It is certainly not unreasonable to try to account for patterns of variation in nature in terms of evolution by natural selection. But there are more or less “avid” proponents of this way of understanding nature who are, according to critics, more or less reasonable in inverse proportion. Attempts to understand nature in terms of evolution by natural selection can, according to these critics, be pushed too far. Just how far such pursuits can reasonably be pushed is the central problem of this paper.I will discuss this issue in the context of an episode in the history of evolutionary biology that has been labelled by Stephen Gould, “the hardening of the evolutionary synthesis” (Gould 1980, 1982, 1983). In the course of the hardening of the synthesis, evolutionists attributed a greater and greater role to natural selection, and correspondingly less and less a role to alternative evolutionary agents—minimizing in particular the role of so-called “random drift”.