By JonathanSwift, from "On Poetry, A Rhapsody".
So, naturalists observe, a flea Hath smaller fleas that on him prey; And these have smaller still to bite 'em; And so proceed ad infinitum. Thus every poet, in his kind, Is bit by him that comes behind.
The more familiar form comes from AugustusDeMorgan, in "A Budget of Paradoxes":
Great fleas have little fleas upon their backs to bite 'em, And little fleas have lesser fleas, and so ad infinitum. And the great fleas themselves, in turn, have greater fleas to go on; While these again have greater still, and greater still, and so on.