Ernst Heinrich Philipp August Haeckel (February 16, 1834 – August 9, 1919), also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in biology, including anthropogeny, ecology, phylum, phylogeny, and the kingdom Protista. Haeckel promoted and popularized Charles Darwin's work in Germany and developed the controversial recapitulation theory ("ontogeny recapitulates phylogeny") claiming that an individual organism's biological development, or ontogeny, parallels and summarizes its species' entire evolutionary development, or phylogeny.

The published artwork of Haeckel includes over 100 detailed, multi-colour illustrations of animals and sea creatures (Kunstformen der Natur, "Artforms of Nature"). As a philosopher, Ernst Haeckel wrote Die Welträtsel (1895 – 1899, in English, The Riddle of the Universe, 1901), the genesis for the term "world riddle" (Welträtsel); and Freedom in Science and Teaching to support teaching evolution. In the United States, Mount Haeckel, a 13,418 ft (4,090 m) summit in the Eastern Sierra Nevada, overlooking the Evolution Basin, is named in his honor, as is another Mount Haeckel, a 2,941 m (9,649 ft) summit in New Zealand; and the asteroid 12323 Haeckel. The Ernst Haeckel house ("Villa Medusa") in Jena, Germany, contains a historic library.

Ernst Haeckel was born on February 16, 1834, in Potsdam (then part of Prussia). In 1852, Haeckel completed studies at Cathedral High School (Domgymnasium) of Merseburg. He then studied medicine in Berlin and biology at Würzburg, particularly with Albert von Kölliker, Franz Leydig, Rudolf Virchow (with whom he later worked briefly as assistant), and with anatomist - physiologist Johannes Peter Müller (1801 – 1858). Together with Hermann Steudner he attended botany lectures in Würzburg. In 1857, Haeckel achieved a doctorate in biology (D.Sc.), and then he received a PhD in Zoology at faculty of Biology. He decided not to practice medicine, but to use his degree in Biology. Haeckel studied under Karl Gegenbaur at the University of Jena for three years, earning a doctorate in zoology, before becoming a professor of comparative anatomy at the University of Jena, where he remained for 47 years, from 1862 to 1909. Between 1859 and 1866, Haeckel worked on many invertebrate groups, including radiolarians, poriferans (sponges) and annelids (segmented worms). During a trip to the Mediterranean, Haeckel named nearly 150 new species of radiolarians. Haeckel named thousands of new species from 1859 to 1887.

From 1866 to 1867, Haeckel made an extended journey to the Canary Islands with Hermann Fol and during this period, met with Charles Darwin, in 1866 at Down House in Kent, Thomas Huxley and Charles Lyell. In 1867, he married Agnes Huschke. Their son Walter was born in 1868, their daughters Elizabeth in 1871 and Emma in 1873. In 1869, he traveled as a researcher to Norway, in 1871 to Dalmatia, and in 1873 to Egypt, Turkey, and to Greece. Haeckel retired from teaching in 1909, and in 1910 he withdrew from the Evangelical church. Haeckel's wife, Agnes, died in 1915, and Haeckel became substantially more frail, with a broken leg (thigh) and broken arm. He sold the mansion Medusa ("Villa Medusa") in 1918 to the Carl Zeiss foundation. Haeckel died on August 9, 1919.

Haeckel's political beliefs were influenced by his affinity for the German Romantic movement coupled with his acceptance of a form of Lamarckism. Rather than being a strict Darwinian, Haeckel believed that racial characteristics were acquired through interactions with the environment and that ontogeny directly followed phylogeny. He believed the social sciences to be instances of "applied biology". Most of these arguments have been shown to be over-generalizations at best and flatly incorrect at worst in modern biology and social studies. In 1905, Haeckel founded a group called the "Monist League" to promote his religious and political beliefs. This group lasted until 1933 and included such notable members as Wilhelm Ostwald, Georg von Arco, Helene Stöcker and Walter Arthur Berendsohn.

Haeckel was the first person known to use the term "First World War". Shortly after the start of the war Haeckel wrote:

Indianapolis Star, September 20, 1914

The "European War" became known as "The Great War", and it was not until 1931, with the beginning realization that another global war might be possible, that there is any other recorded use of the term "First World War".

Haeckel was a zoologist, an accomplished artist and illustrator, and later a professor of comparative anatomy. Although Haeckel's ideas are important to the history of evolutionary theory, and he was a competent invertebrate anatomist most famous for his work on radiolaria, many speculative concepts that he championed are now considered incorrect. For example, Haeckel described and named hypothetical ancestral microorganisms that have never been found.

He was one of the first to consider psychology as a branch of physiology. He also proposed many now ubiquitous terms including "anthropogeny", "phylum", "phylogeny", "ecology" ("oekologie"), and proposed the kingdom Protista in 1866. His chief interests lay in evolution and life development processes in general, including development of nonrandom form, which culminated in the beautifully illustrated Kunstformen der Natur (Art forms of nature). Haeckel did not support natural selection, rather believing in a Lamarckian inheritance of acquired characteristics (Lamarckism).

Haeckel advanced a version of the earlier "recapitulation theory", previously set out by Étienne Serres in the 1820s and supported by followers of Geoffroy including Robert Edmond Grant, which proposed a link between ontogeny (development of form) and phylogeny (evolutionary descent), summed up by Haeckel in the phrase "ontogeny recapitulates phylogeny". His concept of recapitulation has been refuted in the form he gave it (now called "strong recapitulation"), in favour of the ideas first advanced by Karl Ernst von Baer. "Strong" recapitulation hypothesis views ontogeny as repeating forms of the ancestors, while "weak" recapitulation means that what is repeated (and built upon) is the ancestral embryonic development process. He supported the theory with embryo drawings that have since been shown to be oversimplified and in part inaccurate, and the theory is now considered an oversimplification of quite complicated relationships. Haeckel introduced the concept of "heterochrony", which is the change in timing of embryonic development over the course of evolution.

Haeckel was a flamboyant figure. He 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 (1859), 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. (Richard and Oskar Hertwig were two of Haeckel's many important students.) One student did find the remains: 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.

The creationist polygenism of Samuel George Morton and Louis Agassiz, which presented human races as separately created species, was rejected by Charles Darwin, who argued for the monogenesis of the human species and the recent African origin of modern humans. In contrast to most of Darwin's supporters, Haeckel put forward a doctrine of evolutionary polygenism based on the ideas of the linguist August Schleicher, in which several different language groups had arisen separately from speechless prehuman Urmenschen, which themselves had evolved from simian ancestors. These separate languages had completed the transition from animals to man, and, under the influence of each main branch of languages, humans had evolved — in a kind of Lamarckian use-inheritance — as separate species, which could be subdivided into races. From this Haeckel drew the implication that languages with the most potential formed human species with the most potential, led by the Semitic and Indo - Germanic groups, with Berber, Jewish, Greco - Roman and Germanic varieties to the fore. As Haeckel stated:

We must mention here one of the most important results of the comparative study of languages, which for the Stammbaum of the species of men is of the highest significance, namely that human languages probably had a multiple or polyphyletic origin. Human language as such probably developed only after the species of speechless Urmenschen or Affenmenschen had split into several species or kinds. With each of these human species, language developed on its own and independently of the others. At least this is the view of Schleicher, one of the foremost authorities on this subject.… If one views the origin of the branches of language as the special and principal act of becoming human, and the species of humankind as distinguished according to their language stem, then one can say that the different species of men arose independently of one another.

Haeckel's view can be seen as a forerunner of the multi-regional hypothesis, which until the 1990s remained in contention with developments of Darwin's hypothesis of a recent African origin of modern humans. The multiregional view then fell from favour, and Darwin's view has more recently been validated by the decipherment of the human genome. Haeckel also applied the hypothesis of polygenism to the modern diversity of human groups. He became a leading proponent of scientific racism, stating for instance:

The Caucasian, or Mediterranean man (Homo Mediterraneus), has from time immemorial been placed at the head of all the races of men, as the most highly developed and perfect. It is generally called the Caucasian race, but as, among all the varieties of the species, the Caucasian branch is the least important, we prefer the much more suitable appellation proposed by Friedrich Müller, namely, that of Mediterranese. For the most important varieties of this species, which are moreover the most eminent actors in what is called "Universal History," first rose to a flourishing condition on the shores of the Mediterranean.… This species alone (with the exception of the Mongolian) has had an actual history; it alone has attained to that degree of civilization which seems to raise men above the rest of nature.

When Haeckel was a student in the 1850s he showed great interest in embryology, attending the rather unpopular lectures twice and in his notes sketched the visual aids: textbooks had few illustrations, and large format plates were used to show students how to see the tiny forms under a reflecting microscope, with the translucent tissues seen against a black background. Developmental series were used to show stages within a species, but inconsistent views and stages made it even more difficult to compare different species. It was agreed by all European evolutionists that all vertebrates looked very similar at an early stage, in what was thought of as a common ideal type, but there was a continuing debate from the 1820s between the Romantic recapitulation theory that human embryos developed through stages of the forms of all the major groups of adult animals, literally manifesting a sequence of organisms on a linear chain of being, and Karl Ernst von Baer's opposing view that the early general forms diverged into four major groups of specialised forms without ever resembling the adult of another species, showing affinity to an archetype but no relation to other types or any transmutation of species. By the time Haeckel was teaching he was able to use a textbook with woodcut illustrations written by his own teacher Albert von Kölliker, which purported to explain human development while also using other mammalian embryos to claim a coherent sequence. Despite the significance to ideas of transformism, this was not really polite enough for the new popular science writing, and was a matter for medical institutions and for experts who could make their own comparisons.