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HAECKEL'S THEORY 's Embryo Drawings .]] 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. 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 Slit s are not the same as Gill s and are ''not functional''. Gill slits are the Invagination s between the Gill Pouch es or pharyngeal pouches, and they open the Pharynx to the outside. Gill pouches appear in all Tetrapod animal embryos. In Mammal s, 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 {Link without Title} . 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 (though 'reptiles' are now known to be a composite group and the ancestors of mammals split off before todays' reptiles evolved), 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 ( Nephron ). The fact that the literal form of recapitulation theory is rejected by modern biologists has sometimes been used as an argument against and Phylogeny , explains them using evolutionary theory without recourse to Haeckel's specific views, and considers them as supporting evidence for that theory. Unfortunately, many science teachers and textbooks in the United States still erroneously cite recapitulation theory as evidence in support of evolution, in effect providing creationists with a very convincing counter-argument to the popular, facile notion of evolutionary theory. Because the actual scientific theory is difficult to explain in Layman's Terms , expressions of doubt about evolution that point to the flaws in Haeckel's arguments typically go unchallenged in popular media. Historical impact 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 fit nicely with 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. An early form of the law was devised by the 19th-century Estonian Zoologist Karl Ernst Von Baer , who observed that embryos resemble the embryos, but not the adults, of other species. MODERN OBSERVATIONS Generally, if a structure pre-dates another structure in evolutionary terms, then it also appears earlier than the other in the embryo. Species which have an evolutionary relationship typically share the early stages of embryonal development and differ in later stages. Examples include:
If a structure vanished in an evolutionary sequence, then one can often observe a corresponding structure appearing at one stage during embryonic development, only to disappear or become modified in a later stage. Examples include:
Modern theory One can explain connections between phylogeny and ontogeny if one assumes that one species changes into another by a sequence of small modifications to its developmental program (specified by the 's discredited recapitulation law, fails. REFERENCES |
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