Ernst Haeckel was among the first German biologists to adopt Charles Darwin's ideas about evolution in the latter half of the nineteenth century. He popularized evolutionary thought on the European continent and added many novel concepts to biology and the social sciences. Haeckel is most often remembered for his "law of recapitulation," the belief that the evolutionary history of any given animal is repeated during its embryological development.
Haeckel was born in Potsdam, Germany, to middle-class parents, Carl and Charlotte. He was a curious naturalist from the start, studying botany in grammar school and cultivating his artistic skills to produce sketches and watercolors of living things. He began medical studies at the age of 18 and was licensed as a general practitioner, surgeon, and obstetrician at the age of 25. But soon after Charles Darwin's Origin of Species was published in 1859, Haeckel read a German translation of it and abandoned his medical practice. After three years of intense study he became a professor of comparative anatomy at the University of Jena.
The chief objects of study for Haeckel were invertebrates—small animals without backbones, including sponges and segmented worms. He traveled widely in search of unusual forms, exploring throughout Europe, North Africa, and Asia. In one trip to the Mediterranean he named almost 150 new species of radiolarian, marine protozoa with spiny shells. Haeckel's careful embryological studies of invertebrates formed the core of his evolutionary theories.
Haeckel met Charles Darwin (1809-1882) in 1866, and they corresponded for many years. The German embryologist was one of the most prolific supporters of evolution in continental Europe, but he diverged from Darwin's thoughts in many important respects. While Darwin proposed natural selection as the mechanism of evolution, each species changing gradually through the selective survival of advantageous traits, Haeckel's concept of evolution was steeped in his embryological discoveries. He asserted that new species evolved through novel steps at the end of their embryos' development. This integral role of embryology in the evolution of animal life was at the base of Haeckel's law of recapitulation.
As Haeckel put it, and has been quoted ever since, "ontogeny recapitulates phylogeny." Ontogeny, being the development of an individual animal, is an instant replay in fast-motion of phylogeny, the evolution of that animal's lineage through time. This was not a completely new idea; it was common among embryologists who had long observed the early worm-like or fishlike embryos of higher animals. But Haeckel did more than anyone to popularize and refine the concept. He even proposed an ancestral organism, a "gastraea," similar to the first hollow ball of cells that make up an embryo, as the hypothetical ancestor of all multi-celled animals, stimulating other hypotheses in this direction.
Haeckel's vision of evolution also diverged from Darwin's in its overall shape. While Darwin imagined a tree or bush, with all species more or less connected by spreading branches, Haeckel proposed an unbranching line. Each animal was related to the next up the chain, he reasoned, by added changes in embryonic development. Haeckel's concept was a scientific rendering of the Great Chain of Being, a ranking of living things that had occupied much of Western thought for centuries and inevitably put human beings literally at the top.
The law of recapitulation gained some popularity in Europe during Haeckel's generation but was largely discredited by the time of his death. Haeckel's thoughts have had a more lasting effect on the public perception of evolution, and on the social sciences. His linear vision of evolution not only supported a ranking of species, with humans at the top, but a ranking of humans from the more "primitive" (typically non-Caucasian) to the more "evolved" European races. Haeckel and his followers gave scientific validity to racial prejudices, ultimately leading to the tragic genocidal policy of Adolf Hitler and the Third Reich during the 1930s and 1940s. Despite its flaws, obvious to us now, recapitulation remained a central paradigm of biology into modern times.