Darwinists" worst predicament is their inability to find a single piece of evidence for a transition from invertebrates to fish, or from fish to reptiles, from reptiles to mammals and birds. The late evolutionist paleontologist Stephen Jay Gould describes the lack of evidence for these imaginary transitions as a disquieting problem: "The absence of fossil evidence for intermediary stages between major transitions in organic design, indeed our inability, even in our imagination, to construct functional intermediates in many cases, has been a persistent and nagging problem for gradualistic accounts of evolution." (Stephen Jay Gould, "Is a New and General Theory of Evolution Emerging?", Paleobiology 6, 1980, pp. 119-130, reprinted in the collection Evolution Now: A Century After Darwin, ed. Maynard Smith, Freeman, 1982, p. 140)