The Australian phytogeographic region is defined as including the Australian mainland and Tasmania. This region may be subdivided into the Tropical Zone in the north and east, the Temperate Zone in the south and east, and the Eremaean Zone in the arid centre. Delineation of these zones is closely linked with present day climates but their floristic constituents also reflect selection resulting from past climatic and geographic conditions. The following areas are of special phytogeographic interest: South-West Province of Western Australia, Tasmania, Sorth-East Queensland, and the MacPherson–Macleay Overlap where the Tropical and Temperate Zones coincide. Three interzone areas have been defined where special circumstances prevent the drawing of zonal boundaries. In Parts A and B floristic analyses covering the distribution of Australian phanerogamic genera are provided for the Australian Region and for the special areas listed above. The analyses deal with family representation, endemism, and the estimated number of species present. The flora of the South-West Province reveals the highest proportion of endemism though there is a close relationship with the flora of the eastern part of the Temperate Zone. The existing west-east affinities and discontinuities are discussed in relation to climatic changes. The Province is not regarded as the "cradle" of the autochthonous elements of the Australian flora though it is apparently an asylum for many relict forms. Affinities with the flora of South Africa are not higher than those for other areas of the Australian Region. the Temperate Zone. The existing west-east affinities and discontinuities are discussed in relation to climatic changes. The Province is not regarded as the "cradle" of the autochthonous elements of the Australian flora though it is apparently an asylum for many relict forms. Affinities with the flora of South Africa are not higher than those for other areas of the Australian Region. The flora of Tasmania is not highly endemic at the generic level but it is of special interest because of certain affinities with the flora of Malaysia and with genera otherwise found along the Malaysia–New Zealand arc or genera also in South America. In some cases the genera are unknown on the mainland, in others they are known from Australian Tertiary deposits. The Tasmanian flora is considered to include (1) relicts from early Tertiary floras, (2) survivors from the cold climate regimes of the Pleistocene, and (3) Australian elements which have mingled with the flora during periods of land continuity. The North-East Queensland area is defined as including the high rainfall habitats of the eastern parts of Cape York Peninsula. An outstanding number of families, genera, and species are restricted to this area so far as the Australian Region is concerned. Many genera are limited to a single representative which may also occur in Malaysia. It is suggested that a proportion of these may be recent arrivals while others, known as fossils from southern deposits, have a more restricted distribution than in the past. The close affinity with the flora of New Guinea is obvious but there are certain affinities with the floras of New Caledonia and New Zealand which may be independent of those of other parts of the Australian Region. Consideration is given to the problems of migration of temperate elements between the northern and southern hemispheres and the passage of such elements through the tropical belt. The MacPherson–Macleay Overlap is defined as that area of eastern Australia where the Tropical and Temperate Zones overlap. It includes part of south-east Queensland and part of north-east New South Wales. Within this area tropical elements predominate in the wetter habitats of the eastern slopes of the ranges and temperate elements in the drier or cooler and more open sites. Many of the tropical elements are to be found in discontinuous areas to the south of the Overlap but there is no similar pattern of temperate communities to the north. The Overlap is of special interest in the discussion of discontinuous distributions in eastern Australia and the significance of these. The development of the Australian flora is discussed in Part C. Its present composition is regarded as primarily due to climatic selection both within the region and from the biotypes available as a result of migration. Migration by communities rather than by chance dispersal of individuals is considered a prime factor. Though the data concerning Tertiary floras are not extensive certain significant facts emerge. First there is a stronger affinity with the flora of South America and this must be contrasted with the marked lack of data suggesting a similarly increased relationship with southern Africa. The relationship between fossil and modern representatives of such genera as Podocarpus, Dacrydium, and Nothofagus suggests that there has been a northward migration or a withdrawal to warmer latitudes since the Tertiary. All the main elements of the present day flora are represented In the Tertiary assemblages. If current views on the "Malaysian" or "tropical" nature of some affinities and the "Antarctic" nature of others be accepted then there is no indication that the major migration into the Australian Region came from one particular direction. Possible climatic regimes and changes are discussed. It is suggested that the occurrence of cold pluvial conditions in southern Australia implies a contraction of the arid centre rather than a northward shift of the dry tropical belt. Under such circumstances northern Australia could still have enjoyed a warm wet climate. This opinion is supported by certain distribution patterns and by the pedological data concerning laterite formation. Apart from the Australian, Malaysian, and Antarctic elements in the Australian flora there are also temperate elements with northern hemisphere affinities. Some of these are found in...