darwin and modern science-第84节

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（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　eggs　early　or　late；　according　to　the　physical　conditions　surrounding　them。　　If　they　are　kept　in　water　or　in　proximity　to　water　and　in　a　moist　atmosphere　they　have　a　tendency　to　lay　their　eggs　earlier　and　a　comparatively　high　temperature　enhances　the　tendency　to　shorten　the　period　of　gestation。　　If　the　salamanders　are　kept　in　comparative　dryness　they　show　a　tendency　to　lay　their　eggs　rather　late　and　a　low　temperature　enhances　this　tendency。

Since　Salamandra　atra　is　found　in　rather　dry　alpine　regions　with　a　relatively　low　temperature　and　Salamandra　maculosa　in　lower　regions　with　plenty　of　water　and　a　higher　temperature；　the　fact　that　S。　atra　bears　young　which　are　already　developed　and　beyond　the　stage　of　aquatic　life；　while　S。　maculosa　bears　young　ones　in　an　earlier　stage；　has　been　termed　adaptation。　　Kammerer's　experiments；　however；　show　that　we　are　dealing　with　the　direct　effects　of　definite　outside　forces。　　While　we　may　speak　of　adaptation　when　all　or　some　of　the　variables　which　determine　a　reaction　are　unknown；　it　is　obviously　in　the　interest　of　further　scientific　progress　to　connect　cause　and　effect　directly　whenever　our　knowledge　allows　us　to　do　so。

VII。　　CONCLUDING　REMARKS。

The　discovery　of　De　Vries；　that　new　species　may　arise　by　mutation　and　the　wide　if　not　universal　applicability　of　Mendel's　Law　to　phenomena　of　heredity；　as　shown　especially　by　Bateson　and　his　pupils；　must；　for　the　time　being；　if　not　permanently；　serve　as　a　basis　for　theories　of　evolution。　　These　discoveries　place　before　the　experimental　biologist　the　definite　task　of　producing　mutations　by　physico…chemical　means。　　It　is　true　that　certain　authors　claim　to　have　succeeded　in　this；　but　the　writer　wishes　to　apologise　to　these　authors　for　his　inability　to　convince　himself　of　the　validity　of　their　claims　at　the　present　moment。　　He　thinks　that　only　continued　breeding　of　these　apparent　mutants　through　several　generations　can　afford　convincing　evidence　that　we　are　here　dealing　with　mutants　rather　than　with　merely　pathological　variations。

What　was　said　in　regard　to　the　production　of　new　species　by　physico…　chemical　means　may　be　repeated　with　still　more　justification　in　regard　to　the　second　problem　of　transformation；　namely　the　making　of　living　from　inanimate　matter。　　The　purely　morphological　imitations　of　bacteria　or　cells　which　physicists　have　now　and　then　proclaimed　as　artificially　produced　living　beings；　or　the　plays　on　words　by　which；　e。g。　the　regeneration　of　broken　crystals　and　the　regeneration　of　lost　limbs　by　a　crustacean　were　declared　identical；　will　not　appeal　to　the　biologist。　　We　know　that　growth　and　development　in　animals　and　plants　are　determined　by　definite　although　complicated　series　of　catenary　chemical　reactions；　which　result　in　the　synthesis　of　a　DEFINITE　compound　or　group　of　compounds；　namely；　NUCLEINS。

The　nucleins　have　the　peculiarity　of　acting　as　ferments　or　enzymes　for　their　own　synthesis。　　Thus　a　given　type　of　nucleus　will　continue　to　synthesise　other　nuclein　of　its　own　kind。　　This　determines　the　continuity　of　a　species；　since　each　species　has；　probably；　its　own　specific　nuclein　or　nuclear　material。　　But　it　also　shows　us　that　whoever　claims　to　have　succeeded　in　making　living　matter　from　inanimate　will　have　to　prove　that　he　has　succeeded　in　producing　nuclein　material　which　acts　as　a　ferment　for　its　own　synthesis　and　thus　reproduces　itself。　　Nobody　has　thus　far　succeeded　in　this；　although　nothing　warrants　us　in　taking　it　for　granted　that　this　task　is　beyond　the　power　of　science。

XV。　　THE　VALUE　OF　COLOUR　IN　THE　STRUGGLE　FOR　LIFE。

By　E。B。　POULTON。　Hope　Professor　of　Zoology　in　the　University　of　Oxford。

INTRODUCTION。

The　following　pages　have　been　written　almost　entirely　from　the　historical　stand…point。　　Their　principal　object　has　been　to　give　some　account　of　the　impressions　produced　on　the　mind　of　Darwin　and　his　great　compeer　Wallace　by　various　difficult　problems　suggested　by　the　colours　of　living　nature。　　In　order　to　render　the　brief　summary　of　Darwin's　thoughts　and　opinions　on　the