 (a) Michael Ruse (1971) and David Hull (1974) have argued that the relation between classical and molecular genetics is one of theory replacement, not reduction in Schaffner’s sense. Although Schaffner’s account permits a correction of the theory to be reduced (TB* instead of TB), it is not clear when a modified version TB* (corrected “classical genetics”) still counts as being largely similar to TB (classical genetics). This leaves an ambiguity of whether one should conclude that TB was reduced because TB* is similar enough or that TB has been replaced by another theory because the two are so dissimilar (Hull 1976, Ruse 1976). Schaffner only requires that TB and TB* are “strongly analogous” (1993, 429), without offering a specification of how much similarity is required for reduction rather than replacement (Winther 2009). William Wimsatt (1976b) emphasizes that while interlevel, compositional reductions are transitive, successional reductions through history are intransitive, so that a continuum between genuine successional reduction and replacement obtains. Any sequence of successive reductions can be expected to evolve into an instance of replacement given enough time.