  In the absence of information about the initial conditions of the divergence, there is only one way empirically to choose between alternatives (a)–(c). This way requires that there are ecological fitness differences, and that we can detect them. Suppose that fitness differences were matters of probabilistic differential reproductive success (no matter what interpretation of probability is adopted). Then the only access to fitness differences is via population censuses in previous generations (since these form the bases of the probabilities). Suppose that this census does indeed show a 7:3 ratio between as and bs in the recent past. In order to exclude the absence of fitness differences instead of drift, as the source of the current generation’s 5:5 outcome, we need to be able to establish that the 7:3 differences in previous populations were not themselves the result of drift. But this is the first step in a regress, since we began with the problem of discriminating drift from mismeasures of fitness. To solve the initial problem, we now have to assure ourselves that the 7:3 ratios in the past were not the result of drift. Whence the regress. Of course the problem does not arise if we have access to fitness differences independently of previous population censuses. And this access we have, at least in principle, if fitness is a matter of differences in the solution of identifiable design problems, that is, if there is such a thing as ecological fitness and it is (fallibly) measured by probabilistic propensities to leave offspring.