Haeckel, Ernst Heinrich (1834–1919), a German zoologist and philosopher. Haeckel was the chief popularizer on the European continent of Charles Darwin's theory of evolution. Later Haeckel created controversy by denying the immortality of the soul, freedom of the will, and existence of a personal God.

Haeckel believed he had discovered proof of evolution in his biogenetic law, or theory of recapitulation. This theory states that a developing organism passes through stages in which it resembles, in sequence, its ancestors. He traced human development in 26 stages from the simplest forms of animal life. His theories have been discarded or sharply modified, but his careful observations of single-celled organisms still provide useful information.

Haeckel attended the universities of Berlin, Vienna, and Würzburg. He received a medical degree at Würzburg before becoming a zoologist. While serving as professor of zoology at the University of Jena, 1862–1909, he lectured throughout Germany.

Haeckel's works include: General Morphology of Organisms (1866); The Evolution of Man (1874); The Riddle of the Universe (1899).