Ernst Haeckel, zoologist and artist, was the foremost proponent of evolutionary theory in 19th-century Europe.  His observations of nature led to the discovery of almost 4,000 species of radiolarian, and his iconic visual forms helped to make complex information accessible to other scientists and to the public.

In his 1866 book General Morphology of Organisms, he illustrated the first genealogical tree showing how organisms might be related through descent and included hypotheses about evolutionary relationships based fossils. His use of the tree as a graphic device was quite original and became the standard mode for such depictions until at least the 1970s.

In trying to shore up the evidence for his theories Haeckel manipulated several of his most famous drawings by exaggerating the resemblance among various organisms. He proposed that the development of the embryo repeats the organism's evolutionary history, or "ontogeny recapitulates phylogeny" with the human embryo progressing through fish, amphibian, and other precursor forms while still in the womb.  Much of his work is now discredited because of this manipulation.