Institutionalists have emphasized the importance of law for the spread of bureaucracy and examined its effects; but they have not examined the evolution of law as an institution in its own right, particularly from a Weberian standpoint. In this paper, we investigate whether or not there is an inexorable proliferation and refinement of rational legal rules within a law, as we have found to be the case with bureaucratic rules. In other words, are the same tendencies toward proliferation and refinement associated with the 'closing of the iron cage' found in the context of legal rules? An examination of all sections of a regional water law over a 90-year period shows that the number of law sections and the text covered by the sections actually declines over time, through alternating phases of gradual expansion followed by rapid collapse; that is via punctuated equilibrium. Most of the expansion is due to revisions of existing sections, rather than to births of new sections. Poisson models of births and event history models of revisions show that the sources of the proliferation within the law are, in fact, some of the same ones anticipated by Weber: the interpretation of the law by the courts, changes in political parties, and shock events such as war. But, in contrast to Weberian predictions, the result of this evolutionary process appears to be a law that is smaller, tighter and more functionally differentiated.