 Even though Schaffner and Hull were engaged in a debate over theory reduction, they simultaneously admitted that the question of formal theory reduction was rather peripheral to what scientists actually did and studied (Schaffner 1974b; Hull 1974). And indeed, while the theory reduction debate was playing out, a number of philosophers of biology switched attention from scientific theories to the stuff in nature that scientists investigated. William Wimsatt (1976) argued for a shift in the reduction debate from talk of relations between theories to talk of decompositional explanation via mechanisms. And Lindley Darden and Nancy Maull (1977) focused attention on the bridges between fields formed by part-whole relations, structure-function relations, and cause-effect relations.