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Ontogeny recapitulates phylogeny, also called the biogenetic law or the theory of recapitulation, is a now-discredited hypothesis in biology first espoused in 1866 by the German zoologist Ernst Haeckel, a contemporary of Charles Darwin. Ontogeny is the development of the embryos of a given species; phylogeny is the evolutionary history of a species. The theory claims that the development of the embryo of every species repeats the evolutionary development of that species fully. Or otherwise put: each successive stage in the development of an individual represents one of the adult forms that appeared in its evolutionary history. Haeckel formulated his theory as such: "Ontogeny recapitulates phylogeny". This notion later became simply known as recapitulation.
In order to support his theory, Haeckel produced several embryo drawings which overemphasized similarities between embryos of related species and found their way into many biology textbooks. For example, the human embryo with gill slits in the neck was believed by Haeckel to not only signify a fishlike ancestor, but it represented a total fishlike stage in development. Gill slits are not the same as gills and are not functional. Gill slits are the invaginations between the gill pouches or pharyngeal pouches, and they open the pharynx to the outside. Gill pouches appear in all tetrapod animal embryos. In mammals, the first gill bar (in the first gill pouch) develops into the lower jaw (Meckel's cartilage ), the malleus and the stapes. In a later stage, all gill slits close, with only the ear opening remaining open. For a technical discussion on the topic, see .
Modern biology rejects the literal form of Haeckel's theory. While for instance the phylogeny of humans as having evolved from fish through reptiles to mammals is accepted, no cleanly defined "fish", "reptile" and "mammal" stages of human embryonal development can be discerned. There is no linearity in the development. For instance in kidney development, at one given time, the anterior region of the kidney is less developed (nephridium ) than the posterior region (nefron ).
The fact that the literal form of recapitulation theory is rejected by modern biologists has sometimes been used as an argument against evolution by creationists. The argument is: "Haeckel's theory was presented as supporting evidence for evolution, Haeckel's theory is wrong, therefore evolution has less support". This argument is not only an oversimplification but misleading because modern biology does recognize numerous connections between ontogeny and phylogeny, explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory.
Although Haeckel's specific form of recapitulation theory is now discredited among biologists, it did have a strong impact in social and educational theories of the late 19th century. The maturationist theory of G. Stanley Hall was based on the premise that growing children would recapitulate evolutionary stages of development as they grew up and that there was a one to one correspondence between childhood stages and evolutionary history, and that it was counterproductive to push a child ahead of its development stage. The whole notion fitted nicely in other social darwinist concepts, such as the idea that "primitive" societies needed guidance by more advanced societies, i.e. Europe and North America, which were the pinnacle of evolution.
- Haeckel, E. 1899. Riddle of the Universe at the Close of the Nineteenth Century. Cited at http://www.ucmp.berkeley.edu/history/haeckel.html.
- Division of Biology and Medicine, Brown University, Evolution and Development I: Size and shape, http://biomed.brown.edu/Courses/BIO48/30.S&S.HTML
- Rebecca Irwin, Ontogeny and Phylogeny, http://www.utm.edu/~rirwin/391OntogPhylog.htm
- Stephen Jay Gould, Ontogeny and Phylogeny, Cambridge Mass.: Belknap Press of Harvard University Press, 1977 ISBN 0-674-63941-3
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