darwin and modern science-第80节

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The　experiments　of　Driesch　and　others　have　taught　us　that　twins　originate　from　one　egg　in　this　manner；　namely；　that　the　first　two　cells　into　which　the　egg　divides　after　fertilisation　become　separated　from　each　other。　　This　separation　can　be　brought　about　by　a　change　in　the　chemical　constitution　of　the　sea…water。　　Herbst　observed　that　if　the　fertilised　eggs　of　the　sea…　urchin　are　put　into　sea…water　which　is　freed　from　calcium；　the　cells　into　which　the　egg　divides　have　a　tendency　to　fall　apart。　　Driesch　afterwards　noticed　that　eggs　of　the　sea…urchin　treated　with　sea…water　which　is　free　from　lime　have　a　tendency　to　give　rise　to　twins。　　The　writer　has　recently　found　that　twins　can　be　produced　not　only　by　the　absence　of　lime；　but　also　through　the　absence　of　sodium　or　of　potassium；　in　other　words；　through　the　absence　of　one　or　two　of　the　three　important　metals　in　the　sea…water。　　There　is；　however；　a　second　condition；　namely；　that　the　solution　used　for　the　production　of　twins　must　have　a　neutral　or　at　least　not　an　alkaline　reaction。

The　procedure　for　the　production　of　twins　in　the　sea…urchin　egg　consists　simply　in　this：the　eggs　are　fertilised　as　usual　in　normal　sea…water　and　then；　after　repeated　washing　in　a　neutral　solution　of　sodium　chloride　（of　the　concentration　of　the　sea…water）；　are　placed　in　a　neutral　mixture　of　potassium　chloride　and　calcium　chloride；　or　of　sodium　chloride　and　potassium　chloride；　or　of　sodium　chloride　and　calcium　chloride；　or　of　sodium　chloride　and　magnesium　chloride。　　The　eggs　must　remain　in　this　solution　until　half　an　hour　or　an　hour　after　they　have　reached　the　two…cell　stage。　　They　are　then　transferred　into　normal　sea…water　and　allowed　to　develop。　　From　50　to　90　per　cent　of　the　eggs　of　Strongylocentrotus　purpuratus　treated　in　this　manner　may　develop　into　twins。　　These　twins　may　remain　separate　or　grow　partially　together　and　form　double　monsters；　or　heal　together　so　completely　that　only　slight　or　even　no　imperfections　indicate　that　the　individual　started　its　career　as　a　pair　of　twins。　　It　is　also　possible　to　control　the　tendency　of　such　twins　to　grow　together　by　a　change　in　the　constitution　of　the　sea…water。　　If　we　use　as　a　twin…producing　solution　a　mixture　of　sodium；　magnesium　and　potassium　chlorides　（in　the　proportion　in　which　these　salts　exist　in　the　sea…water）　the　tendency　of　the　twins　to　grow　together　is　much　more　pronounced　than　if　we　use　simply　a　mixture　of　sodium　chloride　and　magnesium　chloride。

The　mechanism　of　the　origin　of　twins；　as　the　result　of　altering　the　composition　of　the　sea…water；　is　revealed　by　observation　of　the　first　segmentation　of　the　egg　in　these　solutions。　　This　cell…division　is　modified　in　a　way　which　leads　to　a　separation　of　the　first　two　cells。　　If　the　egg　is　afterwards　transferred　back　into　normal　sea…water；　each　of　these　two　cells　develops　into　an　independent　embryo。　　Since　normal　sea…water　contains　all　three　metals；　sodium；　calcium；　and　potassium；　and　since　it　has　besides　an　alkaline　reaction；　we　perceive　the　reason　why　twins　are　not　normally　produced　from　one　egg。　　These　experiments　suggest　the　possibility　of　a　chemical　cause　for　the　origin　of　twins　from　one　egg　or　of　double　monstrosities　in　mammals。　　If；　for　some　reason；　the　liquids　which　surround　the　human　egg　a　short　time　before　and　after　the　first　cell…division　are　slightly　acid；　and　at　the　same　time　lacking　in　one　of　the　three　important　metals；　the　conditions　for　the　separation　of　the　first　two　cells　and　the　formation　of　identical　twins　are　provided。

In　conclusion　it　may　be　pointed　out　that　the　reverse　result；　namely；　the　fusion　of　normally　double　organs；　can　also　be　brought　about　experimentally　through　a　change　in　the　chemical　constitution　of　the　sea…water。　　Stockard　succeeded　in　causing　the　eyes　of　fish　embryos　（Fundulus　heteroclitus）　to　fuse　into　a　single　cyclopean　eye　through　the　addition　of　magnesium　chloride　to　the　sea…water。　　When　he　added　about　6　grams　of　magnesium　chloride　to　100　cubic　centimetres　of　sea…water　and　placed　the　fertilised　eggs　in　the　mixture；　about　50　per　cent　of　the　eggs　gave　rise　to　one…eyed　embryos。　　〃When　the　embryos　were　studied　the　one…eyed　condition　was　found　to　result　from　the　union　or　fusion　of　the　'anlagen'　of　the　two　eyes。　　Cases　were　observed　which　showed　various　degrees　in　this　fusion；　it　appeared　as　though　the　optic　vessels　were　formed　too　far　forward　and　ventral；　so　that　their　antero…ventro…median　surfaces　fused。　　This　produces　one　large　optic　cup；　which　in　all　cases　gives　more　or　less　evidence　of　its　double　nature。〃　　（Stockard；　〃Archiv　f。　Entwickelungsmechanik〃；　Vol。　23；　page　249；　1907。）

We　have　confined　ourselves　to　a　discussion　of　rather　simple　effects　of　the　change　in　the　constitution　of　the　sea…water　upon　development。　　It　is　a　priori　obvious；　however；　that　an　unlimited　number　of　pathological　variations　might　be　produced　by　a　variation　in　the　concentration　and　constitution　of　the　sea…water；　and　experience　confirms　this　statement。　　As　an　example　we　may　mention　the　abnormalities　observed　by　Herbst　in　the　development　of　sea…urchins　through　the　addition　of　lithium　to　sea…water。　　It　is；　however；　as　yet　impossible　to　connect　in　a　rational　way　the　effects　produced　in　this　and　similar　cases　with　the　cause　which　produced　them；　and　it　is　also　impossible　to　define　in　a　simple　way　the　character　of　the　change　produced。

III。　　THE　INFLUENCE　OF　TEMPERATURE。

（a）　　THE　INFLUENCE　OF　TEMPERATURE　UPON　THE　DENSITY　OF　PELAGIC　ORGANISMS　AND　THE　DURATION　OF　LIFE。

It　has　often　been　noticed　by　explorers　who　have　had　a　chance　to　compare　the　faunas　in　different　climates　that　in　polar　seas　such　species　as　thrive　at　all　in　those　regions　occur；　as　a　rule；　in　much　greater　density　than　they　do　in　the　moderate　or　warmer　regions　of　the　ocean。　　This　refers　to　those　members　of　the　fauna　which　live　at　or　near　the　surface；　since　they　alone　lend　themselves　to　a　statistical　comparison。　　In　his　account　of　the　Valdivia　expedition；　Chun　（Chun；　〃Aus　den　Tiefen　des　Weltmeeres〃；　page　225；　Jena；　1903。）　calls　especial　attention　to　this　quantitative　difference　in　the　surface　fauna　and　flora　of　different　regions。　　〃In　the　icy　water　of　the　Antarctic；　the　temperature　of　which　is　below　0　deg　C。；　we　find　an　astonishingly　rich　animal　and　plant　life。　　The　same　condition　with　which　we　are　familiar　in　the　Arctic　seas　is　repeated　here；　namely；　that　the　quantity　of　plankton　material　exceeds　that　of　the　temperate　and　warm　seas。〃　　And　again；　in　regard　to　the　pelagic　fauna　in　the　region　of　the　Kerguelen　Islands；　he　states：　　〃The　ocean　is　alive　with　transparent　jelly　fish；　Ctenophores　（Bolina　and　Callianira）　and　of　Siphonophore　colonies　of　the　genus　Agalma。〃

The　paradoxical　character　of　this　general　observation　lies　in　the　fact　that　a　low　temperature　retards　development；　and　hence　should　be　expected　to　have　the　opposite　effect　from　that　mentioned　by　Chun。　　Recent　investigations　have　led　to　the　result　that　life…phenomena　are　affected　by　temperature　in　the　same　sense　as　the　velocity　of　chemical　reactions。　　In　the　case　of　the　latter　van't　Hoff　had　shown　that　a　decrease　in　temperature　by　10　degrees　reduces　their　velocity　to　one　half　or　less；　and　the　same　has　been　found　for　the　influence　of　temperature　on　the　velocity　of　physiological　processes。　　Thus　Snyder　and　T。B。　Robertson　found　that　the　rate　of　heartbeat　in　the　tortoise　and　in　Daphnia　is　reduced　to　about　one…half　if　the　temperature　is　lowered　10　deg　C。；　and　Maxwell；　Keith　Lucas；　and　Snyder　found　the　same　influence　of　temperature　for　the　rate　with　which　an　impulse　travels　in　the　nerve。　　Peter　observed　that　the　rate　of　development　in　a　sea…urchin's　egg　is　reduced　to　less　than　one…half　if　the　temperature　（within　certain　limits）　is　reduced　by　10　degrees。　　The　same　effect　of　temperature　upon　the　rate　of　development　holds　for　the　egg　of　the　frog；　as　Cohen　and　Peter　calculated　from　the　experiments　of　O。　Hertwig。　　The　writer　found　the　same　temperature…　coefficient　for　the　rate　of　maturation　of　the　egg　of　a　mollusc　（Lottia）。

All　these　facts　prove　that　the　velocity　of　development　of　animal　life　in　Arctic　regions；　where　the　temperature　is　near　the　freezing　point　of　water；　must　be　from　two　to　three　times　smaller　than　in　regions　where　the　temperature　of　the　ocean　is　about　10　deg　C。　and　from　four　to　nine　times　smaller　than　in　seas　the　temperature　of　which　is　about　20　deg　C。　　It　is；　therefore；　exactly　the　reverse　of　what　we　should　expect　when　authors　state　that　the　density　of　organisms　at　or　near　the　surface　of　the　ocean　in　polar　regions　is　greater　than　in　more　temperate　regions。

The　writer　believes　that　this　paradox　finds　its　explanation　in　experiments　which　he　has　recently　made　on　the　influence　of　temperature　on　the　duration　of　life　of　cold…blooded　marine　animals。　　The　experiments　were　made　on　the　fertilised　and　unfertilised　eggs　of　the　sea…urchin；　and　yielded　the　result　that　for　the　lowering　of　temperature　by　1　deg　C。　the　duration　of　life　was　about　doubled。　　Lowering　the　temperature　by　10　degrees　therefore　prolongs　the　life　of　the　organism　2　to　the　power　10；　i。e。　over　a　thousand　times；　and　a　lowering　by　20　degrees　prolongs　it　about　one　million　times。　　Since　this　prolongation　of　life　is　far　in　excess　of　the　retardation　of　development　through　a　lowering　of　temperature；　it　is　obvious　that；