While the MDC account leaves open the possibility that some mechanisms are stochastic, it clearly rules out mechanisms that usually fail to produce their phenomena. Skipper and Millstein (2005) press this point to argue that the MDC account cannot accommodate the idea that natural selection is a mechanism. If, as Gould (1990) argued, one could not reproduce the history of life by rewinding the tapes and letting things play forward again, then natural selection would not be an MDC mechanism (see also  Section 2.6 below). It is unclear why  MDC would allow for the possibility of stochastic mechanisms and rule out, by definition, the possibility that they might fail more often than they work. Whether any biological mechanisms are truly irregular in this sense (i.e., all the causally relevant factors are the same but the product of the mechanism differs) is a separate question from whether they are mechanisms simpliciter (see Bogen 2005; Machamer 2004; Steel 2008 develops a stochastic account of mechanisms).