Haeckel was a physician and, later, a professor of comparative anatomy; Haeckel was one of the first to consider psychology as a branch of physiology. He also proposed many now ubiquitous terms including "phylum" and "ecology."
Haeckel's observations on the link between ontogeny and phylogeny have been named the "recapitulation law," summed up in the phrase, ontogeny recapitulates phylogeny. As Haeckel's effort to prove the law was itself provably misguided and inaccurate, he became a favorite target of creationists seeking to discredit the theory of evolution.
Haeckel was a flamboyant figure whose popularity with the public was substantially higher than it was with many of his scientific peers. Although Haeckel's ideas are important to the history of evolutionary theory and he was a competent invertebrate anatomist, almost all the speculative concepts that he championed have turned out to be incorrect. Haeckel hypothesized, described and named, hypothetical ancestral micro-organisms that have not been found and almost certainly do not exist. His concept of recapitulation has been disproved. Haeckel did not support "survival of the fittest" instead believing in a Lamarckian inheritance of acquired characteristics. On top of picking the wrong theories to support, he was caught using doctored data in some of his papers.
