Evolution is as much a fact as the earth turning on its axis and going around
the sun. At one time this was called the Copernican theory; but, when
evidence for a theory becomes so overwhelming that no informed person can
doubt it, it is customary for scientists to call it a fact. That all present
life descended from earlier forms, over vast stretches of geologic time, is
as firmly established as Copernican cosmology. Biologists differ only with
respect to theories about how the process operates.
                -- Martin Gardner, "Irving Kristol and the Facts of Life".