As discussed in Chapter 1, Aristotle divided all life into two taxonomic categories, plant and animal, a view that, as Section 5.3.2 recounts, dominated biology until less than two hundred years ago. When one considers that Aristotle’s observations were limited to what could be seen by means of unaided human vision, namely, plants, animals, and certain fungi, for example, mushrooms (which he classified as plants), this is hardly surprising. In the seventeenth century, Antonie van Leeuwenhoek, who first observed and described them under a microscope of his own devising, classified microorganisms as tiny animals (“animalcules”). It was not until the mid-nineteenth century that unicellular organisms were placed in their own (a third) taxonomic category, Protista, by Ernst Haeckel. What is surprising is how long Aristotle’s classification system survived in the face of mounting empirical evidence that unicellular organisms defy classification as plant or animal.