darwin and modern science-第77节

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he　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。　　After　a　few　years　of　cultivation　this　〃good〃　and　constant　species　becomes　highly　variable。　　The　specimens　on　which　the　experiments　were　made　were　three　modified　inflorescence　cuttings；　the　parent…plants　of　which　certainly　exhibited　no　striking　abnormalities。　　In　a　short　time　many　hitherto　latent　potentialities　became　apparent；　so　that　characters；　never　previously　observed；　or　at　least　very　rarely；　were　exhibited；　such　as　scattered　leaf…　arrangement；　torsion；　terminal　or　branched　inflorescences；　the　conversion　of　the　inflorescence　into　foliage…shoots；　every　conceivable　alteration　in　the　colour　of　flowers；　the　assumption　of　a　green　colour　by　parts　of　the　flowers；　the　proliferation　of　flowers。

All　this　points　to　some　disturbance　in　the　species　resulting　from　methods　of　cultivation。　　It　has；　however；　not　yet　been　possible　to　produce　constant　races　with　any　one　of　these　modified　characters。　　But　variations　appeared　among　the　seedlings；　some　of　which；　e。g。　yellow　variegation；　were　not　inheritable；　while　others　have　proved　constant。　　This　holds　good；　so　far　as　we　know　at　present；　for　a　small　rose…coloured　form　which　is　to　be　reckoned　as　a　mutation。　　Thus　the　prospect　of　producing　new　races　by　cultivation　appears　to　be　full　of　promise。

So　long　as　the　view　is　held　that　good　nourishment；　i。e。　a　plentiful　supply　of　water　and　salts；　constitutes　the　essential　characteristic　of　garden…　cultivation；　we　can　hardly　conceive　that　new　mutations　can　be　thus　produced。　　But　perhaps　the　view　here　put　forward　in　regard　to　the　production　of　form　throws　new　light　on　this　puzzling　problem。

Good　manuring　is　in　the　highest　degree　favourable　to　vegetative　growth；　but　is　in　no　way　equally　favourable　to　the　formation　of　flowers。　　The　constantly　repeated　expression；　good　or　favourable　nourishment；　is　not　only　vague　but　misleading；　because　circumstances　favourable　to　growth　differ　from　those　which　promote　reproduction；　for　the　production　of　every　form　there　are　certain　favourable　conditions　of　nourishment；　which　may　be　defined　for　each　species。　　Experience　shows　that；　within　definite　and　often　very　wide　limits；　it　does　not　depend　upon　the　ABSOLUTE　AMOUNT　of　the　various　food　substances；　but　upon　their　respective　degrees　of　concentration。　　As　we　have　already　stated；　the　production　of　flowers　follows　a　relative　increase　in　the　amount　of　carbohydrates　formed　in　the　presence　of　light；　as　compared　with　the　inorganic　salts　on　which　the　formation　of　albuminous　substances　depends。　　（Klebs；　〃Kunstliche　Metamorphosen〃；　page　117。）　　The　various　modifications　of　flowers　are　due　to　the　fact　that　a　relatively　too　strong　solution　of　salts　is　supplied　to　the　rudiments　of　these　organs。　　As　a　general　rule　every　plant　form　depends　upon　a　certain　relation　between　the　different　chemical　substances　in　the　cells　and　is　modified　by　an　alteration　of　that　relation。

During　long　cultivation　under　conditions　which　vary　in　very　different　degrees；　such　as　moisture；　the　amount　of　salts；　light　intensity；　temperature；　oxygen；　it　is　possible　that　sudden　and　special　disturbances　in　the　relations　of　the　cell　substances　have　a　directive　influence　on　the　inner　organisation　of　the　sexual　cells；　so　that　not　only　inconstant　but　also　constant　varieties　will　be　formed。

Definite　proof　in　support　of　this　view　has　not　yet　been　furnished；　and　we　must　admit　that　the　question　as　to　the　cause　of　heredity　remains；　fundamentally；　as　far　from　solution　as　it　was　in　Darwin's　time。　　As　the　result　of　the　work　of　many　investigators；　particularly　de　Vries；　the　problem　is　constantly　becoming　clearer　and　more　definite。　　The　penetration　into　this　most　difficult　and　therefore　most　interesting　problem　of　life　and　the　creation　by　experiment　of　new　races　or　elementary　species　are　no　longer　beyond　the　region　of　possibility。


XIV。　　EXPERIMENTAL　STUDY　OF　THE　INFLUENCE　OF　ENVIRONMENT　ON　ANIMALS。

By　JACQUES　LOEB；　M。D。　Professor　of　Physiology　in　the　University　of　California。

I。　INTRODUCTORY　REMARKS。

What　the　biologist　calls　the　natural　environment　of　an　animal　is　from　a　physical　point　of　view　a　rather　rigid　combination　of　definite　forces。　　It　is　obvious　that　by　a　purposeful　and　systematic　variation　of　these　and　by　the　application　of　other　forces　in　the　laboratory；　results　must　be　obtainable　which　do　not　appear　in　the　natural　environment。　　This　is　the　reasoning　underlying　the　modern　development　of　the　study　of　the　effects　of　environment　upon　animal　life。　　It　was　perhaps　not　the　least　important　of　Darwin's　services　to　science　that　the　boldness　of　his　conceptions　gave　to　the　experimental　biologist　courage　to　enter　upon　the　attempt　of　controlling　at　will　the　life…phenomena　of　animals；　and　of　bringing　about　effects　which　cannot　be　expected　in　Nature。

The　systematic　physico…chemical　analysis　of　the　effect　of　outside　forces　upon　the　form　and　reactions　of　animals　is　also　our　only　means　of　unravelling　the　mechanism　of　heredity　beyond　the　scope　of　the　Mendelian　law。　　The　manner　in　which　a　germ…cell　can　force　upon　the　adult　certain　characters　will　not　be　understood　until　we　succeed　in　varying　and　controlling　heredit