The International Code of Zoological Nomenclature (ICZN or ICZN Code) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. The rules principally regulate:
Written nomenclatural rules in zoology were compiled in various countries since the late 1830s, such as Merton's Rules[1] and Strickland's codes[2] going back to 1843.[3] On the first and second International Zoological Congresses (Paris 1889, Moscow 1892) zoologists saw the need to establish commonly accepted international rules to be followed in all disciplines and in all countries, to substitute the various different conventions and unwritten rules used in various disciplines and which differed also among countries in which English, French or German was either spoken or used as the international scientific language in these times.
Compiling "International Rules on Zoological Nomenclature" was first proposed in 1895 in Leiden (3rd International Congress for Zoology) and officially published in three languages in 1905 (French, English, German; only French was official).[4] From then on amendments and modifications were subsequently passed on various zoological congresses (Boston 1907, Graz 1910, Monaco 1913, Budapest 1927, Padova 1930, Paris 1948, Copenhagen 1953 and London 1958). These were only published in English and can only be found in the reports of these congresses or in other official publications. The 1905 Rules became increasingly outdated, they were soon sold out and it became increasingly difficult to obtain to complete set of the Rules with all its amendments.[5] In Copenhagen 1953 the French and English texts of the Rules were declared to be of equivalent official force and a declaration was approved to prepare a new compilation of the rules. An Editorial Committee was composed in London 1958 to elaborate a completely new version of the nomenclatural rules which should then be called the ICZN Code. This text was finally published as the first edition of the ICZN Code on 9 November 1961.
The 2nd edition of the Code (only weakly modified) was from 1963. The last zoological congress to deal with nomenclatural problems took place in Monte Carlo 1972, since then the official zoological organs did not derive its power from zoological congresses any more.[6] The 3rd edition of the Code was from 1985, the present edition is the 4th edition (effective since 2000). These Code editions were elaborated by Editorial Committees[7] appointed by the International Commission on Zoological Nomenclature. The ICZN Commission takes its power from a general biological congress (IUBS, International Union of Biological Sciences). The Editorial Committee for the 4th edition was composed of seven persons. Such new editions of the ICZN Code are not democratically approved by those taxonomists who are forced to follow the Code's provisions, neither do taxonomists have the right to vote for the members of the Commission or the Editorial Committee.
As the Commission may alter the Code (by declarations and amendments) without issuing a new edition of the book, the current edition does not necessarily contain the actual provision that applies in a particular case. Currently the Code consists of the original text of the 4th edition and Declaration 44. The Code is published in an English and a French[8] version; both versions are official and equivalent in force, meaning and authority.[9] This means that if something in the English Code is unclear or its interpretation ambiguous, the French version is decisive, and if there is something unclear in the French Code, the English version is decisive.
The Code deals with zoological nomenclature, which is defined in the Glossary as
Zoological nomenclature is independent of other systems of nomenclature, for example botanical nomenclature. This implies that animals can have the same generic names as plants.
The rules and recommendations have one fundamental aim: to provide the maximum universality and continuity in the naming of all animals, except where taxonomic judgment dictates otherwise. The Code is meant to guide only the nomenclature of animals, while leaving zoologists freedom in classifying new taxa. In other words, whether a species itself is or is not an entity to be recognized is a subjective decision, but what name should be applied to it is not; the Code applies only to the latter, not to the former. A new animal name published without adherence to the Code may be deemed simply "unavailable" if it fails to meet certain criteria, or fall entirely out of the province of science (e.g., the "scientific name" for the Loch Ness Monster).
The rules in the Code determine what names are valid for any taxon in the family group, genus group, and species group. It has additional (but more limited) provisions on names in higher ranks. The Code recognizes no case law. Any dispute is to be decided first by applying the Code directly, and not by reference to precedent. The Code is also retroactive or retrospective, which means that previous editions of the Code, or previous other rules and conventions have no force any more today,[10] and the nomenclatural acts published back in the old times are to be evaluated only under the present edition of the Code. In cases of disputes concerning the interpretation, the usual procedure is to consult the French Code, lastly a case can be brought to the Commission who has the right to publish a final decision.[11]
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In regulating the names of animals it holds by six central principles, which were first set out (as Principles) in the third edition of the Code (1985):
This is the principle that the scientific name of a species, and not of a taxon at any other rank, is a combination of two names; the use of a trinomen for the name of a subspecies and of uninominal names for taxa above the species group is in accord with this Principle.[12]
This means that in the system of nomenclature for animals the name of a species is composed of a combination of a generic name and a specific name (a "binomen"). No other rank can have a name composed of two names. Example:
In botanical nomenclature, the equivalent for "binominal nomenclature" is "binary nomenclature" (or sometimes "binomial nomenclature").
This is the principle that the correct formal scientific name for an animal taxon, the name that is to be used, called the valid name, is the oldest available name that applies to it.[12]
It is certainly the most important principle and the fundamental guiding precept that preserves the stability of zoological nomenclature. It was first formulated in 1842 by a committee appointed by the British Association to consider the rules of zoological nomenclature; the committee's report was written by Hugh Edwin Strickland.
Example:
There are approximately 2-3 million cases of this kind for which this Principle is applied in zoology.
This is the principle that within the family group, genus group and species group a name established for a taxon at any rank in the group is deemed to be simultaneously established with the same author and date for taxa based on the same name-bearing type at other ranks in the corresponding group.[12]
In other words, the act of publishing a new zoological name thereby automatically and simultaneously establishes all the corresponding names in the relevant other ranks, with the same type.
In the species-group, publishing a species name (the binomen) Giraffa camelopardalis Linnaeus, 1758 also establishes the subspecies name (the trinomen) Giraffa camelopardalis camelopardalis Linnaeus, 1758. The same applies to the name of a subspecies; this establishes the corresponding species name.
In the genus-group, similarly, publishing the name of a genus also establishes the corresponding name of a subgenus (or vice versa): genus Giraffa Linnaeus, 1758 and subgenus Giraffa (Giraffa) Linnaeus, 1758.
In the family-group, publication of the name of a family, subfamily, superfamily (or any other such rank) also establishes the names in all the other ranks in the family group (family Giraffidae, superfamily Giraffoidea, subfamily Giraffinae).
Author citations for such names (for example a subgenus) are the same as for the name actually published (for example a genus). It is immaterial if there is an actual taxon to which the automatically established name applies; if ever such a taxon is recognised, there is a name available for it.
This is the principle that in cases of conflicts between simultaneously published divergent acts, the first subsequent author can decide which one shall be regarded as the one that should have precedence.
It supplements the Principle of Priority, which states that the first published name takes precedence. The Principle of the First Reviser deals with situations that cannot be resolved by priority. These items may be two or more different names for the same taxon, two or more names with the same spelling used for different taxa, two or more different spellings of a particular name, etc. In such cases the first subsequent author who deals with the matter, makes a choice and publishes the decision in the required manner, the First Reviser, is to be followed.[13]
Example:
Linnæus 1758 established Strix scandiaca and Strix noctua (Aves), for which he gave different descriptions and referred to different types, but both taxa later turned out to refer to the same species, the snowy owl. The two names are subjective synonyms. Lönnberg 1931 acted as First Reviser, cited both names and selected Strix scandiaca to have precedence.
A problem is that sometimes the First Reviser is unknown. For the sperm whale Linnæus 1758 established three subjective synonyms, Physeter macrocephalus, Physeter catodon and Physeter microps. The First Reviser remains unknown, currently both names Ph. macrocephalus and Ph. catodon are used.
This is the principle that the name of each taxon must be unique. Consequently a name that is a junior homonym of another name must not be used as a valid name.[12]
It means that any one animal name, in one particular spelling, may be used only once (within its group). This will usually be the first-published name; any later name with the same spelling (a homonym) is barred from being used. The Principles of Priority and the First Reviser apply here. For family-group names the termination (which is rank-bound) is not taken into account.
Genera can either be homonyms or not. A one-letter difference is not tolerated.
Examples:
In species there is a difference between primary and secondary homonyms. There can also be double homonyms (same genus and species). A slight difference in spelling is tolerated if Art. 58 applies.
Primary homonyms are those which have the same genus and the same species in their original combination. The difference between a primary junior homonym and a subsequent use of a name is undefined, but it is commonly accepted that if the name referred to another species or form, and if there is in addition no evidence the author knew that the name was previously used, it is considered as a junior homonym.
Examples:
Secondary homonyms can be produced if taxa with the same specific name but different original genus are later classified in the same genus (Art. 57.3, 59). A secondary synonym is only a temporary state, it is only effective in this classification. If another classification is applied, the secondary homonymy may not be produced, and the involved name can be used again (Art. 59.1). A name does not become unavailable or unusable if it was once in the course of history placed in such a genus where it produced a secondary homonymy with another name. This is one of the rare cases where a zoological species does not have a stable specific name and a unique species-author-year combination, it can have two names at the same time.
Example:
Art. 59.3 states that in exceptional cases junior secondary homonyms which were replaced before 1961 by substitute names can become invalid "unless the substitute name is not in use", an exception of the exception. But the ICZN Code does not give an example for such a case. It seems that this passage in the ICZN Code is widely ignored. It is also undefined what the expression "is not in use" should mean.
Example:
Double homonymy (genus and species) is no homonymy: if the genera are homonyms and belong to different animal groups, the same specific names can be used in both groups.
Examples:
For disambiguating one genus-group name from its homonym, it is important to cite author and year. Citing the author alone is often not sufficient.
Examples:
The name Ansa can only be used for a lepidopteran taxon. If that name cannot be used (for example because an older name established prior to 1858 takes precedence), this does not mean that the 1868 name can be used for a hemipteran genus. The only option to use the 1868 name for the hemipteran taxon is to get the 1858 name officially suppressed by the Commission.
In some cases, the same genus-group or species-group name was published in the same year by the same author. In these cases it would be useful to cite the page on which the name was established.
There are cases where two homonyms were established by the same author in the same year on the same page:
Animal, plant and Fungi nomenclature are entirely independent from each other. The most evident shortcoming of this situation (for their use in biodiversity informatics) is that the same generic name can be used for animals and plants at the same time. For this kind of homonyms the expression "hemihomonyms" is sometimes used. Far more than 1000 such names are known.[15]
Examples:
For names above the family level, the Principle of Homonymy does not apply.
Examples:
Homonyms occur relatively rarely in families (only if generic names are identical or very similar and adding an ending -idae produces identical results). Discovering such a homonymy usually produces the same problems as if there were no rules: conflicts between entirely independent and unconnected groups of taxonomists working in different animal groups. Very often the Commission must be asked to take a decision.
Examples:
This is the principle that each nominal taxon in the family group, genus group or species group has, actually or potentially, a name-bearing type fixed to provide the objective standard of reference by which the application of the name is determined.
This means that any named taxon would have a name-bearing type which allows the objective application of that name. Any family-group name must have a type genus, any genus-group name must have a type species, and any species-group name can (not must) have one or more type specimens (holotype, lectotype, neotype, syntypes or others), usually deposited in a museum collection.
The type genus for a family-group name is simply the genus that provided the stem to which was added the ending -idae (for families).
Example:
The type species for a genus-group name is more complicated and follows exactly defined provisions in Art. 67-69. Type species are very important, and there is no general zoological database that has recorded the type species for all genera. Except in fishes and some minor groups, type species are rarely reliably recorded in online animal databases. In 60 % of the cases the type species can be determined in the original publication. The type species is always the original name of the taxon (and not the currently used combination).
Example:
Designation and fixation have different meanings. A designation is the proposal of the type species. It is not necessary to have spelled the name of the genus or species correctly with correct authors (Art. 67.2.1, 67.6, 67.7), type species will always be the correctly spelled name. If the designation is valid, the type species is fixed.
A designation can also be invalid and ineffective, for example if the genus had already a previously fixed type species, or if a type species was proposed that was not originally included, or contradicted the description or figure for a genus for which no species had originally been included.
There are various possible modes of type species designation. This is their order of legal importance, with approximate proportions of occurrence[16] and examples:
A species-group name can have a name-bearing type specimen, but this is no must. In many cases species-group names have no type specimens, or they are lost. In those cases the application of the species-group name is usually based on common acceptance. If there is no common acceptance, there are provisions in the Code to fix a name-bearing type specimen which will be binding for the users of that name. Fixing such a name-bearing type should only be done if this is taxonomically necessary (Art. 74.7.3, 75.2, 75.3).
Examples:
Links to the separate articles (these are redundant and can eventually be removed):
The Code divides names in the following manner:
The names above the family group are regulated only as to the requirements for publication; there is no restriction to the number of ranks and the use of names is not restricted by priority.
The names in the family group, the genus group and the species group are fully regulated by the provisions in the Code. There is no limitation to the number of ranks allowed in the family group. In the genus group there are only two ranks: the genus and the subgenus. In the species group there are only two ranks: the species and the subspecies.
In the species group gender agreement applies. The name of a species, in two parts, a binomen, say, Loxodonta africana, and of a subspecies, in three parts, a trinomen, say Canis lupus albus, originally is a Latin phrase, and must be grammatically correct Latin. If the second part, the specific name (or the third part, the subspecific name) is adjectival in nature, its ending must agree in gender with the name of the genus. If it is a noun, or an arbitrary combinations of letters, this does not apply.
If a species is moved, therefore, the spelling of an ending may need to be changed. If Gryllus migratorius is moved to the genus Locusta, it becomes Locusta migratoria. Confusion over proper Latin grammar have led to many incorrectly formed names appearing in print. An improper automated search may fail to find all the variant spellings of a given name (e.g., the spellings atra and ater may refer to the same species).
Accordingly, many laymen and some scientists object to continued adherence to this rule, especially those who work with butterflies and moths. This has historical reasons. In 1758, Linnæus placed all butterflies in the genus Papilio, which after a few decades contained thousands of species.[18] From the beginning on the gender of Papilio was unclear, undecided and disputed. Some authors regarded it as masculine, others as feminine. Linnæus knew this problem and avoided any statement. All his 250 specific names in the genus Papilio were either nouns, unchangeable adjectives or adjectives ending in -is. He strictly did not use a single adjective ending in -us, -a, -um. P. Brown, Cramer, Fabricius, Fueßlin, Goeze, Poda and Schrank regarded Papilio as masculine, Ménétriés, Pontoppidan and most modern authors as feminine. In ICZN Opinion 278 from 1954 it was regarded as masculine. In many cases lepidopterists would not change the ending of a name as used by the author who established a name. So we find for example Papilio fuscus or Papilio macilentus, but also Papilio osmana and Papilio paradoxa. Only in few cases both versions are found in the web (an example is Papilio multicaudatus and Papilio multicaudata). This works also with other butterfly genera of which the gender is undisputed. Graphium is clearly neuter, but only the incorrect versions Graphium angolanus and Graphium mandarinus are used, while Graphium sandawanum can only be found in the correct declination. Likewise pairs are more frequently found in genera of which the gender is not obvious: Delias castaneus and Delias gigantea, Belenois albumaculatus and Belenois rubrosignata, Mylothris arabicus and Mylothris ruandana. Even in moths such pairs can be found: Xylophanes obscurus and Xylophanes turbata, Manduca boliviana and Manduca caribbeus, Sphinx caligineus and Sphinx formosana, Macroglossum albolineata and Macroglossum vicinum. It may also occur that a lepidopteran subspecies can have a different gender than the name of the species, as for example in Papilio multicaudata pusillus Austin & Emmel, 1998, or Papilio torquatus flavida Oberthür, 1879.
The rules in the Code are to be followed by all users of zoological names. However, its provisions can be interpreted, 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 International Commission on Zoological Nomenclature, acting on behalf of all zoologists. The Commission takes such action in response to proposals submitted to it.