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." According to this hypothesis a structure or organ that has been lost or discarded through the process of evolution will not reappear in exactly the same form in that line of organisms. 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 direction." Stephen Gould viewed the idea less strictly, suggesting that "irreversibility" forecloses certain evolutionary pathways once broad forms have emerged: ", once you adopt the ordinary body plan of a reptile, hundreds of options are forever closed, and future possibilities must unfold within the limits of inherited design."
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“Escalus. What do you think of the trade, Pompey? Is it a lawful trade?
Pompey. If the law would allow it, sir.
Escalus. But the law will not allow it, Pompey; nor it shall not be allowed in Vienna.
Pompey. Does your worship mean to geld and spay all the youth of the city?
Escalus. No, Pompey.
Pompey. Truly, sir, in my poor opinion they will to’t then. If your worship will take order for the drabs and the knaves, you need not to fear the bawds.”
—William Shakespeare (1564–1616)