Morepork though he followed Mees in treating them as a single species. Janette Norman and colleagues tested the cytochrome "b" DNA of three subspecies (as well as the powerful and rufous owls) to ascertain whether the closest relative was used in breeding with the last surviving female of the Norfolk boobook. They discovered that although the Norfolk boobook was similar in plumage to the Tasmanian boobook, it was genetically much closer to the New Zealand subspecies. In fact, the two were so close genetically that they considered whether the Norfolk boobook should be recognised as a separate taxon at all, although
Failure to refer Failure to refer Failure to refer, also reference failure or failure of reference, is the concept that names can fail to name a real object. According to Bertrand Russell's theory of truth, there is only one actual world, and a statement's truth value depends on whether the statement obtains in the actual world. Continuing the tradition of Gottlob Frege, Bertrand Russell posited that a name picks out, or refers to, a real object in the world (Russell's correspondence theory of truth). The name "Genghis Khan" thus picks out the 12th and 13th century Mongol leader we know by that name.