Ernst Haeckel was a German biologist who lived from 1834 until 1919. He is primarily remembered today as a fierce proponent of Charles Darwin's theory of evolution by natural selection. Together with Englishman Thomas Henry Huxley, Haeckel persuaded a generation of scientists to accept the new paradigm, and fought those who opposed it on religious or other grounds.
Haeckel was a professor of comparative anatomy at the University of Jena. Much of his original research was on invertebrate species, including poriferans (sponges), annelids (segmented worms), and especially radiolarians.
Haeckel sometimes took great (and non-scientific) leaps from available evidence. For example, at the time that Darwin first published On the Origin of Species by Means of Natural Selection, no remains of human ancestors had yet been found. Haeckel postulated that evidence of human evolution would be found in the Dutch East Indies (now Indonesia), and described these theoretical remains in great detail. He even named the as-of-yet unfound species, Pithecanthropus alalus, and charged his students to go find it!
Remarkably, one of them did so -- a young Dutchman named Eugene Dubois went to the East Indies and dug up the remains of Java Man, the first human ancestral remains ever found. (These remains originally carried Haeckel's Pithecanthropus label, though they were later reclassified as Homo erectus.)
On the flip side, Haeckel's theory that ontogeny recapitulates phylogeny turned out to be wrong, and his graphs comparing embryonic development in different species turned out to be fake. Harvard biologist Stephen Jay Gould often took credit for discovering this fraud, but it had been widely discussed in the German scientific press as early as 1874, over one hundred years before Gould wrote about it.
Haeckel left his mark on language -- he coined the terms ontogeny, ecology, phylum, and many others. However his scientific work is typically ignored today, and may even have been consciously purged from textbooks. This may be because he extrapolated a new religion or philosophy called Monism from evolutionary science. In Monism, all economics, politics, and ethics are reduced to "applied biology."
His writings and lectures on Monism provided scientific (or quasi-scientific) justifications for racism, nationalism and social darwinism. Monism thus became the de facto religion of Nazi Germany.
Ironically, to the extent that Haeckel's work is remembered today, it is primarily in the arts. His glorious engravings of radiolarians and other creatures are studied in design and architecture programs. | 
