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The requested URL /mbrowse/msdb/OriginOfSpecies/description/is/Close+investigation,+in+many+cases,+will+no+doubt+bring+naturalists+to+agree+how+to+rank+doubtful+forms. Yet+it+must+be+confessed+that+it+is+in+the+best-known+countries+that+we+find+the+greatest+number+of+them. I+have+been+struck+with+the+fact,+that+if+any+animal+or+plant+in+a+state+of+nature+be+highly+useful+to+man,+or+from+any+cause+closely+attracts+his+attention,+varieties+of+it+will+almost+universally+be+found+recorded. These+varieties,+moreover,+will+often+be+ranked+by+some+authors+as+species.+Look+at+the+common+oak,+how+closely+it+has+been+studied;+yet+a+German+author+makes+more+than+a+dozen+species+out+of+forms,+which+are+almost+universally+considered+by+other+botanists+to+be+varieties;+and+in+this+country+the+highest+botanical+authorities+and+practical+men+can+be+quoted+to+show+that+the+sessile+and+pedunculated+oaks+are+either+good+and+distinct+species+or+mere+varieties. I+may+here+allude+to+a+remarkable+memoir+lately+published+by+A.+de+Candolle,+on+the+oaks+of+the+whole+world.+No+one+ever+had+more+ample+materials+for+the+discrimination+of+the+species,+or+could+have+worked+on+them+with+more+zeal+and+sagacity. [[[[[images/OakTree.jpg|oak]]]]] He+first+gives+in+detail+all+the+many+points+of+structure+which+vary+in+the+several+species,+and+estimates+numerically+the+relative+frequency+of+the+variations.+He+specifies+above+a+dozen+characters+which+may+be+found+varying+even+on+the+same+branch,+sometimes+according+to+age+or+development,+sometimes+without+any+assignable+reason. Such+characters+are+not+of+course+of+specific+value,+but+they+are,+as+Asa+Gray+has+remarked+in+commenting+on+this+memoir,+such+as+generally+enter+into+specific+definitions. De+Candolle+then+goes+on+to+say+that+he+gives+the+rank+of+species+to+the+forms+that+differ+by+characters+never+varying+on+the+same+tree,+and+never+found+connected+by+intermediate+states. After+this+discussion,+the+result+of+so+much+labour,+he+emphatically+remarks:+"They+are+mistaken,+who+repeat+that+the+greater+part+of+our+species+are+clearly+limited,+and+that+the+doubtful+species+are+in+a+feeble+minority. This+seemed+to+be+true,+so+long+as+a+genus+was+imperfectly+known,+and+its+species+were+founded+upon+a+few+specimens,+that+is+to+say,+were+provisional.+Just+as+we+come+to+know+them+better,+intermediate+forms+flow+in,+and+doubts+as+to+specific+limits+augment."+He+also+adds+that+it+is+the+best+known+species+which+present+the+greater+number+of+spontaneous+varieties+and+sub-varieties. Thus+Quercus+robur+has+twenty-eight+varieties,+all+of+which,+excepting+six,+are+clustered+round+three+sub-species,+namely,+Q.+pedunculata,+sessiliflora,+and+pubescens. The+forms+which+connect+these+three+sub-species+are+comparatively+rare;+and,+as+Asa+Gray+again+remarks,+if+these+connecting+forms+which+are+now+rare,+were+to+become+wholly+extinct,+the+three+sub-species+would+hold+exactly+the+same+relation+to+each+other,+as+do+the+four+or+five+provisionally+admitted+species+which+closely+surround+the+typical+[[Quercus+robur|Quercus+Robur]]. [[[[[images/Quercus+Robur.jpg|Quercus+Robur+(Common+Oak)]]]]] Finally,+De+Candolle+admits+that+out+of+the+300+species,+which+will+be+enumerated+in+his+Prodromus+as+belonging+to+the+oak+family,+at+least+two-thirds+are+provisional+species,+that+is,+are+not+known+strictly+to+fulfil+the+definition+above+given+of+a+true+species.+It+should+be+added+that+De+Candolle+no+longer+believes+that+species+are+immutable+creations,+but+concludes+that+the+derivative+theory+is+the+most+natural+one,+"and+the+most+accordant+with+the+known+facts+in+palaeontology,+geographical+botany+and+zoology,+of anatomical+structure+and+classification."/ was not found on this server.