German zoologist Ernst Haeckel famously—and inaccurately—uttered, "Ontogeny recapitulates phylogeny." While investigating the developing embryos of a variety of vertebrates, Haeckel thought they all closely resembled one another. This observation led to his conclusion that embryonic development echoed morphological evolution. Specifically, gill depressions in human embryos led Haeckel to conclude that humans were derived from fishes. He therefore felt that the study of embryonic development, or ontogeny, retold the story of evolution, or phylogeny. As he wrote in 1866, "During its rapid evolution, an individual repeats the most important changes in form evolved by its ancestors during their long and slow paleontological development." (Haeckel)

There are a number of flaws with Haeckel's theory. For example, Haeckel confused a fish embryo with a young human one. Haeckel's drawings strongly suggest that a variety of vertebrates share a common developmental...