Ricklefs (1987) pointed out that many large-scale contrasts in species richness and composition cannot be explained by local processes such as competition and predation. Ricklefs proposed that differences in history and geography can leave a unique stamp on regional biotas. The mammals of Australia, including kangaroos, koalas, and duck-billed platypuses, are one of the best-known examples of a unique biota. How have history and geography, as opposed to local processes, combined to produce this unique assemblage of mammals?