Understanding when new forms will emerge constitutes a core theoretical issue for organizational theory. The ecological theory of form emergence falls short of providing a full explanation because it treats legitimation as a primitive (unexplained) concept. Here, we use Hannan, Pólos, and Carroll’s (2007) revised theory of organizational evolution to interpret and respecify the legitimation part of the density dependence model. Among other advantages, the respecification allows us to incorporate the insights of the “cultural-frame” institutional perspective. We study early Dutch accounting, an industry setting where form legitimation was fiercely contested by several professional associations in the period 1884–1939. We develop an analytical narrative about the historical legitimation process, and we also present systematic tests of the theory examining predictions about “fuzzy” density and population contrast. Estimated models of firm exit support the revised theory and reveal that fuzziness, induced from fragmented collective action, hampers it.