Leda and the Swan, by William Butler Yeats, first appeared in Dial, June 1924, before being refined for publication in A Vision, 1937 (the version shown above, with identical last verse).

Leda and the Swan

A rush, a sudden wheel, and hovering still
The bird descends, and her frail thighs are pressed.
By the webbed toes, and that all-powerful bill
Has laid her helpless face upon his breast.

How can those terrified vague fingers push
The feathered glory from her loosening thighs!*
All the stretched body's laid on the white rush
And feels the strange heart beating where it lies;

A shudder in the loins engenders there
The broken wall, the burning roof and tower
And Agamemnon dead.
Being so caught up,
So mastered by the brute blood of the air,
Did she put on his knowledge with his power
Before the indifferent beak could let her drop?

* In The Tower, 1928, this line was 'The feathered glory from her loosening thighs?', different still from the final version.

The story of Leda and the Swan is found in Greek mythology. Zeus is thought to have disguised himself as a swan, raping Leda ( a Queen of Sparta). From this encounter she gave birth to an egg that contained Pollux and Helen of Troy. Some versions tell that her other children Castor and Clytemnestra were born - also via egg - through the violent coupling.