This chapter empirically tests the “convergence hypothesis” central to the party models thesis, which predicts that environmental pressures will force European parties into a uniform model. Utilizing the POV index across eleven democracies from the 1970s to the 2010s, the analysis reveals that the “Iron Cage” of institutional isomorphism has failed to materialize. Contrary to expectations of homogenization, the data demonstrates a persistent and often widening diversity in organizational forms, challenging the teleological assumptions of modernization theory. While the dimension of resources shows partial convergence due to widespread reliance on state funding, the analysis identifies a dramatic explosion of variance in internal structures and strategies. Case studies, such as the contrast between the coercive isomorphism of the German system and the competitive fragmentation of the Austrian system, highlight the decisive role of political agency. The chapter concludes that organizational form has become a critical tool for strategic differentiation, resulting in a “multi-speed” European landscape where distinct organizational species coexist rather than converge.