darwin and modern science-第27章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


e　hybrid　is　not　totally　sterile　but　only　partially　so；　and　thus　is　able　to　form　some　good　germ…cells　which　develop　into　new　individuals；　the　sterility　of　these　daughter…individuals　is　sensibly　reduced　or　may　be　entirely　absent。　　The　fertility　once　re…established；　the　sterility　does　not　return　in　the　later　progeny；　a　fact　strongly　suggestive　of　segregation。　　Now　if　the　sterility　of　the　cross…bred　be　really　the　consequence　of　the　meeting　of　two　complementary　factors；　we　see　that　the　phenomenon　could　only　be　produced　among　the　divergent　offspring　of　one　species　by　the　acquisition　of　at　least　TWO　new　factors；　for　if　the　acquisition　of　a　single　factor　caused　sterility　the　line　would　then　end。　　Moreover　each　factor　must　be　separately　acquired　by　distinct　individuals；　for　if　both　were　present　together；　the　possessors　would　by　hypothesis　be　sterile。　　And　in　order　to　imitate　the　case　of　species　each　of　these　factors　must　be　acquired　by　distinct　breeds。　　The　factors　need　not；　and　probably　would　not；　produce　any　other　perceptible　effects；　they　might；　like　the　colour…factors　present　in　white　flowers；　make　no　difference　in　the　form　or　other　characters。　　Not　till　the　cross　was　actually　made　between　the　two　complementary　individuals　would　either　factor　come　into　play；　and　the　effects　even　then　might　be　unobserved　until　an　attempt　was　made　to　breed　from　the　cross…bred。

Next；　if　the　factors　responsible　for　sterility　were　acquired；　they　would　in　all　probability　be　peculiar　to　certain　individuals　and　would　not　readily　be　distributed　to　the　whole　breed。　　Any　member　of　the　breed　also　into　which　BOTH　the　factors　were　introduced　would　drop　out　of　the　pedigree　by　virtue　of　its　sterility。　　Hence　the　evidence　that　the　various　domesticated　breeds　say　of　dogs　or　fowls　can　when　mated　together　produce　fertile　offspring；　is　beside　the　mark。　　The　real　question　is；　Do　they　ever　produce　sterile　offspring？　　I　think　the　evidence　is　clearly　that　sometimes　they　do；　oftener　perhaps　than　is　commonly　supposed。　　These　suggestions　are　quite　amenable　to　experimental　tests。　　The　most　obvious　way　to　begin　is　to　get　a　pair　of　parents　which　are　known　to　have　had　any　sterile　offspring；　and　to　find　the　proportions　in　which　these　steriles　were　produced。　　If；　as　I　anticipate；　these　proportions　are　found　to　be　definite；　the　rest　is　simple。

In　passing；　certain　other　considerations　may　be　referred　to。　　First；　that　there　are　observations　favouring　the　view　that　the　production　of　totally　sterile　cross…breds　is　seldom　a　universal　property　of　two　species；　and　that　it　may　be　a　matter　of　individuals；　which　is　just　what　on　the　view　here　proposed　would　be　expected。　　Moreover；　as　we　all　know　now；　though　incompatibility　may　be　dependent　to　some　extent　on　the　degree　to　which　the　species　are　dissimilar；　no　such　principle　can　be　demonstrated　to　determine　sterility　or　fertility　in　general。　　For　example；　though　all　our　Finches　can　breed　together；　the　hybrids　are　all　sterile。　　Of　Ducks　some　species　can　breed　together　without　producing　the　slightest　sterility；　others　have　totally　sterile　offspring；　and　so　on。　　The　hybrids　between　several　genera　of　Orchids　are　perfectly　fertile　on　the　female　side；　and　some　on　the　male　side　also；　but　the　hybrids　produced　between　the　Turnip　（Brassica　napus）　and　the　Swede　（Brassica　campestris）；　which；　according　to　our　estimates　of　affinity　should　be　nearly　allied　forms；　are　totally　sterile。　　（See　Sutton；　A。W。；　〃Journ。　Linn。　Soc。〃　XXXVIII。　page　341；　1908。）　　Lastly；　it　may　be　recalled　that　in　sterility　we　are　almost　certainly　considering　a　meristic　phenomenon。　　FAILURE　TO　DIVIDE　is；　we　may　feel　fairly　sure；　the　immediate　〃cause〃　of　the　sterility。　　Now；　though　we　know　very　little　about　the　heredity　of　meristic　differences；　all　that　we　do　know　points　to　the　conclusion　that　the　less…divided　is　dominant　to　the　more…divided；　and　we　are　thus　justified　in　supposing　that　there　are　factors　which　can　arrest　or　prevent　cell…division。　　My　conjecture　therefore　is　that　in　the　case　of　sterility　of　cross…breds　we　see　the　effect　produced　by　a　complementary　pair　of　such　factors。　　This　and　many　similar　problems　are　now　open　to　our　analysis。

The　question　is　sometimes　asked；　Do　the　new　lights　on　Variation　and　Heredity　make　the　process　of　Evolution　easier　to　understand？　　On　the　whole　the　answer　may　be　given　that　they　do。　　There　is　some　appearance　of　loss　of　simplicity；　but　the　gain　is　real。　　As　was　said　above；　the　time　is　not　ripe　for　the　discussion　of　the　origin　of　species。　　With　faith　in　Evolution　unshakenif　indeed　the　word　faith　can　be　used　in　application　to　that　which　is　certainwe　look　on　the　manner　and　causation　of　adapted　differentiation　as　still　wholly　mysterious。　　As　Samuel　Butler　so　truly　said：　　〃To　me　it　seems　that　the　'Origin　of　Variation；'　whatever　it　is；　is　the　only　true　'Origin　of　Species'〃　（〃Life　and　Habit〃；　London；　page　263；　1878。）；　and　of　that　Origin　not　one　of　us　knows　anything。　　But　given　Variationand　it　is　given：　　assuming　further　that　the　variations　are　not　guided　into　paths　of　adaptationand　both　to　the　Darwinian　and　to　the　modern　school　this　hypothesis　appears　to　be　sound　if　unprovenan　evolution　of　species　proceeding　by　definite　steps　is　more；　rather　than　less；　easy　to　imagine　than　an　evolution　proceeding　by　the　accumulation　of　indefinite　and　insensible　steps。　　Those　who　have　lost　themselves　in　contemplating　the　miracles　of　Adaptation　（whether　real　or　spurious）　have　not　unnaturally　fixed　their　hopes　rather　on　the　indefinite　than　on　the　definite　changes。　　The　reasons　are　obvious。　　By　suggesting　that　the　steps　through　which　an　adaptative　mechanism　arose　were　indefinite　and　insensible；　all　further　trouble　is　spared。　　While　it　could　be　said　that　species　arise　by　an　insensible　and　imperceptible　process　of　variation；　there　was　clearly　no　use　in　tiring　ourselves　by　trying　to　perceive　that　process。　　This　labour…saving　counsel　found　great　favour。　　All　that　had　to　be　done　to　develop　evolution…　theory　was　to　discover　the　good　in　everything；　a　task　which；　in　the　complete　absence　of　any　control　or　test　whereby　to　check　the　truth　of　the　discovery；　is　not　very　onerous。　　The　doctrine　〃que　tout　est　au　mieux〃　was　therefore　preached　with　fresh　vigour；　and　examples　of　that　illuminating　principle　were　discovered　with　a　facility　that　Pangloss　himself　might　have　envied；　till　at　last　even　the　spectators　wearied　of　such　dazzling　performances。

But　in　all　seriousness；　why　should　indefinite　and　unlimited　variation　have　been　regarded　as　a　more　probable　account　of　the　origin　of　Adaptation？　　Only；　I　think；　because　the　obstacle　was　shifted　one　plane　back；　and　so　looked　rather　less　prominent。　　The　abundance　of　Adaptation；　we　all　grant；　is　an　immense；　almost　an　unsurpassable　difficulty　in　all　non…Lamarckian　views　of　Evolution；　but　if　the　steps　by　which　that　adaptation　arose　were　fortuitous；　to　imagine　them　insensible　is　assuredly　no　help。　　In　one　most　important　respect　indeed；　as　has　often　been　observed；　it　is　a　multiplication　of　troubles。　　For　the　smaller　the　steps；　the　less　could　Natural　Selection　act　upon　them。　　Definite　variationsand　of　the　occurrence　of　definite　variations　in　abundance　we　have　now　the　most　convincing　proofhave　at　least　the　obvious　merit　that　they　can　make　and　often　do　make　a　real　difference　in　the　chances　of　life。

There　is　another　aspect　of　the　Adaptation　problem　to　which　I　can　only　allude　very　briefly。　　May　not　our　present　ideas　of　the　universality　and　precision　of　Adaptation　be　greatly　exaggerated？　　The　fit　of　organism　to　its　environment　is　not　after　all　so　very　closea　proposition　unwelcome　perhaps；　but　one　which　could　be　illustrated　by　very　copious　evidence。　　Natural　Selection　is　stern；　but　she　has　her　tolerant　moods。

We　have　now　most　certain　and　irrefragable　proof　that　much　definiteness　exists　in　living　things　apart　from　Selection；　and　also　much　that　may　very　well　have　been　preserved　and　so　in　a　sense　constituted　by　Selection。　　Here　the　matter　is　likely　to　rest。　　There　is　a　passage　in　the　sixth　edition　of　the　〃Origin〃　which　has　I　think　been　overlooked。　　On　page　70　Darwin　says　〃The　tuft　of　hair　on　the　breast　of　the　wild　turkey…cock　cannot　be　of　any　use；　and　it　is　doubtful　whether　it　can　be　ornamental　in　the　eyes　of　the　female　bird。〃　　This　tuft　of　hair　is　a　most　definite　and　unusual　structure；　and　I　am　afraid　that　the　remark　that　it　〃cannot　be　of　any　use〃　may　have　been　made　inadvertently；　but　it　may　have　been　intended；　for　in　the　first　edition　the　usual　qualification　was　given　and　must　therefore　have　been　deliberately　excised。　　Anyhow　I　should　like　to　think　that　Darwin　did　throw　over　that　tuft　of　hair；　and　that　he　felt　relief　when　he　had　done　so。　　Whether　however　we　have　his　great　authority　for　such　a　course　or　not；　I　feel　quite　sure　that　we　shall　be　rightly　interpreting　the　facts　of　nature　if　we　cease　to　expect　to　find　purposefulness　wherever　we　meet　with　definite　structures　or　patterns。　　Such　things　are；　as　often　as　not；　I　suspect　rather　of　the　nature　of　tool…marks；　mere　incidents　of　manufacture；　benefiting　their　possessor　not　more　than　the　wire…marks　in　a　sheet　of　paper；　or　the　ribbing　on　the　bottom　of　an　oriental　plate　renders　those　objects　more　attractive　in　our　eyes。

If　Variation　may　be　in　any　way　definite；　the　question　once　more　arises；　may　it　not　be　definite　in　direction？　　The　belief　that　it　is　has　had　many　supporters；　from　Lamarck　onwards；　who　held　that　it　was　guided　by　need；　and　others　who；　like　Nageli；　while　laying　no　emphasis　on　need；　yet　were　convinced　that　there　was　guidance　of　some　kind。　　The　latter　view　under　the　name　of　〃Orthogenesis；〃　devised　I　believe　by　Eimer；　at　the　present　day　commends　itself　to　some　naturalists。　　The　objection　to　such　a　suggestion　is　of　course　that　no　fragment　of　real　evidence　can　be　produced　in　its　support。　　On　the　other　hand；　with　the　experimental　proof　that　variation　consists　largely　in　the　unpacking　and　repacking　of　an　original　complexity；　it　is　not　so　certain　as　we　might　like　to　think　that　the　order　of　these　events　is　not　pre…determined。　　For　instance　the　original　〃pack〃　may　have　been　made　in　such　a　way　that　at　the　nth　division　of　the　germ…cells　of　a　Sweet　Pea　a　colour…factor　might　be　dropped；　and　that　at　the　n　plus　n　prime　division　the　hooded　variety　be　given　off；　and　so　on。　　I　see　no　ground　whatever　for　holding　such　a　view；　but　in　fairness　the　possibility　should　not　be　forgotten；　and　in　the　light　of　modern　research　it　scarcely　looks　so　absurdly　improbable　as　before。

No　one　can　survey　the　work　of　recent　years　without　perceiving　that　evolutionary　orthodoxy　developed　too　fast；　and　that　a　great　deal　has　got　to　come　down；　but　this　satisfaction　at　least　remains；　that　in　the　experimental　methods　which　Mendel　inaugurated；　we　have　means　of　reaching　certainty　in　regard　to　the　physiology　of　Heredity　and　Variation　upon　which　a　more　lasting　structure　may　be　built。


VI。　　THE　MINUTE　STRUCTURE　OF　CELLS　IN　RELATION　TO　HEREDITY。

By　EDUARD　STRASBURGER；　Professor　of　Botany　in　the　University　of　Bonn。

Since　1875　an　unexpected　insight　has　been　gained　into　the　internal　structure　of　cells。　　Those　who　are　familiar　with　the　results　of　investigations　in　this　branch　of　Science　are　convinced　that　any　modern　theory　of　heredity　must　rest　on　a　basis　of　cytology　and　cannot　be　at　variance　with　cytological　facts。　　Many　histological　discoveries；　both　such