the burden of proof to those who claim that creationism is a better explanation. Falsifiability Philosopher of science Karl R. Popper set out the concept of falsifiability as a way to distinguish science and pseudoscience: testable theories are scientific, but those that are untestable are not. In Unended Quest, Popper declared "I have come to the conclusion that Darwinism is not a testable scientific theory but a metaphysical research programme, a possible framework for testable scientific theories," while pointing out it had "scientific character." In what one sociologist derisively called "Popper-chopping," opponents of evolution seized upon Popper's definition to claim evolution was not a science, and claimed creationism was an equally valid metaphysical research program. For example, Duane Gish, a leading Creationist proponent, wrote in a letter to Discover magazine (July 1981): "Stephen Jay Gould states that creationists claim creation is a scientific theory. This is a false accusation. Creationists have repeatedly stated that neither creation nor evolution is a scientific theory (and each is equally religious)." Popper responded to news that his conclusions were being used by anti-evolutionary forces by affirming that evolutionary theories regarding the origins of life on earth were scientific because "their hypotheses can in many cases be tested." Creationists claimed that a key evolutionary concept, that all life on Earth is descended from a single common ancestor, was not mentioned as testable by Popper, and claimed it never would be. In fact, Popper wrote admiringly of the value of Darwin's theory. Only a few years later, Popper wrote, "I have in the past described the theory as 'almost tautological' ... I still believe that natural selection works in this way as a research programme. Nevertheless, I have changed my mind about the testability and logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation." His conclusion, later in the article is "The theory of natural selection may be so formulated that it is far from tautological. In this case it is not only testable, but it turns out to be not strictly universally true." Debate among some scientists and philosophers of science on the applicability of falsifiability in science continues. Simple falsifiability tests for common descent have been offered by some scientists: for instance, biologist and prominent critic of creationism Richard Dawkins and J. B. S. Haldane both pointed out that if fossil rabbits were