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This law was first stated by Dollo in this way: "An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors." (Dollo, quoted in "Ammonites, Indicates Reversal," in Nature, March 21, 1970).
According to this hypothesis a structure or organ that has been lost or discarded through the process of evolution will not reappear in that line of organisms.