Plant geography is the science of the distribution of plants. In this science the subject matter of one discipline, botany, is considered from the standpoint of another, geography. Paradoxically, the new thinking also proved the bane of the geographical tradition, particularly in North America and Britain, leading to its partial demise for some forty years from the turn of the century with certain useful contributions excepted, especially those of Willis, Guppy, Ridley, de Vries and Diels. Historical plant geography is essentially the science of area applied to plants. Its first task is to establish the distribution of plant taxa in geographically defined areas, and its second is to interpret the origins and present status of these areas. Man represents only one of the major disruptions to plant areas. Geological instability, climatic change, the evolution of new and more vigorous competitors, disease, and animal activity all help to destroy, separate, alter or enlarge plant areas.