Ernst Haeckel was a very versatile and complete biologist,equally at home with imaginative leaps of conceptualization,serious natural history in the wild, and meticulous experimentationin embryology. His work shaped the developmentof a holistic evolutionary perspective that brought ecology,ontogeny, phylogeny, and biogeography together into a unifiedexplanation of the patterns of diversity seen in the livingworld. He, along with Darwin and Mendel, was perhaps oneof the three most consequential biologists of the nineteenth century,in many ways the golden age of biology.