　shoots。　　From　the　seeds　of　such　plants　he　obtained　races；　a　large　percentage　of　which　exhibited　these　abnormalities。　　Starting　from　a　male　Maize　plant　with　a　fasciated　inflorescence；　on　which　a　proportion　of　the　flowers　had　become　male；　a　new　race　was　bred　in　which　hermaphrodite　flowers　were　frequently　produced。　　In　the　same　way　Blaringhem　obtained；　among　other　similar　results；　a　race　of　barley　with　branched　ears。　　These　races；　however；　behaved　in　essentials　like　those　which　have　been　demonstrated　by　de　Vries　to　be　inconstant；　e。g。　Trifolium　pratense　quinquefolium　and　others。　　The　abnormality　appears　in　a　proportion　of　the　individuals　and　only　under　very　special　conditions。　　It　must　be　remembered　too　that　Blaringhem　worked　with　old　cultivated　plants；　which　from　the　first　had　been　disposed　to　split　into　a　great　variety　of　races。　　It　is　possible；　but　difficult　to　prove；　that　injury　contributed　to　this　result。

A　third　method　has　been　adopted　by　MacDougal　（MacDougal；　〃Heredity　and　Origin　of　species〃；　〃Monist〃；　1906；　〃Report　of　department　of　botanical　research〃；　〃Fifth　Year…book　of　the　Carnegie　Institution　of　Washington〃；　page　119；　1907。）　who　injected　strong　（10　percent）　sugar　solution　or　weak　solutions　of　calcium　nitrate　and　zinc　sulphate　into　young　carpels　of　different　plants。　　From　the　seeds　of　a　plant　of　Raimannia　odorata　the　carpels　of　which　had　been　thus　treated　he　obtained　several　plants　distinguished　from　the　parent…forms　by　the　absence　of　hairs　and　by　distinct　forms　of　leaves。　　Further　examination　showed　that　he　had　here　to　do　with　a　new　elementary　species。　　MacDougal　also　obtained　a　more　or　less　distinct　mutant　of　Oenothera　biennis。　　We　cannot　as　yet　form　an　opinion　as　to　how　far　the　effect　is　due　to　the　wound　or　to　the　injection　of　fluid　as　such；　or　to　its　chemical　properties。　　This；　however；　is　not　so　essential　as　to　decide　whether　the　mutant　stands　in　any　relation　to　the　influence　of　external　factors。　　It　is　at　any　rate　very　important　that　this　kind　of　investigation　should　be　carried　further。

If　it　could　be　shown　that　new　and　inherited　races　were　obtained　by　MacDougal's　method；　it　would　be　safe　to　conclude　that　the　same　end　might　be　gained　by　altering　the　conditions　of　the　food…stuff　conducted　to　the　sexual　cells。　　New　races　or　elementary　species；　however；　arise　without　wounding　or　injection。　　This　at　once　raises　the　much　discussed　question；　how　far　garden…cultivation　has　led　to　the　creation　of　new　races？　　Contrary　to　the　opinion　expressed　by　Darwin　and　others；　de　Vries　（〃Mutationstheorie〃；　Vol。　I。　pages　412　et　seq。）　tried　to　show　that　garden…races　have　been　produced　only　from　spontaneous　types　which　occur　in　a　wild　state　or　from　sub…races；　which　the　breeder　has　accidentally　discovered　but　not　originated。　　In　a　small　number　of　cases　only　has　de　Vries　adduced　definite　proof。　　On　the　other　side　we　have　the　work　of　Korschinsky　（Korschinsky；　〃Heterogenesis　und　Evolution〃；　〃Flora〃；　1901。）　which　shows　that　whole　series　of　garden…races　have　made　their　appearance　only　after　years　of　cultivation。　　In　the　majority　of　races　we　are　entirely　ignorant　of　their　origin。

It　is；　however；　a　fact　that　if　a　plant　is　removed　from　natural　conditions　into　cultivation；　a　well…marked　variation　occurs。　　The　well…known　plant…　breeder　L。　de　Vilmorin　（L。　de　Vilmorin；　〃Notices　sur　l'amelioration　des　plantes〃；　Paris；　1886；　page　36。）；　speaking　from　his　own　experience；　states　that　a　plant　is　induced　to　〃affoler；〃　that　is　to　exhibit　all　possible　variations　from　which　the　breeder　may　make　a　further　selection　only　after　cultivation　for　several　generations。　　The　effect　of　cultivation　was　particularly　striking　in　Veronica　chamaedrys　（Klebs；　〃Kunstliche　Metamorphosen〃；　Stuttgart；　1906；　page　152。）　which；　in　spite　of　its　wide　distribution　in　nature；　varies　very　little。　　Afte