Sanjie said that after the invention of the microscope, biologists found that some single-celled organisms have chlorophyll, which can make their eyes open; With flagella, it can swim freely and take food from the outside, such as euglena (also known as Euglena). Therefore, it is not clear whether they belong to plants or animals. 1866, German scholar haeckel put forward the theory of three realms, which divided the biological world into plant kingdom, animal kingdom and protozoa kingdom, among which the protozoa kingdom included bacteria, cyanobacteria, protozoa and myxomycetes.

There are various opinions about the three realms, namely 1. Three realms of Buddhism, 2. Nine days, three days. Biology, 4. Calligraphy and 5. Lacan