Dollo's law

Dollo's Law is a hypothesis proposed by French-born Belgian paleontologist Louis Dollo (1857-1931) in 1890 that states that evolution is not reversible.

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.