darwin and modern science-第64章

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　adult　certain　characters　will　not　be　understood　until　we　succeed　in　varying　and　controlling　hereditary　characteristics；　and　this　can　only　be　accomplished　on　the　basis　of　a　systematic　study　of　the　effects　of　chemical　and　physical　forces　upon　living　matter。

Owing　to　limitation　of　space　this　sketch　is　necessarily　very　incomplete；　and　it　must　not　be　inferred　that　studies　which　are　not　mentioned　here　were　considered　to　be　of　minor　importance。　　All　the　writer　could　hope　to　do　was　to　bring　together　a　few　instances　of　the　experimental　analysis　of　the　effect　of　environment；　which　indicate　the　nature　and　extent　of　our　control　over　life…phenomena　and　which　also　have　some　relation　to　the　work　of　Darwin。　　In　the　selection　of　these　instances　preference　is　given　to　those　problems　which　are　not　too　technical　for　the　general　reader。

The　forces；　the　influence　of　which　we　shall　discuss；　are　in　succession　chemical　agencies；　temperature；　light；　and　gravitation。　　We　shall　also　treat　separately　the　effect　of　these　forces　upon　form　and　instinctive　reactions。

II。　　THE　EFFECTS　OF　CHEMICAL　AGENCIES。

（a）　　HETEROGENEOUS　HYBRIDISATION。

It　was　held　until　recently　that　hybridisation　is　not　possible　except　between　closely　related　species　and　that　even　among　these　a　successful　hybridisation　cannot　always　be　counted　upon。　　This　view　was　well　supported　by　experience。　　It　is；　for　instance；　well　known　that　the　majority　of　marine　animals　lay　their　unfertilised　eggs　in　the　ocean　and　that　the　males　shed　their　sperm　also　into　the　sea…water。　　The　numerical　excess　of　the　spermatozoa　over　the　ova　in　the　sea…water　is　the　only　guarantee　that　the　eggs　are　fertilised；　for　the　spermatozoa　are　carried　to　the　eggs　by　chance　and　are　not　attracted　by　the　latter。　　This　statement　is　the　result　of　numerous　experiments　by　various　authors；　and　is　contrary　to　common　belief。　As　a　rule　all　or　the　majority　of　individuals　of　a　species　in　a　given　region　spawn　on　the　same　day；　and　when　this　occurs　the　sea…water　constitutes　a　veritable　suspension　of　sperm。　　It　has　been　shown　by　experiment　that　in　fresh　sea…water　the　sperm　may　live　and　retain　its　fertilising　power　for　several　days。　　It　is　thus　unavoidable　that　at　certain　periods　more　than　one　kind　of　spermatozoon　is　suspended　in　the　sea…water　and　it　is　a　matter　of　surprise　that　the　most　heterogeneous　hybridisations　do　not　constantly　occur。　　The　reason　for　this　becomes　obvious　if　we　bring　together　mature　eggs　and　equally　mature　and　active　sperm　of　a　different　family。　　When　this　is　done　no　egg　is；　as　a　rule；　fertilised。　　The　eggs　of　a　sea…urchin　can　be　fertilised　by　sperm　of　their　own　species；　or；　though　in　smaller　numbers；　by　the　sperm　of　other　species　of　sea…urchins；　but　not　by　the　sperm　of　other　groups　of　echinoderms；　e。g。　starfish；　brittle…stars；　holothurians　or　crinoids；　and　still　less　by　the　sperm　of　more　distant　groups　of　animals。　　The　consensus　of　opinion　seemed　to　be　that　the　spermatozoon　must　enter　the　egg　through　a　narrow　opening　or　canal；　the　so…called　micropyle；　and　that　the　micropyle　allowed　only　the　spermatozoa　of　the　same　or　of　a　closely　related　species　to　enter　the　egg。

It　seemed　to　the　writer　that　the　cause　of　this　limitation　of　hybridisation　might　be　of　another　kind　and　that　by　a　change　in　the　constitution　of　the　sea…water　it　might　be　possible　to　bring　about　heterogenous　hybridisations；　which　in　normal　sea…water　are　impossible。　　This　assumption　proved　correct。　Sea…water　has　a　faintly　alkaline　reaction　（in　terms　of　the　physical　chemist　its　concentration　of　hydroxyl　ions　is　about　（10　to　the　power　minus　six）N　at　Pacific　Grove；　California；　and　about　（10　to　the　power　minus　5）N　at　Woods　Hole；　Massachusetts）。　　If　we　slightly　raise　the　alkalinity　of　the　sea…water　by　adding　to　it　a　small　but　definite　quantity　of　sodium　hydroxide　or　some　other　alkali；　the　eggs　of　the　sea…urchin　can　be　fertilised　with　the　sperm　of　widely　different　groups　of　animals；　possibly　with　the　sperm　of　any　marine　animal　which　sheds　it　into　the　ocean。　　In　1903　it　was　shown　that　if　we　add　from　about　0。5　to　0。8　cubic　centimetre　N/10　sodium　hydroxide　to　50　cubic　centimetres　of　sea…water；　the　eggs　of　Strongylocentrotus　purpuratus　（a　sea…urchin　which　is　found　on　the　coast　of　California）　can　be　fertilised　in　large　quantities　by　the　sperm　of　various　kinds　of　starfish；　brittle…　stars　and　holothurians；　while　in　normal　sea…water　or　with　less　sodium　hydroxide　not　a　single　egg　of　the　same　female　could　be　fertilised　with　the　starfish　sperm　which　proved　effective　in　the　hyper…alkaline　sea…water。　　The　sperm　of　the　various　forms　of　starfish　was　not　equally　effective　for　these　hybridisations；　the　sperm　of　Asterias　ochracea　and　A。　capitata　gave　the　best　results；　since　it　was　possible　to　fertilise　50　per　cent　or　more　of　the　sea…urchin　eggs；　while　the　sperm　of　Pycnopodia　and　Asterina　fertilised　only　2　per　cent　of　the　same　eggs。

Godlewski　used　the　same　method　for　the　hybridisation　of　the　sea…urchin　eggs　with　the　sperm　of　a　crinoid　（Antedon　rosacea）。　　Kupelwieser　afterwards　obtained　results　which　seemed　to　indicate　the　possibility　of　fertilising　the　eggs　of　Strongylocentrotus　with　the　sperm　of　a　mollusc　（Mytilus。）　　Recently；　the　writer　succeeded　in　fertilising　the　eggs　of　Strongylocentrotus　franciscanus　with　the　sperm　of　a　molluscChlorostoma。　　This　result　could　only　be　obtained　in　sea…water　the　alkalinity　of　which　had　been　increased　（through　the　addition　of　0。8　cubic　centimetre　N/10　sodium　hydroxide　to　50　cubic　centimetres　of　sea…water）。　　We　thus　see　that　by　increasing　the　alkalinity　of　the　sea…water　it　is　possible　to　effect　heterogeneous　hybridisations　which　are　at　present　impossible　in　the　natural　environment　of　these　animals。

It　is；　however；　conceivable　that　in　former　periods　of　the　earth's　history　such　heterogeneous　hybridisations　were　possible。　　It　is　known　that　in　solutions　like　sea…water　the　degree　of　alkalinity　must　increase　when　the　amount　of　carbon…dioxide　in　the　atmosphere　is　diminished。　　If　it　be　true；　as　Arrhenius　assumes；　that　the　Ice　age　was　caused　or　preceded　by　a　diminution　in　the　amount　of　carbon…dioxide　in　the　air；　such　a　diminution　must　also　have　resulted　in　an　increase　of　the　alkalinity　of　the　sea…water；　and　one　result　of　such　an　increase　must　have　been　to　render　possible　heterogeneous　hybridisations　in　the　ocean　which　in　the　present　state　of　alkalinity　are　practically　excluded。

But　granted　that　such　hybridisations　were　possible；　would　they　have　influenced　the　character　of　the　fauna？　　In　other　words；　are　the　hybrids　between　sea…urchin　and　starfish；　or　better　still；　between　sea…urchin　and　mollusc；　capable　of　development；　and　if　so；　what　is　their　character？　　The　first　experiment　made　it　appear　doubtful　whether　these　heterogeneous　hybrids　could　live。　　The　sea…urchin　eggs　which　were　fertilised　in　the　laboratory　by　the　spermatozoa　of　the　starfish；　as　a　rule；　died　earlier　than　those　of　the　pure　breeds。　　But　more　recent　results　indicate　that　this　was　due　merely　to　deficiencies　in　the　technique　of　the　earlier　experiments。　　The　writer　has　recently　obtained　hybrid　larvae　between　the　sea…urchin　egg　and　the　sperm　of　a　mollusc　（Chlorostoma）　which；　in　the　laboratory；　developed　as　well　and　lived　as　long　as　the　pure　breeds　of　the　sea…urchin；　and　there　was　nothing　to　indicate　any　difference　in　the　vitality　of　the　two　breeds。

So　far　as　the　question　of　heredity　is　concerned；　all　the　experiments　on　heterogeneous　hybridisation　of　the　egg　of　the　sea…urchin　with　the　sperm　of　starfish；　brittle…stars；　crinoids　and　molluscs；　have　led　to　the　same　result；　namely；　that　the　larvae　have　purely　maternal　characteristics　and　differ　in　no　way　from　the　pure　breed　of　the　form　from　which　the　egg　is　taken。　　By　way　of　illustration　it　may　be　said　that　the　larvae　of　the　sea…　urchin　reach　on　the　third　day　or　earlier　（according　to　species　and　temperature）　the　so…called　pluteus　stage；　in　which　they　possess　a　typical　skeleton；　while　neither　the　larvae　of　the　starfish　nor　those　of　the　mollusc　form　a　skeleton　at　the　corresponding　stage。　　It　was；　therefore；　a　matter　of　some　interest　to　find　out　whether　or　not　the　larvae　produced　by　the　fertilisation　of　the　sea…urchin　egg　with　the　sperm　of　starfish　or　mollusc　would　form　the　normal　and　typical　pluteus　skeleton。　　This　was　invariably　the　case　in　the　experiments　of　Godlewski；　Kupelwieser；　Hagedoorn；　and　the　writer。　　These　hybrid　larvae　were　exclusively　maternal　in　character。

It　might　be　argued　that　in　the　case　of　heterogeneous　hybridisation　the　sperm…nucleus　does　not　fuse　with　the　egg…nucleus；　and　that；　therefore；　the　spermatozoon　cannot　transmit　its　hereditary　substances　to　the　larvae。　　But　these　objections　are　refuted　by　Godlewski's　experiments；　in　which　he　showed　definitely　that　if　the　egg　of　the　sea…urchin　is　fertilised　with　the　sperm　of　a　crinoid　the　fusion　of　the　egg…nucleus　and　sperm…nucleus　takes　place　in　the　normal　way。　　It　remains　for　further　experiments　to　decide　what　the　character　of　the　adult　hybrids　would　be。

（b）。　　ARTIFICIAL　PARTHENOGENESIS。

Possibly　in　no　other　field　of　Biology　has　our　ability　to　control　life…　phenomena　by　outside　conditions　been　proved　to　such　an　extent　as　in　the　domain　of　fertilisation。　　The　reader　knows　that　the　eggs　of　the　overwhelming　majority　of　animals　cannot　develop　unless　a　spermatozoon　enters　them。　　In　this　case　a　living　agency　is　the　cause　of　development　and　the　problem　arises　whether　it　is　possible　to　accomplish　the　same　result　through　the　application　of　well…known　physico…chemical　agencies。　　This　is；　indeed；　true；　and　during　the　last　ten　years　living　larvae　have　been　produced　by　chemical　agencies　from　the　unfertilised　eggs　of　sea…urchins；　starfish；　holothurians　and　a　number　of　annelids　and　molluscs；　in　fact　this　holds　true　in　regard　to　the　eggs　of　practically　all　forms　of　animals　with　which　such　experiments　have　been　tried　long　enough。　　In　each　form　the　method　of　procedure　is　somewhat　different　and　a　long　series　of　experiments　is　often　required　before　the　successful　method　is　found。

The　facts　of　Artificial　Parthenogenesis；　as　the　chemical　fertilisation　of　the　egg　is　called；　have；　perhaps；　some　bearing　on　the　problem　of　evolution。　If　we　wish　to　form　a　mental　image　of　the　process　of　evolution　we　have　to　reckon　with　the　possibility　that　parthenogenetic　propagation　may　have　preceded　sexual　reproduction。　　This　suggests　also　the　possibility　that　at　that　period　outside　forces　may　have　supplied　the　conditions　for　the　development　of　the　egg　which　at　present　the　spermatozoon　has　to　supply。　　For　this；　if　for　no　other　reason；　a　brief　consideration　of　the　means　of　artificial　parthenogenesis　may　be　of　interest　to　the　student　of　evolution。

It　seemed　necessary　in　these　experiments　to　imitate　as　completely　as　possible　by　chemical　agencies　the　effects　of　the　spermatozoon　upon　the　egg。　When　a　spermatozoon　enters　the　egg　of　a　sea…urchin　or　certain　starfish　or　annelids；　the　immediate　effect　is　a　characteristic　change　of　the　surface　of　the　egg；　namely　the　formation　of　the　so…called　membrane　of　fertilisation。　　The　writer　found　that　we　can　produce　this　membrane　in　the　unfertilised　egg　by　certain　acids；　especially　the　monobasic　acids　of　the　fatty　series；　e。g。　formic；　acetic；　propionic；　butyric；　etc。　　Carbon…dioxide　is　also　very　efficient　in　this　direction。　　It　was　also　found　that　the　higher　acids　are　more　efficient　than　th