Who wrote great fleas have lesser fleas?

Augustus de Morgan did. A very similar poem was also written in the 1700s by Jonathan Swift. A. de Morgan's version can be viewed as a modernization.

de Morgan:

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.

Swift:

So, naturalists observe, a flea

Has smaller fleas that on him prey;

And these have smaller still to bite 'em;

And so proceed ad infinitum.