In the context of modern biology, a pseudo-scientific and/or out-dated ide derived from the medieval and pre-Darwin Christian concept of "chain of being," i.e., the hierarchy of permanent unchangable life-forms created by God. The idea became important when the study of fossils began in ernest in 18th Cent Europe. The fossils seem to appear in a range of progressions from simpler ones to those that appear highly complex, but with gaps in between some of them. Those yet to be discovered fossils that would have filled the 'gaps' were called "missing links." Modern concepts of evolution has done away with the idea of such a progress and, therefore, the idea of the "missing links."
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