The rules in the Code determine what names are potentially valid for any taxon including the ranks of Subspecies and Superfamily . Its provisions can be waived or modified in their application to a particular case when strict adherence would cause confusion. Such exceptions are not made by an individual scientist, no matter how well-respected within his or her field, but only by the (ICZN), acting on behalf of all zoologists. The Commission takes such action in response to proposals submitted to it. Note that, formally, the acronym "ICZN" refers to the Commission, and not the Code. Misapplications of the acronym are pervasive, however, and even taxonomists will use the acronym occasionally when referring to the Code.
The Code recognizes no Case Law . Any dispute is decided by applying the Code directly, and never by reference to precedent.
The first published name of an organism or group takes priority; later names for that organism or group are and are not considered valid.
- John Edward Gray published the name ''Antilocapra anteflexa'' in 1855 for a species of Pronghorn , based on a pair of horns. However, it is now thought that his specimen was an unusual individual of the species ''Antilocapra americana'' published by George Ord in 1815. Ord's name thus takes priority, with ''Antilocapra anteflexa'' being a junior synonym.
- Johann Jakob Kaup published the name ''Leptocephalus brevirostris'' in 1856 for a species of Eel . However, it was realized in 1893 that the organism described by Kaup was in fact the juvenile form of the European Eel (see Eel Life History for the full story). The European eel was named ''Muraena anguilla'' by Carolus Linnaeus in 1758 and moved to the genus ''Anguilla'' by Franz Paula Von Schrank in 1798. So ''Anguilla anguilla'' is now the valid name for the species, and ''Leptocephalus brevirostris'' is considered a junior synonym.
The first published use of a name takes priority; later uses of a name spelled the same but used to refer to different organisms are and must be given ''' Replacement Names '''.
The first published description of a species fixes the species epithet; if the species is later moved to another genus, it retains the first-published epithet unless that would create a homonym.
- The Common Chimpanzee was named ''Simia troglodytes'' by Johann Friedrich Blumenbach in 1799; Lorenz Oken moved it to the new genus '' Pan '' in 1816, so the valid name is now ''Pan troglodytes''.
- Two species of Madagascar snake were given the species epithet ''madagascariensis'' by André Marie Constant Duméril and Gabriel Bibron in 1844 — ''Pelophilus madagascariensis'' and ''Xiphosoma madagascariensis''. George Albert Boulenger moved the former to the genus '' Boa '' in 1893, giving it the name ''Boa madagascariensis''. This meant that when Arnold G. Kluge of the Museum of Zoology at the University Of Michigan moved ''Xiphosoma madagascariensis'' to the genus ''Boa'' in 1991, the name ''Boa madagascariensis'' was invalid — a junior secondary homonym. So Kluge gave the species the replacement name '' Boa Manditra ''. This example also demonstrates a case where rules of come into play (a tradition which has become more controversial in recent years). That is, an adjectival species epithet must - with very few exceptions - agree in gender with the name of the genus in which it is placed. If a species is moved, therefore, often it must have its spelling changed as a result. The genus name ''Xiphosoma'' is neuter in gender, and therefore the original spelling of the species should have been ''madagascariense'', which is the neuter form - the spelling change to ''madagascariensis'' would occur only after being placed in ''Boa''. Epithets that are nouns, or arbitrary combinations of letters, are not changed, but this is not always obvious from the appearance of a name, as in ''manditra'', which is a noun, and would not change if, for example, it were moved to the genus ''Pelophilus'' (it would become ''Pelophilus manditra'' and not ''Pelophilus manditrus''). Changes in placement, or confusion over proper Latin grammar, lead to many incorrectly-formed names appearing in print, and automated searches failing to find all the variant spellings of a given name (e.g., the epithets ''atra'' and ''ater'' may refer to the same species). Accordingly, many laymen and some scientists object to the continued adherence to this long-standing practice.
In the interests of stability of nomenclature, the rule of priority can be reversed if a junior name has been used very widely and for a long period of time.
- ''. Taxonomists who consider the domesticated cat a subspecies of the wild cat should use ''F. silvestris catus''; the name ''F. catus'' remains available for the domestic cat where it is considered to be a separate species.
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