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Ernst Haeckel was a German biologist, naturalist, philosopher, and artist who founded the school of thought known as "monism". Haeckel's most famous work is Art Forms of Nature, a collection of 100 color lithographs of various organisms, including plants, animals, and protists. The lithographs were intended to illustrate Haeckel's belief that all life is derived from a single ancestor, and that all organisms are related to each other.