Ernst Haeckel was a naturalist who spent much of his career examining the relationship between evolutionary development (phylogeny) and the development of the embryo (ontogeny). Although the pursuit of this relationship often led Haeckel to advance some rather extreme theories, the environment of controversy and debate that surrounded him produced a number of genuine advances in the field of biology.

Haeckel was born in Potsdam, Prussia (now Germany), in 1834. His father, a lawyer and government official, decided early on that young Ernst would be a physician. Haeckel himself displayed little interest in medicine, preferring the biological sciences; he chose, however, to honor his father's wishes, enrolling at the University of Berlin's college of medicine. While a student, he took part in an expedition to the North Sea to study tiny sea creatures and, though he continued his medical coursework, his passion for biology was reignited. He obtained his...