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Rules of thumb: Oldest name has priority.

A younger name can be valid instead of an older name if:
- The older name is a primary homonym (there is another species with the same original name that is older; so two authers used the same name for different species);
- The older name is a secondary homonym (there is another species with the same combination of genus and species name that is older; so the same name for different species is caused by the fact that the species were placed in the same genus at a later date);
- The older name was not validly published;
- The older name was discovered much later or has mostly been overlooked and a younger name has been used so often and in most literature that the Internationl Commission for Zoological Nomeclature decides to give priority to the younger name by suppressing the older one.