Comments on a article by Dewsbury (February-March 2009) in which he stated, "Darwin provided a viable mechanism for evolutionary change, natural selection" (p. 67). Although this view is consistent with the modern synthesis, the author argues that (a) the natural selection "mechanism" provided by Darwin was not initially accepted by scientists because (b) natural selection is a functional explanation, and functional explanations are always incomplete because they describe how things work but not why they work; that is, they lack mechanism information. This explanatory deficiency led to what Bowler (1983), uncited by Dewsbury (2009), described (in a book of the same name) as "The eclipse of Darwinism," the initial rejection of natural selection by most scientists. The importance of the distinction the author is making between functional explanations and mechanism information for the future of psychology is that efforts to advance psychological science by clarifying causal mechanisms must first understand how mechanism explanations differ from functional explanations.