‘These evolutionary happenings are unique, unrepeatable, and irreversible. It is as impossible to turn a land vertebrae into a fish as it is to effect the reverse transformation. The applicability of the experimental method to the study of such unique historical processes is severely restricted before all else by the time intervals involved, which far exceed the lifetime of any human experimenter. And it is just such impossibility that is demanded by anti-evolutionists when they ask for “proofs” of evolution which they would magnanimously accept as satisfactory.’
Theodosius Dobzhansky (late Emeritus Professor of Zoology and Biology, Rockefeller University) American Scientist, vol. 45, p.388 1957