Not as illogical as it sounds. If you go back a thousand years or so, “fish” meant almost anything that mostly lived in water. That would have included muskrats if they’d been known.
And it’s actually no worse a way of looking at things than the current one, because according to at least one notable biologist, Stephen Jay Gould, there’s no such thing as a fish (see, e.g., the BBC QI episode “H for Hoax”). Lots of “fish” are more closely related to land animals than to each other. The modern definition basically boils down to the old one, with caveats of “but not those!”
Not as illogical as it sounds. If you go back a thousand years or so, “fish” meant almost anything that mostly lived in water. That would have included muskrats if they’d been known.
And it’s actually no worse a way of looking at things than the current one, because according to at least one notable biologist, Stephen Jay Gould, there’s no such thing as a fish (see, e.g., the BBC QI episode “H for Hoax”). Lots of “fish” are more closely related to land animals than to each other. The modern definition basically boils down to the old one, with caveats of “but not those!”