darwin and modern science-第87章

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gart；　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　hereditary　characteristics；　and　this　can　only　be　accomplished　on　the　basis　of　a　systematic　study　of　the　effects　of　chemical　and　physical　forces　upon　living　matter。

Owing　to　limitation　of　space　this　sketch　is　necessarily　very　incomplete；　and　it　must　not　be　inferred　that　studies　which　are　not　mentioned　here　were　considered　to　be　of　minor　importance。　　All　the　writer　could　hope　to　do　was　to　bring　together　a　few　instances　of　the　experimental　analysis　of　the　effect　of　environment；　which　indicate　the　nature　and　extent　of　our　control　over　life…phenomena　and　which　also　have　some　relation　to　the　work　of　Darwin。　　In　the　selection　of　these　instances　preference　is　given　to　those　problems　which　are　not　too　technical　for　the　general　reader。

The　forces；　the　influence　of　which　we　shall　discuss；　are　in　succession　chemical　agencies；　temperature；　light；　and　gravitation。　　We　shall　also　treat　separately　the　effect　of　these　forces　upon　form　and　instinctive　reactions。

II。　　THE　EFFECTS　OF　CHEMICAL　AGENCIES。

（a）　　HETEROGENEOUS　HYBRIDISATION。

It　was　held　until　recently　that　hybridisation　is　not　possible　except　between　closely　related　species　and　that　even　among　these　a　successful　hybridisation　cannot　always　be　counted　upon。　　This　view　was　well　supported　by　experience。　　It　is；　for　instance；　well　known　that　the　majority　of　marine　animals　lay　their　unfertilised　eggs　in　the　ocean　and　that　the　males　shed　their　sperm　also　into　the　sea…water。　　The　numerical　excess　of　the　spermatozoa　over　the　ova　in　the　sea…water　is　the　only　guarantee　that　the　eggs　are　fertilised；　for　the　spermatozoa　are　carried　to　the　eggs　by　chance　and　are　not　attracted　by　the　latter。　　This　statement　is　the　result　of　numerous　experiments　by　various　authors；　and　is　contrary　to　common　belief。　As　a　rule　all　or　the　majority　of　individuals　of　a　species　in　a　given　region　spawn　on　the　same　day；　and　when　this　occurs　the　sea…water　constitutes　a　veritable　suspension　of　sperm。　　It　has　been　shown　by　experiment　that　in　fresh　sea…water　the　sperm　may　live　and　retain　its　fertilising　power　for　several　days。　　It　is　thus　unavoidable　that　at　certain　periods　more　than　one　kind　of　spermatozoon　is　suspended　in　the　sea…water　and　it　is　a　matter　of　surprise　that　the　most　heterogeneous　hybridisations　do　not　constantly　occur。　　The　reason　for　this　becomes　obvious　if　we　bring　together　mature　eggs　and　equally　mature　and　active　sperm　of　a　different　family。　　When　this　is　done　no　egg　is；　as　a　rule；　fertilised。　　The　eggs　of　a　sea…urchin　can　be　fertilised　by　sperm　of　their　own　species；　or；　though　in　smaller　numbers；　by　the　sperm　of　other　species　of　sea…urchins；　but　not　by　the　sperm　of　other　groups　of　echinoderms；　e。g。　starfish；　brittle…stars；　holothurians　or　crinoids；　and　still　less　by　the　sperm　of　more　distant　groups　of　animals。　　The　consensus　of　opinion　seemed　to　be　that　the　spermatozoon　must　enter　the　egg　through　a　narrow　opening　or　canal；　the　so…called　micropyle；　and　that　the　micropyle　allowed　only　the　spermatozoa　of　the　same　or　of　a　closely　related　species　to　enter　the　egg。

It　seemed　to　the　writer　that