  Some philosophers have argued that fitnesses are precisely such objective chances. Brandon and Carson (1996) held that here, as in quantum mechanics, we find a brute unanalyzable probabilistic dispositional property of a particular item, which generates the long run relative frequencies. It is indeed the case that among philosophers of quantum mechanics some hold that probabilistic propensities can explain actual frequencies [cf. Railton 1978, 216], and some hold that they do so via a detour into long run relative frequencies. But few are comfortable with such arguments and adopt them only because, at the level of the quantum mechanical probabilistic propensities are indispensable and irreducible [cf. Lewis, 1984]. Proponents of probabilistic propensities in biology may envision two possibilities here. One is that probabilistic propensities at the levels of phenomena that constitute the biological are the result of quantum probabilities “percolating up” in Sober (1984) and Brandon’s and Carson’s (1996) phrase; the second is that there are brute unexplainable probabilistic propensities at the level of fitness differences (Brandon and Carson 1996). No one doubts the potential biological significance of quantum percolation. It may well be an important source of mutation [cf. Stamos 1999 for a discussion]. But the claim that it has a significant role in most fitness differences is not supported by any independent evidence [cf. Millstein 2000 and Glymour 2001 for a discussion]. The claim that there are brute probabilistic propensities at the level of organismal fitness differences is only slightly more tenable. No one has adduced any evidence that, for instance, the probabilistic generalizations about the behaviour of animals that ethology and behavioural biology provide, are irreducibly probabilistic, instead of simply expressions of the current state of our knowledge and ignorance of the causes and conditions of the behaviour in question.