The German zoologist Ernst Haeckel (1834-1919) first proposed the kingdom Protista in 1866. This early classification included any microorganism that was not a plant or an animal. Biologists did not readily accept this kingdom, and even after the American botanist Herbert F. Copeland again tried to establish its use 90 years later, there was not much support from the scientific community. Around 1960, R.Y. Stanier and C.B. Van Niel (1897-1985) proposed the division of all organisms into two groups, the prokaryotes and the eukaryotes. Eukaryotes are organisms that have membrane-bound organelles in which metabolic processes take place, while prokaryotes lack these structures. In 1969, Robert Whittaker proposed the five-kingdom system of classification. The kingdom Protista was one of the five proposed kingdoms. At this time, only unicellular eukaryotic organisms were considered protists. Since then, the kingdom has expanded to include multicellular organisms, although biologists still disagree about what exactly makes an organism a protist.