From Aristotle (384–322B.C.E.) to Carolus Linnaeus (1707–1778), scientists who proposed the earliest classification systems divided living organisms into two kingdoms—plants and animals. During the nineteenth century, Ernst Haeckel (1834–1919) proposed establishing a third kingdom—Protista—for simple organisms that do not appear to fit in either the plant or animal kingdom. In 1969 R. H. Whitaker (1920–1980) proposed a system of classification based on five different kingdoms. The groups Whitaker suggested were the bacteria group Prokaryotae (originally called Monera), Protista, Fungi (for multicellular forms of nonphotosynthetic heterotrophs and single-celled yeasts), Plantae, and Animalia. This classification system is still widely accepted; however a six-kingdom system of classification was proposed in 1977 by Carl Woese (1928–). The groups proposed in the six-kingdom approach are Archaebacteria and Eubacteria (both for bacteria), Protista, Fungi, Plantae, and Animalia. In 1981 Woese proposed a classification system based on three domains (a level of classification higher than kingdom): Bacteria, Archaea, and Eukarya. The domain Eukarya is subdivided into four kingdoms: Protista, Fungi, Plantae, and Animalia.