Recapitulation Theory - Haeckel

Haeckel

Ernst Haeckel attempted to synthesize the ideas of Lamarckism and Goethe's Naturphilosophie with Charles Darwin's concepts. While often seen as rejecting Darwin's theory of branching evolution for a more linear Lamarckian "biogenic law" of progressive evolution, this is not accurate: Haeckel used the Lamarckian picture to describe the ontogenic and phylogenic history of the individual species, but agreed with Darwin about the branching nature of all species from one, or a few, original ancestors. Since around the start of the twentieth century, Haeckel's "biogenetic law" has been refuted on many fronts.

Haeckel formulated his theory as "Ontogeny recapitulates phylogeny". The notion later became simply known as the recapitulation theory. Ontogeny is the growth (size change) and development (shape change) of an individual organism; phylogeny is the evolutionary history of a species. Haeckel's recapitulation theory claims that the development of advanced species passes through stages represented by adult organisms of more primitive species. Otherwise put, each successive stage in the development of an individual represents one of the adult forms that appeared in its evolutionary history.

For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches in the neck of the human embryo resembled gill slits of fish, thus representing an adult "fishlike" developmental stage as well as signifying a fishlike ancestor. Embryonic pharyngeal slits, which form in many animals when the thin branchial plates separating pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes. But these embryonic pharyngeal arches, grooves, pouches, and slits in human embryos could not at any stage carry out the same function as the gills of an adult fish.

Haeckel produced several embryo drawings that often overemphasized similarities between embryos of related species. The misinformation was propagated through many biology textbooks, and popular knowledge, even today. Modern biology rejects the literal and universal form of Haeckel's theory.

Haeckel's drawings were disputed by Wilhelm His, who had developed a rival theory of embryology. His developed a "causal-mechanical theory" of human embryonic development.

Darwin's view, that early embryonic stages are similar to the same embryonic stage of related species but not to the adult stages of these species, has been confirmed by modern evolutionary developmental biology.