From satirist Jonathan Swift:
So nat’ralists observe, a flea
Hath smaller fleas that on him prey,
And these have smaller fleas that bite ’em,
And so proceed ad infinitum.
Logician Augustus De Morgan goes him one better:
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.