 On the other hand, Wright’s later incorporation of fluctuations in mutation rate, fluctuations in migration, and fluctuations in selection (see, e.g., Wright 1949) as types of drift was challenged by Cain and Currey, who asserted that “the worker on actual examples must classify processes according to their biological significance” and that such lumping together would produce confusion and prevent proper analysis of actual situations (Cain &amp; Currey 1963: 59). They thus urged the use of the term “sampling drift”, which Wright adopted in the fourth volume of his 1978 magnum opus, Evolution and the Genetics of Populations.