Stephen Jay Gould was an American geologist, paleontologist, biologist and popular-science author who spent most of his career teaching at Harvard University and working at the American Museum of Natural History in New York. At some point (it’s not clear exactly when) Gould — who had spent a lifetime studying evolutionary biology — declared that “there’s no such thing as a fish.” This comment became the name of a popular podcast spun off from the QI TV series, in which the hosts discuss interesting facts. But what did Gould mean? Obviously there are things called fish. Was this some attempt to be funny, like the guy who tried to convince people that birds aren’t real?
Not exactly. What Gould meant was that the term “fish” doesn’t really have any scientific or categorical meaning per se. In other words, lots of things that are defined as fish — many of which even have the term “fish” in their name, like the hagfish — are not really similar enough to be considered part of the same category of living things. As the Wikipedia entry for the podcast notes, a salmon is more closely related to a camel than it is to a hagfish, for example. All the things that we might believe to be common to fish — living underwater, having gills, fins, giving birth via eggs, etc. — are not universally true for everything that is usually thought of as a fish (also, there are lots of things called fish that aren’t, including the cuttlefish, starfish, crayfish, and jellyfish).
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