This principle states that a particular name can be used only once in zoological nomenclature. Hence a generic name or a family-group name can only be used once in the animal kingdom – it must be unique. Thus, when the generic name Pan Richmond, 1899, applied to the Toucan Barbet, was shown to be a junior homonym of Pan von Oken, 1816 (for the hominid Chimpanzee), it had to be replaced – by Semnornis Richmond, 1900. The rules for generic homonyms have changed over the years. Formerly, genera differing only in their gender terminations (e.g. Cinclidia Gould, 1838; Cinclidium Blyth, 1842) were regarded as homonyms, and the junior name was replaced. This interpretation of homonymy is no longer valid.
Similarly, under this principle, a specific and subspecific name can be used only once within a genus. If, through error or omission, two species or subspecies within a genus bear the same name, or if taxonomic research results in the submergence of one genus into another resulting in two species taxa bearing the same name, the name proposed later becomes the junior homonym and that taxon must take the next available name by precedence of the date of publication or be given a new name.
Despite more than two hundred and fifty years of scientific study, the family limits and relationships of birds and, to a lesser extent, generic and specific limits and relationships are still unresolved and the subject of considerable investigation. Most attempts to achieve some degree of consensus founder on conservatism, individual interpretation of the scientific evidence, and the swift emergence of new or competing methodologies. Subspecies are incipient species, and hence may be evolving intrinsic isolating mechanisms and other attributes which separate fully evolved species. These geographic races range from poorly differentiated forms to well-marked or isolated geographic entities, often considered to be allospecies – members of a superspecies. While one ornithologist considers a geographic form to be only a subspecies, another ornithologist may consider the same taxon to be a full species.