darwin and modern science-第36节

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gether　and　eventually　assuming　a　definite　form　as　special　organs　of　the　cell。　　It　may　be　also　assumed　that　in　the　protoplasm　and　in　the　primitive　types　of　nucleus；　the　carriers　of　the　same　hereditary　unit　were　represented　in　considerable　quantity；　they　became　gradually　differentiated　to　an　extent　commensurate　with　newly　acquired　characters。　　It　was　also　necessary　that；　in　proportion　as　this　happened；　the　mechanism　of　nuclear　division　must　be　refined。　　At　first　processes　resembling　a　simple　constriction　would　suffice　to　provide　for　the　distribution　of　all　hereditary　units　to　each　of　the　products　of　division；　but　eventually　in　both　organic　kingdoms　nuclear　division；　which　alone　insured　the　qualitative　identity　of　the　products　of　division；　became　a　more　marked　feature　in　the　course　of　cell…multiplication。

Where　direct　nuclear　division　occurs　by　constriction　in　the　higher　organisms；　it　does　not　result　in　the　halving　of　hereditary　units。　　So　far　as　my　observations　go；　direct　nuclear　division　occurs　in　the　more　highly　organised　plants　only　in　cells　which　have　lost　their　specific　functions。　　Such　cells　are　no　longer　capable　of　specific　reproduction。　　An　interesting　case　in　this　connection　is　afforded　by　the　internodal　cells　of　the　Characeae；　which　possess　only　vegetative　functions。　　These　cells　grow　vigorously　and　their　cytoplasm　increases；　their　growth　being　accompanied　by　a　correspondingly　direct　multiplication　of　the　nuclei。　　They　serve　chiefly　to　nourish　the　plant；　but；　unlike　the　other　cells；　they　are　incapable　of　producing　any　offspring。　　This　is　a　very　instructive　case；　because　it　clearly　shows　that　the　nuclei　are　not　only　carriers　of　hereditary　characters；　but　that　they　also　play　a　definite　part　in　the　metabolism　of　the　protoplasts。

Attention　was　drawn　to　the　fact　that　during　the　reducing　division　of　nuclei　which　contain　chromosomes　of　unequal　size；　gemini　are　constantly　produced　by　the　pairing　of　chromosomes　of　the　same　size。　　This　led　to　the　conclusion　that　the　pairing　chromosomes　are　homologous；　and　that　one　comes　from　the　father；　the　other　from　the　mother。　　（First　stated　by　T。H。　Montgomery　in　1901　and　by　W。S。　Sutton　in　1902。）　　This　evidently　applies　also　to　the　pairing　of　chromosomes　in　those　reduction…divisions　in　which　differences　in　size　do　not　enable　us　to　distinguish　the　individual　chromosomes。　　In　this　case　also　each　pair　would　be　formed　by　two　homologous　chromosomes；　the　one　of　paternal；　the　other　of　maternal　origin。　　When　the　separation　of　these　chromosomes　and　their　distribution　to　both　daughter…nuclei　occur　a　chromosome　of　each　kind　is　provided　for　each　of　these　nuclei。　　It　would　seem　that　the　components　of　each　pair　might　pass　to　either　pole　of　the　nuclear　spindle；　so　that　the　paternal　and　maternal　chromosomes　would　be　distributed　in　varying　proportion　between　the　daughter…nuclei；　and　it　is　not　impossible　that　one　daughter…nucleus　might　occasionally　contain　paternal　chromosomes　only　and　its　sister…nucleus　exclusively　maternal　chromosomes。

The　fact　that　in　nuclei　containing　chromosomes　of　various　sizes；　the　chromosomes　which　pair　together　in　reduction…division　are　always　of　equal　size；　constitutes　a　further　and　more　important　proof　of　their　qualitative　difference。　　This　is　supported　also　by　ingenious　experiments　which　led　to　an　unequal　distribution　of　chromosomes　in　the　products　of　division　of　a　sea…urchin's　egg；　with　the　result　that　a　difference　was　induced　in　their　further　development。　　（Demonstrated　by　Th。　Boveri　in　1902。）

The　recently　discovered　fact　that　in　diploid　nuclei　the　chromosomes　are　arranged　in　pairs　affords　additional　evidence　in　favour　of　the　unequal　value　of　the　chromosomes。　　This　is　still　more　striking　in　the　case　of　chromosomes　of　different　sizes。　　It　has　been　shown　that　in　the　first　division…figure　in　the　nucleus　of　the　fertilised　egg　the　chromosomes　of　corresponding　size　form　pairs。　　They　appear　with　this　arrangement　in　all　subsequent　nuclear　divisions　in　the　diploid　generation。　　The　longitudinal　fissions　of　the　chromosomes　provide　for　the　unaltered　preservation　of　this　condition。　　In　the　reduction　nucleus　of　the　gonotokonts　the　homologous　chromosomes　being　near　together　need　not　seek　out　one　another；　they　are　ready　to　form　gemini。　　The　next　stage　is　their　separation　to　the　haploid　daughter…nuclei；　which　have　resulted　from　the　reduction　process。

Peculiar　phenomena　in　the　reduction　nucleus　accompany　the　formation　of　gemini　in　both　organic　kingdoms。　　（This　has　been　shown　more　particularly　by　the　work　of　L。　Guignard；　M。　Mottier；　J。B。　Farmer；　C。B。　Wilson；　V。　Hacker　and　more　recently　by　V。　Gregoire　and　his　pupil　C。A。　Allen；　by　the　researches　conducted　in　the　Bonn　Botanical　Institute；　and　by　A。　and　K。E。　Schreiner。）　　Probably　for　the　purpose　of　entering　into　most　intimate　relation；　the　pairs　are　stretched　to　long　threads　in　which　the　chromomeres　come　to　lie　opposite　one　another。　　（C。A。　Allen；　A。　and　K。E。　Schreiner；　and　Strasburger。）　　It　seems　probable　that　these　are　homologous　chromomeres；　and　that　the　pairs　afterwards　unite　for　a　short　time；　so　that　an　exchange　of　hereditary　units　is　rendered　possible。　　（H。　de　Vries　and　Strasburger。）　　This　cannot　be　actually　seen；　but　certain　facts　of　heredity　point　to　the　conclusion　that　this　occurs。　　It　follows　from　these　phenomena　that　any　exchange　which　may　be　effected　must　be　one　of　homologous　carriers　of　hereditary　units　only。　　These　units　continue　to　form　exchangeable　segments　after　they　have　undergone　unequal　changes；　they　then　constitute　allelotropic　pairs。　　We　may　thus　calculate　what　sum　of　possible　combinations　the　exchange　of　homologous　hereditary　units　between　the　pairing　chromosomes　provides　for　before　the　reduction　division　and　the　subsequent　distribution　of　paternal　and　maternal　chromosomes　in　the　haploid　daughter…nuclei。　　These　nuclei　then　transmit　their　characters　to　the　sexual　cells；　the　conjugation　of　which　in　fertilization　again　produces　the　most　varied　combinations。　　（A。　　Weismann　gave　the　impulse　to　these　ideas　in　his　theory　on　〃Amphimixis〃。）　　In　this　way　all　the　cooperations　which　the　carriers　of　hereditary　characters　are　capable　of　in　a　species　are　produced；　this　must　give　it　an　appreciable　advantage　in　the　struggle　for　life。

The　admirers　of　Charles　Darwin　must　deeply　regret　that　he　did　not　live　to　see　the　results　achieved　by　the　new　Cytology。　　What　service　would　they　have　been　to　him　in　the　presentation　of　his　hypothesis　of　Pangenesis；　what　an　outlook　into　the　future　would　they　have　given　to　his　active　mind！

The　Darwinian　hypothesis　of　Pangenesis　rests　on　the　conception　that　all　inheritable　properties　are　represented　in　the　cells　by　small　invisible　particles　or　gemmules　and　that　these　gemmules　increase　by　division。　　Cytology　began　to　develop　on　new　lines　some　years　after　the　publication　in　1868　of　Charles　Darwin's　〃Provisional　hypothesis　of　Pangenesis〃　（〃Animals　and　Plants　under　Domestication〃；　London；　1868；　Chapter　XXVII。）；　and　when　he　died　in　1882　it　was　still　in　its　infancy。　　Darwin　would　have　soon　suggested　the　substitution　of　the　nuclei　for　his　gemmules。　　At　least　the　great　majority　of　present…day　investigators　in　the　domain　of　cytology　have　been　led　to　the　conclusion　that　the　nucleus　is　the　carrier　of　hereditary　characters；　and　they　also　believe　that　hereditary　characters　are　represented　in　the　nucleus　as　distinct　units。　　Such　would　be　Darwin's　gemmules；　which　in　conformity　with　the　name　of　his　hypothesis　may　be　called　pangens　（So　called　by　H。　de　Vries　in　1889。）：　　these　pangens　multiply　by　division。　　All　recently　adopted　views　may　be　thus　linked　on　to　this　part　of　Darwin's　hypothesis。　　It　is　otherwise　with　Darwin's　conception　to　which　Pangenesis　owes　its　name；　namely　the　view　that　all　cells　continually　give　off　gemmules；　which　migrate　to　other　places　in　the　organism；　where　they　unite　to　form　reproductive　cells。　　When　Darwin　foresaw　this　possibility；　the　continuity　of　the　germinal　substance　was　still　unknown　（Demonstrated　by　Nussbaum　in　1880；　by　Sachs　in　1882；　and　by　Weismann　in　1885。）；　a　fact　which　excludes　a　transference　of　gemmules。

But　even　Charles　Darwin's　genius　was　confined　within　finite　boundaries　by　the　state　of　science　in　his　day。

It　is　not　my　province　to　deal　with　other　theories　of　development　which　followed　from　Darwin's　Pangenesis；　or　to　discuss　their　histological　probabilities。　　We　can；　however；　affirm　that　Charles　Darwin's　idea　that　invisible　gemmules　are　the　carriers　of　hereditary　characters　and　that　they　multiply　by　division　has　been　removed　from　the　position　of　a　provisional　hypothesis　to　that　of　a　well…founded　theory。　　It　is　supported　by　histology；　and　the　results　of　experimental　work　in　heredity；　which　are　now　assuming　extraordinary　prominence；　are　in　close　agreement　with　it。


VII。　　〃THE　DESCENT　OF　MAN〃

By　G。　SCHWALBE。　Professor　of　Anatomy　in　the　University　of　Strassburg。

The　problem　of　the　origin　of　the　human　race；　of　the　descent　of　man；　is　ranked　by　Huxley　in　his　epoch…making　book　〃Man's　Place　in　Nature〃；　as　the　deepest　with　which　biology　has　to　concern　itself；　〃the　question　of　questions；〃the　problem　which　underlies　all　others。　　In　the　same　brilliant　and　lucid　exposition；　which　appeared　in　1863；　soon　after　the　publication　of　Darwin's　〃Origin　of　Species〃；　Huxley　stated　his　own　views　in　re