The biogenetic law is also known as the recapitulation theory, or the law of recapitulation. The biogenetic law is epitomized in Ernst Haeckel's (1834-1919) phrase "ontogeny recapitulates phylogeny." However, the biogentic law is based on the law of corresponding stages established by Karl Ernst von Baer (1792-1876), one of the founders of modern embryology.
In the course of his comparative studies of embryological development, von Baer discovered the mammalian ovum and the notochord, a transient embryonic structure in vertebrate embryos that eventually develops into the backbone. Von Baer also formulated a powerful descriptive generalization, which he referred to as the law of corresponding stages. This so-called law essentially states that during development the embryo of a higher animal passes through stages that resemble stages in the development of lower animals. As formulated by von Baer, the original law of corresponding stages was associated with four descriptive propositions. First, during development general characters appear before specialized ones. Second, the most general characteristics gradually develop towards the less general and then to the most specific. For example, limb buds become recognizable as limbs that at later stages differentiate into hands, wings, or flippers. Third, during development the embryo of a given species continuously diverges from those of other species. Fourth, the embryo of a higher species goes through stages that resemble the stages of development of lower animals.
Scientists less cautious than von Baer transformed these general principles into the overly simplified and dogmatic biogenetic law. Haeckel, one of the first German biologists to adopt Charles Darwin's (1809-1882) ideas about evolution, was instrumental in popularizing evolutionary theory and the biogenetic law. According to Haeckel, "ontogeny recapitulates phylogeny." The German embryologist argued that ontogeny, the embryological development of the individual, repeats phylogeny, the evolutionary development of the species. Although Haeckel and Darwin corresponded for many years after they met in 1866, Haeckel did not accept Darwin's theory that natural selection was the primary mechanism of evolution. Haeckel believed that new species evolved by means of novel steps added to their embryological development. He suggested that the ancestral organism from which all multicellular animals evolved had been a gastraea, that resembled the hollow ball of cells that make up an early embryo. Whereas Darwin thought of the evolutionary history of living forms would resemble a tree or bush, Haeckel thought that evolution progressed as an unbranched chain of being, with primitive beings at the bottom and human beings at the top. Interpreting embryological history as evidence that all species evolved from common ancestors, Haeckel assumed that during development embryos progressed though stages that were virtually identical to the adult forms of their ancestors.
Many scientists accepted and exploited the biogenetic law or recapitulation theory as a means of incorporating embryology into the body of evolutionary theory. Some proponents of the biogenetic law believed that during gestation the human embryo climbed the ladder of animal forms from lowest to highest: fish, reptile, mammal, human. Von Baer never accepted such oversimplified interpretations of his work.
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