I cited Arthur C. Clarke's 1959 story `Out of the Cradle, Endlessly Orbiting', reprinted in the collection, TALES OF TEN WORLDS, 1963:
`...famous remark of Tsiolkovsky's, which I'd hung up for everyone to see as they entered my office:
EARTH IS THE CRADLE OF THE MIND--BUT YOU CANNOT LIVE IN THE CRADLE FOREVER
Perhaps I should have spelled out a rather obvious play in Clarke's title (obvious, at any rate, to Americans, although maybe not to the rest of us): it's a reference to Walt Whitman's poem `Out of the Cradle Endlessly Rocking' (in LEAVES OF GRASS, 1881 edition), which opens with that line. Its second line, eerily predictive in its own way, perhaps, is `Out of the mocking-bird's throat, the musical shuttle'.