Haeckel Ernst (1834–1919), German zoologist, an impassioned adherent of Darwin’s theory of evolution. His popular work Die Welträtsel (The Riddle of the Universe, 1899) became a best-seller and was very influential in its time. Lenin is said to have admired it. Haeckel’s philosophy, which he called monism, is characterized negatively by his rejection of free will, immortality, and theism, as well as his criticisms of the traditional forms of materialism and idealism. Positively it is distinguished by passionate arguments for the fundamental unity of organic and inorganic nature and a form of pantheism. M.K.