Dollo's law of irreversibility (also known as Dollo's law and Dollo's principle) is a hypothesis proposed in 1893 by French-born Belgian paleontologist Louis Dollo which states that evolution is not reversible. This hypothesis 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." The statement is often misinterpreted as a less stringent hypothesis regarding the likelihood of regain of lost structures or organs, which are unlikely to reappear in the (loosely defined) 'same form.' According to Richard Dawkins, the law is "really just a statement about the statistical improbability of following exactly the same evolutionary trajectory twice (or, indeed, any particular trajectory), in either