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.