Given the diversity of explanatory practices that is typical of the sciences a healthy pluralism would seem to be desirable where theories of explanation are concerned. Nevertheless, I argue that explanations are only unifying in Kitcher's unificationist sense if they are backed by the kind of understanding of underlying mechanisms, dispositions, constitutions, and dependencies that is central to a causalist account of explanation. This case can be made through analysis of Kitcher's account of the conditions under which apparent improvements in (...) unifying power may be judged spurious. But to clarify what is at issue I consider an archaeological case in which debate about the merits of an ambitious explanatory account reproduces exactly the intuitions that divide Salmon and Kitcher. The case in question is the “demic-diffusion” account of contemporary linguistic diversity advanced by Renfrew in the late 1980s: the thesis that the diffusion of agricultural populations, itself attributed to demographic pressure, was responsible for the spread of the ancestral root languages (e.g., proto-Indo-Eurpoean) that account for the existence and distribution of linguistic macrofamilies. The credibility of this powerfully unifying argument pattern depends entirely on the plausibility of its claims about the conditions and mechanisms actually responsible for the explanandum, the spread of agriculture, and not on an elaboration of its unificationist virtues. (shrink)