darwin and modern science-第83节

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　be　proved　very　nicely　in　the　case　of　starving　caterpillars　of　Porthesia。　　The　writer　put　such　caterpillars　into　a　glass　tube　the　axis　of　which　was　at　right　angles　to　the　plane　of　the　window：　　the　caterpillars　went　to　the　window　side　of　the　tube　and　remained　there；　even　if　leaves　of　their　food…plant　were　put　into　the　tube　directly　behind　them。　　Under　such　conditions　the　animals　actually　died　from　starvation；　the　light　preventing　them　from　turning　to　the　food；　which　they　eagerly　ate　when　the　light　allowed　them　to　do　so。　　One　cannot　say　that　these　animals；　which　we　call　positively　helioptropic；　are　attracted　by　the　light；　since　it　can　be　shown　that　they　go　towards　the　source　of　the　light　even　if　in　so　doing　they　move　from　places　of　a　higher　to　places　of　a　lower　degree　of　illumination。

The　writer　has　advanced　the　following　theory　of　these　instinctive　reactions。　　Animals　of　the　type　of　those　mentioned　are　automatically　orientated　by　the　light　in　such　a　way　that　symmetrical　elements　of　their　retina　（or　skin）　are　struck　by　the　rays　of　light　at　the　same　angle。　　In　this　case　the　intensity　of　light　is　the　same　for　both　retinae　or　symmetrical　parts　of　the　skin。

This　automatic　orientation　is　determined　by　two　factors；　first　a　peculiar　photo…sensitiveness　of　the　retina　（or　skin）；　and　second　a　peculiar　nervous　connection　between　the　retina　and　the　muscular　apparatus。　　In　symmetrically　built　heliotropic　animals　in　which　the　symmetrical　muscles　participate　equally　in　locomotion；　the　symmetrical　muscles　work　with　equal　energy　as　long　as　the　photo…chemical　processes　in　both　eyes　are　identical。　　If；　however；　one　eye　is　struck　by　stronger　light　than　the　other；　the　symmetrical　muscles　will　work　unequally　and　in　positively　heliotropic　animals　those　muscles　will　work　with　greater　energy　which　bring　the　plane　of　symmetry　back　into　the　direction　of　the　rays　of　light　and　the　head　towards　the　source　of　light。　　As　soon　as　both　eyes　are　struck　by　the　rays　of　light　at　the　same　angle；　there　is　no　more　reason　for　the　animal　to　deviate　from　this　direction　and　it　will　move　in　a　straight　line。　　All　this　holds　good　on　the　supposition　that　the　animals　are　exposed　to　only　one　source　of　light　and　are　very　sensitive　to　light。

Additional　proof　for　the　correctness　of　this　theory　was　furnished　through　the　experiments　of　G。H。　Parker　and　S。J。　Holmes。　　The　former　worked　on　a　butterfly；　Vanessa　antiope；　the　latter　on　other　arthropods。　　All　the　animals　were　in　a　marked　degree　positively　heliotropic。　　These　authors　found　that　if　one　cornea　is　blackened　in　such　an　animal；　it　moves　continually　in　a　circle　when　it　is　exposed　to　a　source　of　light；　and　in　these　motions　the　eye　which　is　not　covered　with　paint　is　directed　towards　the　centre　of　the　circle。　　The　animal　behaves；　therefore；　as　if　the　darkened　eye　were　in　the　shade。

（b）　　THE　PRODUCTION　OF　POSITIVE　HELIOTROPISM　BY　ACIDS　AND　OTHER　MEANS　AND　THE　PERIODIC　DEPTH…MIGRATIONS　OF　PELAGIC　ANIMALS。

When　we　observe　a　dense　mass　of　copepods　collected　from　a　freshwater　pond；　we　notice　that　some　have　a　tendency　to　go　to　the　light　while　others　go　in　the　opposite　direction　and　many；　if　not　the　majority；　are　indifferent　to　light。　　It　is　an　easy　matter　to　make　the　negatively　heliotropic　or　the　indifferent　copepods　almost　instantly　positively　heliotropic　by　adding　a　small　but　definite　amount　of　carbon…dioxide　in　the　form　of　carbonated　water　to　the　water　in　which　the　animals　are　contained。　　If　the　animals　are　contained　in　50　cubic　centimetres　of　water　it　suffices　to　add　from　three　to　six　cubic　centimetres　of　carbonated　water　to　make　all　the　copepods　energetically　positively　heliotropic。　　This　heliotropism　lasts　about　half　an　hour　（probably　until　all　the　carbon…dioxide　has　again　diffused　into　the　air。）　　Similar　results　may　be　obtained　with　any　other　acid。

The　same　experiments　may　be　made　with　another　freshwater　crustacean；　namely　Daphnia；　with　this　difference；　however；　that　it　is　as　a　rule　necessary　to　lower　the　temperature　of　the　water　also。　　If　the　water　containing　the　Daphniae　is　cooled　and　at　the　same　time　carbon…dioxide　added；　the　animals　which　were　before　indifferent　to　light　now　become　most　strikingly　positively　heliotropic。　　Marine　copepods　can　be　made　positively　heliotropic　by　the　lowering　of　the　temperature　alone；　or　by　a　sudden　increase　in　the　concentration　of　the　sea…water。

These　data　have　a　bearing　upon　the　depth…migrations　of　pelagic　animals；　as　was　pointed　out　years　ago　by　Theo。　T。　Groom　and　the　writer。　　It　is　well　known　that　many　animals　living　near　the　surface　of　the　ocean　or　freshwater　lakes；　have　a　tendency　to　migrate　upwards　towards　evening　and　downwards　in　the　morning　and　during　the　day。　　These　periodic　motions　are　determined　to　a　large　extent；　if　not　exclusively；　by　the　heliotropism　of　these　animals。　　Since　the　consumption　of　carbon…dioxide　by　the　green　plants　ceases　towards　evening；　the　tension　of　this　gas　in　the　water　must　rise　and　this　must　have　the　effect　of　inducing　positive　heliotropism　or　increasing　its　intensity。　　At　the　same　time　the　temperature　of　the　water　near　the　surface　is　lowered　and　this　also　increases　the　positive　heliotropism　in　the　organisms。

The　faint　light　from　the　sky　is　sufficient　to　cause　animals　which　are　in　a　high　degree　positively　heliotropic　to　move　vertically　upwards　towards　the　light；　as　experiments　with　such　pelagic　animals；　e。g。　copepods；　have　shown。　　When；　in　the　morning；　the　absorption　of　carbon…dioxide　by　the　green　algae　begins　again　and　the　temperature　of　the　water　rises；　the　animals　lose　their　positive　heliotropism；　and　slowly　sink　down　or　become　negatively　heliotropic　and　migrate　actively　downwards。

These　experiments　have　also　a　bearing　upon　the　problem　of　the　inheritance　of　instincts。　　The　character　which　is　transmitted　in　this　case　is　not　the　tendency　to　migrate　periodically　upwards　and　downwards；　but　the　positive　heliotropism。　　The　tendency　to　migrate　is　the　outcome　of　the　fact　that　periodically　varying　external　conditions　induce　a　periodic　change　in　the　sense　and　intensity　of　the　heliotropism　of　these　animals。　　It　is　of　course　immaterial　for　the　result；　whether　the　carbon…dioxide　or　any　other　acid　diffuse　into　the　animal　from　the　outside　or　whether　they　are　produced　inside　in　the　tissue　cells　of　the　animals。　　Davenport　and　Cannon　found　that　Daphniae；　which　at　the　beginning　of　the　experiment；　react　sluggishly　to　light　react　much　more　quickly　after　they　have　been　made　to　go　to　the　light　a　few　times。　　The　writer　is　inclined　to　attribute　this　result　to　the　effect　of　acids；　e。g。　carbon…dioxide；　produced　in　the　animals　themselves　in　consequence　of　their　motion。　　A　similar　effect　of　the　acids　was　shown　by　A。D。　Waller　in　the　case　of　the　response　of　nerve　to　stimuli。

The　writer　observed　many　years　ago　that　winged　male　and　female　ants　are　positively　helioptropic　and　that　their　heliotropic　sensitiveness　increases　and　reaches　its　maximum　towards　the　period　of　nuptial　flight。　　Since　the　workers　show　no　heliotropism　it　looks　as　if　an　internal　secretion　from　the　sexual　glands　were　the　cause　of　their　heliotropic　sensitiveness。　　V。　Kellogg　has　observed　that　bees　also　become　intensely　positively　heliotropic　at　the　period　of　their　wedding　flight；　in　fact　so　much　so　that　by　letting　light　fall　into　the　observation　hive　from　above；　the　bees　are　prevented　from　leaving　the　hive　through　the　exit　at　the　lower　end。

We　notice　also　the　reverse　phenomenon；　namely；　that　chemical　changes　produced　in　the　animal　destroy　its　heliotropism。　　The　caterpillars　of　Porthesia　chrysorrhoea　are　very　strongly　positively　heliotropic　when　they　are　first　aroused　from　their　winter　sleep。　　This　heliotropic　sensitiveness　lasts　only　as　long　as　they　are　not　fed。　　If　they　are　kept　permanently　without　food　they　remain　permanently　positively　heliotropic　until　they　die　from　starvation。　　It　is　to　be　inferred　that　as　soon　as　these　animals　take　up　food；　a　substance　or　substances　are　formed　in　their　bodies　which　diminish　or　annihilate　their　heliotropic　sensitiveness。

The　heliotropism　of　animals　is　identical　with　the　heliotropism　of　plants。　　The　writer　has　shown　that　the　experiments　on　the　effect　of　acids　on　the　heliotropism　of　copepods　can　be　repeated　with　the　same　result　in　Volvox。　　It　is　therefore　erroneous　to　try　to　explain　these　heliotropic　reactions　of　animals　on　the　basis　of　peculiarities　（e。g。　vision）　which　are　not　found　in　plants。

We　may　briefly　discuss　the　question　of　the　transmission　through　the　sex　cells　of　such　instincts　as　are　based　upon　heliotropism。　　This　problem　reduces　itself　simply　to　that　of　the　method　whereby　the　gametes　transmit　heliotropism　to　the　larvae　or　to　the　adult。　　The　writer　has　expressed　the　idea　that　all　that　is　necessary　for　this　transmission　is　the　presence　in　the　eyes　（or　in　the　skin）　of　the　animal　of　a　photo…sensitive　substance。　　For　the　transmission　of　this　the　gametes　need　not　contain　anything　more　than　a　catalyser　or　ferment　for　the　synthesis　of　the　photo…sensitive　substance　in　the　body　of　the　animal。　　What　has　been　said　in　regard　to　animal　heliotropism　might；　if　space　permitted；　be　extended；　mutatis　mutandis；　to　geotropism　and　stereotropism。

（c）　　THE　TROPIC　REACTIONS　OF　CERTAIN　TISSUE…CELLS　AND　THE　MORPHOGENETIC　EFFECTS　OF　THESE　REACTIONS。

Since　plant…cells　show　hel