With apologies to Mr. Walton and to so many coastal compatriots in New England, I regret to report that there is surely no such thing as a fish. About 20,000 species of vertebrate have scales and fins and live in water, but they do not form a coherent cladistic group. Some - the lungfishes and the coelacanth in particular - are genealogically close to the creatures that crawled out on land to become amphibians, reptiles, birds, and mammals. In a cladistic ordering of a trout, a lungfish, and any bird or mammal, the lungfish must form a sister group with the sparrow or elephant, leaving the trout in his stream. The characters that form our vernacular concept of "fish" are all shared primitive and do not therefore specify cladistic groups.
At this point, many biologists rebel, and rightly I think. The cladogram of trout, lungfish and elephant is undoubtedly true as an expression of branching order in time. But must classifications be based only on cladistic information? A coelacanth looks like a fish, tastes like a fish, acts like a fish, and therefore - in some legitimate sense beyond hidebound tradition - is a fish.
