‘Gaps among known orders, classes, and phyla are systematic and almost always large.’

George Gaylord Simpson, Ph.D. (Vertebrate Paleontology) (Was Agassiz Professor of Vertebrate Paleontology, Museum of Comparative Zoology, Harvard University and also Professor of Geology at the University of Arizona, Tucson) in ‘The Evolution of Life’, University of Chicago Press, Chicago, 1960, p. 149.