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
This verse is by Jonathan Swift, the 18th century Irish cleric and satirist. He wrote it about the time that Antonie van Leeuwenhoek began looking through the microscope and discovered ‘very little living animalcules’. Van Leeuwenhoek discovered many small life forms, including bacteria, protozoa, and nematodes. His work revealed to scientists that another world of microscopic life existed, which paved the way for the study of bacteria and, eventually, viruses.
I think the verse by Swift is wonderful. It crystallizes in a very amusing way the world of microscopic life that arose from van Leeuwenhoek’s studies.