Summary. The principles of chorological mapping are examined, and the special problems associated with endemic species are discussed. A series of maps is presented, showing the generalized distribution patterns of grass species in the Old World. of factor-climatic, edaphic and biotic. The varying incidence of these factors, acting through the medium of competition, effects a spatial sorting of the species into groups, whose members become blended into more or less close-knit communities. Ecology, in the Anglo-American sense, is concerned with describing and mapping these communities, explaining their relation- ship with the environment, and examining the interactions between their constituent species. Under the concept of succession it takes account of changes induced by short-term fluctuations in the environment. But it generally ignores factors which operate over a very long time span or barriers to the free dispersal of species, for they are seldom relevant to the kind of detailed investigation of comparatively small areas with which it is mainly concerned. The Continental European school of phytosociology covers much the same ground, but has been less concerned with the processes at work than with the precise composition of communities, and with their arrangement in a hierarchical classification according to the degree of fioristic similarity between them. It becomes apparent from such an arrangement that the overall species pools, from which communities are recruited, change significantly from one geographical region to another. These changes do not always seem to be directly related to the environment, but the evidence is difficult to interpret since phytosociological classification tends to break down at the higher levels, there being too few species in common to establish clear relationships. Nevertheless the existence of species pools ('phytochoria') introduces a new facet to the ecological problem, for it hints that species distributions may not be governed solely by environmental factors.