Dollo's law of irreversibility (also recognized as Dollo's principle or Dollo's law), postulated by Belgian paleontologist Louis Dollo in 1893, asserts that an organism never gets back identically to a former state, even if it uncovers itself put in conditions of life equivalent to those wherein it has originally settled. It always keeps some trail of the various intervals through which it has passed.

There are some intermediate steps that the organism has passed through during its lifetime and which it will never be able to return. As a result, the organism undergoes a sort of evolution that cannot be reversed. The fact that the evolutionary element is reversible should not be misinterpreted. The Dollos law is used in morphology, particularly in the study of fossils. This concept can also be applied to the molecular events that must have occurred during an organism's growth. Subsequently, this method can be used to investigate specific mutations as well as gene loss. It is supposedly a definition of the Law of irreversibility.