The Cambridge Botanical Expedition to Nigeria, 1947-48, studied the pattern of distribution of species, the regeneration, and the stability of Rain Forest, particularly in relation to the theory of `mosaic' structure. The Okomu Forest Reserve, a compartment of which was studied by transects, lies on Benin sands and has a rainfall of 80-100 in.; it has been lightly exploited for at least forty years. On well-drained `plateau' sites `Gully-margin Forest', presumably an edaphic variant, is distinguishable both floristically and physiognomically from the more extensive forest which was mainly studied. The plateau-forest is a patchwork of the following serally-related phases, which are more distinct physiognomically than floristically: (a) `High Forest' with continuous middle and lower storeys and numerous emergents. (b) `Broken High Forest', differing from (a) in being interrupted by slight gaps. (c) and (d) `Tall' and `Low Closed Scrub' with a continuous climber-blanketed canopy at 6-12 m. and 3-6 m. respectively and fewer emergents than in (a). (e) `Open Scrub', as (c) and (d), but interrupted by many open glades containing light-demanding herbs, and much frequented by elephants. Scrub has been greatly extended by tree-felling, but it is also formed by wind and is a normal component of unexploited forest. Of the three storeys, that of the emergents is best characterized floristically; 21 per cent of its stems are species which are very strong light-demanders that are scarcely represented in the two lower storeys, 45 per cent are species which are abundant in the lower storey but scarcely represented in the middle storey, and 32 per cent are species which increase progressively in abundance from the emergent to the lower storey. There are striking variations in the abundance of particular species even over distances of a few kilometres, and within the 200 ha. which were sampled by transects there are some species which are randomly distributed, others which are patchily distributed on a large scale though randomly distributed on a small scale, others which are widely distributed but tend to occur in small patches, etc. In general, emergents are distributed more nearly at random than are members of the two lower storeys and the tendency to aggregation increases with decreasing size; nevertheless, some species of medium or small size appear to be randomly distributed, and others of comparable size and abundance are patchily distributed. Very few pairs of species (only Lovoa klaineana with Khaya ivorensis, Macrolobium macrophyllum with Barteria fistulosa, B. fistulosa with Grewia coriacea, and G. coriacea with Xylopia quintasii) could be shown to tend to associate, as they should if floristically well-defined communities were present. Thus despite the existence of a mosaic of physiognomically distinct phases and despite the patchy distribution of certain species, it was not possible to detect any well-marked mosaic of species. Of the emergent species nearly half are wind-dispersed and an equal number are animal-dispersed; in the lower storeys the proportion of wind-dispersed species is small and that of animal-dispersed species high. Seeds with a short life and rapid germination are frequent, but seeds with prolonged dormancy are not rare, especially amongst the animal-dispersed species, and perhaps amongst the members of the understorey. Seedlings of most of the woody species are probably present in the forest, but many are extremely local in their distribution, tending to occur in small patches, e.g. near mature trees. Broadly speaking, all sizes of the shade-tolerant members of the lower and middle storeys are adequately represented, suggesting that they may be regenerating continuously and maintaining the existing population; nevertheless, dead trees of some of these species (e.g. Anonidium, Trichilia prieuriana) are very rare, and dead trees of other species (e.g. Scottellia, Strombosia spp.) are abundant, suggesting that their regeneration may be discontinuous. In contrast, in most of the emergent species stems of medium size tend to be less abundant than large-sized stems, even when small sizes and seedlings are abundant. Some figures for rate of growth of trees of various sizes suggest that this paucity of middle sizes cannot always be explained by fast growth of the deficient size-classes; the alternative explanation must be that of discontinuous recruitment. Seedlings of even the more shade-tolerant emergents survive only in or near small gaps in the canopy. The number of small individuals of emergent species per unit area increases with the degree of degradation of the forest; in High Forest they are certainly not numerous enough to maintain the existing stocking of mature emergents. In the scrub phases they are about twice as numerous and more species are represented, but even here it is doubtful whether they are numerous enough to reconstitute a forest as rich in emergents as that now existing, assuming that the scrub will revert to High Forest. Some emergent species seem to be represented by a disproportionately large number of standing dead trees; dead individuals of other abundant emergent species were not recorded. The death-rate increases rapidly with increasing size above about 7 ft. girth. There is thus much to suggest that the composition of the forest is tending to change, particularly in respect of the emergents. Fragments of pottery and charcoal in the soil show that the land has been farmed. It is probable that the farming was relatively recent rather than prehistoric, but it cannot have been less than about 200 years ago, and it is in accordance with all the evidence to regard the forest as secondary forest of this age which is now breaking up. There are reasons for suspecting that no matter whether scrub is a normal phase in the progression or not, the forest of the true climatic climax might have few large trees, and perhaps fewer species and a simpler structure.