darwin and modern science-第94章

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　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　heliotropic　reactions　identical　with　those　of　animals；　it　is　not　surprising　that　certain　tissue…cells　also　show　reactions　which　belong　to　the　class　of　tropisms。　　These　reactions　of　tissue…cells　are　of　special　interest　by　reason　of　their　bearing　upon　the　inheritance　of　morphological　characters。　　An　example　of　this　is　found　in　the　tiger…like　marking　of　the　yolk…sac　of　the　embryo　of　Fundulus　and　in　the　marking　of　the　young　fish　itself。　　The　writer　found　that　the　former　is　entirely；　and　the　latter　at　least　in　part；　due　to　the　creeping　of　the　chromatophores　upon　the　blood…vessels。　　The　chromatophores　are　at　first　scattered　irregularly　over　the　yolk…sac　and　show　their　characteristic　ramifications。　　There　is　at　that　time　no　definite　relation　between　blood…vessels　and　chromatophores。　　As　soon　as　a　ramification　of　a　chromatophore　comes　in　contact　with　a　blood…　vessel　the　whole　mass　of　the　chromatophore　creeps　gradually　on　the　blood…　vessel　and　forms　a　complete　sheath　around　the　vessel；　until　finally　all　the　chromatophores　form　a　sheath　around　the　vessels　and　no　more　pigment　cells　are　found　in　the　meshes　between　the　vessels。　　Nobody　who　has　not　actually　watched　the　process　of　the　creeping　of　the　chromatophores　upon　the　blood…　vessels　would　anticipate　that　the　tiger…like　colouration　of　the　yolk…sac　in　the　later　stages　of　the　development　was　brought　about　in　this　way。　　Similar　facts　can　be　observed　in　regard　to　the　first　marking　of　the　embryo　itself。　　The　writer　is　inclined　to　believe　that　we　are　here　dealing　with　a　case　of　chemotropism；　and　that　the　oxygen　of　the　blood　may　be　the　cause　of　the　spreading　of　the　chromatophores　around　the　blood…vessels。　　Certain　observations　seem　to　indicate　the　possibility　that　in　the　adult　the　chromatophores　have；　in　some　forms　at　least；　a　more　rigid　structure　and　are　prevented　from　acting　in　the　way　indicated。　　It　seems　to　the　writer　that　such　observations　as　those　made　on　Fundulus　might　simplify　the　problem　of　the　hereditary　transmission　of　certain　markings。

Driesch　has　found　that　a　tropism　underlies　the　arrangement　of　the　skeleton　in　the　pluteus　larvae　of　the　sea…urchin。　　The　position　of　this　skeleton　is　predetermined　by　the　arrangement　of　the　mesenchyme　cells；　and　Driesch　has　shown　that　these　cells　migrate　actively　to　the　place　of　their　destination；　possibly　led　there　under　the　influence　of　certain　chemical　substances。　　When　Driesch　scattered　these　cells　mechanically　before　their　migration；　they　nevertheless　reached　their　destination。

In　the　developing　eggs　of　insects　the　nuclei；　together　with　some　cytoplasm；　migrate　to　the　periphery　of　the　egg。　　Herbst　pointed　out　that　this　might　be　a　case　of　chemotropism；　caused　by　the　oxygen　surrounding　the　egg。　　The　writer　has　expressed　the　opinion　that　the　formation　of　the　blastula　may　be　caused　generally　by　a　tropic　reaction　of　the　blastomeres；　the　latter　being　forced　by　an　outside　influence　to　creep　to　the　surface　of　the　egg。

These　examples　may　suffice　to　indicate　that　the　arrangement　of　definite　groups　of　cells　and　the　morphological　effects　resulting　therefrom　may　be　determined　by　forces　lying　outside　the　cells。　　Since　these　forces　are　ubiquitous　and　constant　it　appears　as　if　we　were　dealing　exclusively　with　the　influence　of　a　gamete；　while　in　reality　all　that　it　is　necessary　for　the　gamete　to　transmit　is　a　certain　form　of　irritability。

（d）　　FACTORS　WHICH　DETERMINE　PLACE　AND　TIME　FOR　THE　DEPOSITION　OF　EGGS。

For　the　preservation　of　species　the　instinct　of　animals　to　lay　their　eggs　in　places　in　which　the　young　larvae　find　their　food　and　can　develop　is　of　paramount　importance。　　A　simple　example　of　this　instinct　is　the　fact　that　the　common　fly　lays　its　eggs　on　putrid　material　which　serves　as　food　for　the　young　larvae。　　When　a　piece　of　meat　and　of　fat　of　the　same　animal　are　placed　side　by　side；　the　fly　will　deposit　its　eggs　upon　the　meat　on　which　the　larvae　can　grow；　and　not　upon　the　fat；　on　which　they　would　starve。　　Here　we　are　dealing　with　the　effect　of　a　volatile　nitrogenous　substance　which　reflexly　causes　the　peristaltic　motions　for　the　laying　of　the　egg　in　the　female　fly。

Kammerer　has　investigated　the　conditions　for　the　laying　of　eggs　in　two　forms　of　salamanders；　e。g。　Salamandra　atra　and　S。　maculosa。　　In　both　forms　the　eggs　are　fertilised　in　the　body　and　begin　to　develop　in　the　uterus。　　Since　there　is　room　only　for　a　few　larvae　in　the　uterus；　a　large　number　of　eggs　perish　and　this　number　is　the　greater　the　longer　the　period　of　gestation。　　It　thus　happens　that　when　the　animals　retain　their　eggs　a　long　time；　very　few　young　ones　are　born；　and　these　are　in　a　rather　advanced　stage　of　development；　owing　to　the　long　time　which　elapsed　since　they　were　fertilised。　　When　the　animal　lays　its　eggs　comparatively　soon　after　copulation；　many　eggs　（from　12　to　72）　are　produced　and　the　larvae　are　of　course　in　an　early　stage　of　development。　　In　the　early　stage　the　larvae　possess　gills　and　can　therefore　live　in　water；　while　in　later　stages　they　have　no　gills　and　breathe　through　their　lungs。　　Kammerer　showed　that　both　forms　of　Salamandra　can　be　induced　to　lay　their