NARRATOR: For instance, on the planet Earth Man had always assumed that he was more intelligent than dolphins because he had achieved so much … the wheel, New York, wars, and so on, whilst all the dolphins had ever done was muck about in the water having a good time. But conversely the dolphins believed themselves to be more intelligent than man for precisely the same reasons.

Douglas Adams (1952-2001) English author, humorist, screenwriter
Hitchhiker’s Guide to the Galaxy, Phase 1, “Fit the 3rd” (BBC Radio) (1978-03-22)
    (Source)

This is rendered the same (save for some punctuation polishing) in the novel form, Hitchhiker's Guide to the Galaxy No. 1, The Hitchhiker's Guide to the Galaxy, ch. 23 (1979):

For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much -- the wheel, New York, wars and so on -- whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man -- for precisely the same reasons.