The document summarizes different systems of microbial classification over time. It describes the Two Kingdom system proposed by Linnaeus, the Three Kingdom system by Haeckel, and the Five Kingdom system by Whittaker. Whittaker's system included the kingdoms Monera, Protista, Fungi, Plantae, and Animalia. Later, the Archaebacteria were separated from Eubacteria, resulting in the Three Domain system of Archaea, Bacteria, and Eukarya. This system is now most widely used to classify organisms based on genetic sequencing and ribosomal RNA structure.