The absence of vital resources, such as adequate water supply or accessible metallic ores, can also hinder innovation, as can endemic diseases and parasites that deplete people’s energy. Topography has played an important role in shaping patterns of intersocietal communications.  Oceans, deserts, and mountain ranges have all prevented or seriously impeded the flow of information between societies, while navigable rivers and open plains have facilitated it.  Considering the importance of diffusion, enormous differences in the rate of innovation can be explained by this factor alone (1991, p. 56).