darwin and modern science-第82节

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t　in　all　directions。　　As　a　rule；　the　centre　of　the　cell　contains　one　or　more　different　pigments　which　under　the　influence　of　nerves　can　spread　out　separately　or　together　into　the　ramifications。　　These　phenomena　of　spreading　and　retraction　of　the　pigments　into　or　from　the　ramifications　of　the　pigment…cells　form　on　the　whole　the　basis　for　the　colour　changes　under　the　influence　of　environment。　　Thus　Keeble　and　Gamble　observed　that　Macromysis　flexuosa　appears　transparent　and　colourless　or　grey　on　sandy　ground。　　On　a　dark　ground　their　colour　becomes　darker。　　These　animals　have　two　pigments　in　their　chromatophores；　a　brown　pigment　and　a　whitish　or　yellow　pigment；　the　former　is　much　more　plentiful　than　the　latter。　　When　the　animal　appears　transparent　all　the　pigment　is　contained　in　the　centre　of　the　cells；　while　the　ramifications　are　free　from　pigment。　　When　the　animal　appears　brown　both　pigments　are　spread　out　into　the　ramifications。　　In　the　condition　of　maximal　spreading　the　animals　appear　black。

This　is　a　comparatively　simple　case。　　Much　more　complicated　conditions　were　found　by　Keeble　and　Gamble　in　other　crustaceans；　e。g。　in　Hippolyte　cranchii；　but　the　influence　of　the　surroundings　upon　the　colouration　of　this　form　was　also　satisfactorily　analysed　by　these　authors。

While　many　animals　show　transitory　changes　in　colour　under　the　influence　of　their　surroundings；　in　a　few　cases　permanent　changes　can　be　produced。　　The　best　examples　of　this　are　those　which　were　observed　by　Poulton　in　the　chrysalids　of　various　butterflies；　especially　the　small　tortoise…shell。　　These　experiments　are　so　well　known　that　a　short　reference　to　them　will　suffice。　　Poulton　（Poulton；　E。B。；　〃Colours　of　Animals〃　（The　International　Scientific　Series）；　London；　1890；　page　121。）　found　that　in　gilt　or　white　surroundings　the　pupae　became　light　coloured　and　there　was　often　an　immense　development　of　the　golden　spots；　〃so　that　in　many　cases　the　whole　surface　of　the　pupae　glittered　with　an　apparent　metallic　lustre。　　So　remarkable　was　the　appearance　that　a　physicist　to　whom　I　showed　the　chrysalids；　suggested　that　I　had　played　a　trick　and　had　covered　them　with　goldleaf。〃　　When　black　surroundings　were　used　〃the　pupae　were　as　a　rule　extremely　dark；　with　only　the　smallest　trace；　and　often　no　trace　at　all；　of　the　golden　spots　which　are　so　conspicuous　in　the　lighter　form。〃　　The　susceptibility　of　the　animal　to　this　influence　of　its　surroundings　was　found　to　be　greatest　during　a　definite　period　when　the　caterpillar　undergoes　the　metamorphosis　into　the　chrysalis　stage。　　As　far　as　the　writer　is　aware；　no　physico…chemical　explanation；　except　possibly　Wiener's　suggestion　of　colour…photography　by　mechanical　colour　adaptation；　has　ever　been　offered　for　the　results　of　the　type　of　those　observed　by　Poulton。

V。　　EFFECTS　OF　GRAVITATION。

（a）　　EXPERIMENTS　ON　THE　EGG　OF　THE　FROG。

Gravitation　can　only　indirectly　affect　life…phenomena；　namely；　when　we　have　in　a　cell　two　different　non…miscible　liquids　（or　a　liquid　and　a　solid）　of　different　specific　gravity；　so　that　a　change　in　the　position　of　the　cell　or　the　organ　may　give　results　which　can　be　traced　to　a　change　in　the　position　of　the　two　substances。　　This　is　very　nicely　illustrated　by　the　frog's　egg；　which　has　two　layers　of　very　viscous　protoplasm　one　of　which　is　black　and　one　white。　　The　dark　one　occupies　normally　the　upper　position　in　the　egg　and　may　therefore　be　assumed　to　possess　a　smaller　specific　gravity　than　the　white　substance。　　When　the　egg　is　turned　with　the　white　pole　upwards　a　tendency　of　the　white　protoplasm　to　flow　down　again　manifests　itself。　　It　is；　however；　possible　to　prevent　or　retard　this　rotation　of　the　highly　viscous　protoplasm；　by　compressing　the　eggs　between　horizontal　glass　plates。　　Such　compression　experiments　may　lead　to　rather　interesting　results；　as　O。　Schultze　first　pointed　out。　　Pflueger　had　already　shown　that　the　first　plane　of　division　in　a　fertilised　frog's　egg　is　vertical　and　Roux　established　the　fact　that　the　first　plane　of　division　is　identical　with　the　plane　of　symmetry　of　the　later　embryo。　　Schultze　found　that　if　the　frog's　egg　is　turned　upside　down　at　the　time　of　its　first　division　and　kept　in　this　abnormal　position；　through　compression　between　two　glass　plates　for　about　20　hours；　a　small　number　of　eggs　may　give　rise　to　twins。　　It　is　possible；　in　this　case；　that　the　tendency　of　the　black　part　of　the　egg　to　rotate　upwards　along　the　surface　of　the　egg　leads　to　a　separation　of　its　first　cells；　such　a　separation　leading　to　the　formation　of　twins。

T。H。　Morgan　made　an　interesting　additional　observation。　　He　destroyed　one　half　of　the　egg　after　the　first　segmentation　and　found　that　the　half　which　remained　alive　gave　rise　to　only　one　half　of　an　embryo；　thus　confirming　an　older　observation　of　Roux。　　When；　however；　Morgan　put　the　egg　upside　down　after　the　destruction　of　one　of　the　first　two　cells；　and　compressed　the　eggs　between　two　glass　plates；　the　surviving　half　of　the　egg　gave　rise　to　a　perfect　embryo　of　half　size　（and　not　to　a　half　embryo　of　normal　size　as　before。）　　Obviously　in　this　case　the　tendency　of　the　protoplasm　to　flow　back　to　its　normal　position　was　partially　successful　and　led　to　a　partial　or　complete　separation　of　the　living　from　the　dead　half；　whereby　the　former　was　enabled　to　form　a　whole　embryo；　which；　of　course；　possessed　only　half　the　size　of　an　embryo　originating　from　a　whole　egg。

（b）　　EXPERIMENTS　ON　HYDROIDS。

A　striking　influence　of　gravitation　can　be　observed　in　a　hydroid；　Antennularia　antennina；　from　the　bay　of　Naples。　　This　hydroid　consists　of　a　long　straight　main　stem　which　grows　vertically　upwards　and　which　has　at　regular　intervals　very　fine　and　short　bristle…like　lateral　branches；　on　the　upper　side　of　which　the　polyps　grow。　　The　main　stem　is　negatively　geotropic；　i。e。　its　apex　continues　to　grow　vertically　upwards　when　we　put　it　obliquely　into　the　aquarium；　while　the　roots　grow　vertically　downwards。　The　writer　observed　that　when　the　stem　is　put　horizontally　into　the　water　the　short　lateral　branches　on　the　lower　side　give　rise　to　an　altogether　different　kind　of　organ；　namely；　to　roots；　and　these　roots　grow　indefinitely　in　length　and　attach　themselves　to　solid　bodies；　while　if　the　stem　had　remained　in　its　normal　position　no　further　growth　would　have　occurred　in　the　lateral　branches。　　From　the　upper　side　of　the　horizontal　stem　new　stems　grow　out；　mostly　directly　from　the　original　stem；　occasionally　also　from　the　short　lateral　branches。　　It　is　thus　possible　to　force　upon　this　hydroid　an　arrangement　of　organs　which　is　altogether　different　from　the　hereditary　arrangement。　　The　writer　had　called　the　change　in　the　hereditary　arrangement　of　organs　or　the　transformation　of　organs　by　external　forces　HETEROMORPHOSIS。　　We　cannot　now　go　any　further　into　this　subject；　which　should；　however；　prove　of　interest　in　relation　to　the　problem　of　heredity。

If　it　is　correct　to　apply　inferences　drawn　from　the　observation　on　the　frog's　egg　to　the　behaviour　of　Antennularia；　one　might　conclude　that　the　cells　of　Antennularia　also　contain　non…miscible　substances　of　different　specific　gravity；　and　that　wherever　the　specifically　lighter　substance　comes　in　contact　with　the　sea…water　（or　gets　near　the　surface　of　the　cell）　the　growth　of　a　stem　is　favoured；　while　contact　with　the　sea…water　of　the　specifically　heavier　of　the　substances；　will　favour　the　formation　of　roots。

VI。　　THE　EXPERIMENTAL　CONTROL　OF　ANIMAL　INSTINCTS。

（a）　　EXPERIMENTS　ON　THE　MECHANISM　OF　HELIOTROPIC　REACTIONS　IN　ANIMALS。

Since　the　instinctive　reactions　of　animals　are　as　hereditary　as　their　morphological　character；　a　discussion　of　experiments　on　the　physico…　chemical　character　of　the　instinctive　reactions　of　animals　should　not　be　entirely　omitted　from　this　sketch。　　It　is　obvious　that　such　experiments　must　begin　with　the　simplest　type　of　instincts；　if　they　are　expected　to　lead　to　any　results；　and　it　is　also　obvious　that　only　such　animals　must　be　selected　for　this　purpose；　the　reactions　of　which　are　not　complicated　by　associative　memory；　or；　as　it　may　preferably　be　termed；　associative　hysteresis。

The　simplest　type　of　instincts　is　represented　by　the　purposeful　motions　of　animals　to　or　from　a　source　of　energy；　e。g。　light；　and　it　is　with　some　of　these　that　we　intend　to　deal　here。　　When　we　expose　winged　aphides　（after　they　have　flown　away　from　the　plant）；　or　young　caterpillars　of　Porthesia　chrysorrhoea　（when　they　are　aroused　from　their　winter　sleep）　or　marine　or　freshwater　copepods　and　many　other　animals；　to　diffused　daylight　falling　in　from　a　window；　we　notice　a　tendency　among　these　animals　to　move　towards　the　source　of　light。　　If　the　animals　are　naturally　sensitive；　or　if　they　are　rendered　sensitive　through　the　agencies　which　we　shall　mention　later；　and　if　the　light　is　strong　enough；　they　move　towards　the　source　of　light　in　as　straight　a　line　as　the　imperfections　and　peculiarities　of　their　locomotor　apparatus　will　permit。　　It　is　also　obvious　that　we　are　here　dealing　with　a　forced　reaction　in　which　the　animals　have　no　more　choice　in　the　direction　of　their　motion　than　have　the　iron　filings　in　their　arrangement　in　a　magnetic　field。　　This　can　be　proved　very　nicely　in　the　case　of　starving　caterpillars　of　Porthesia。　　The　writer　put　such　cater