Two major strands of criticism of Durkheim's writings on punishment con¬cern his evidence for change and development, and his understanding of the nature of modern society. His evidence for progression from repressive to restitutive law as society developed was challenged by Sheleff (1975), who said that as strong a case could be made for progression in the oppo¬site direction, from restitutive to repressive law, as for Durkheim's model. By the time of 'Two Laws of Penal Evolution', however, Durkheim seems to have acknowledged that he had previously underestimated the continu¬ing importance of repressive law in modern society, and this is reflected in his changing theory of solidarity. He continued, however, to insist that generally, punishments become milder as societies develop. Spitzer reviews the evidence for this proposition, which he calls the organic differentiation thesis, and concludes that it 'seems to be built upon a foundation of facts and assumptions for which contemporary investigators can find little sup¬port' (Spitzer 1979: 214).