The Eltonian food-chain, on any reasonable hypotheses of energy transfer and size difference between links, cannot give much diversity; it is subject to an evolutionary tendency to contract and to metaphoetesis or blurring by different stages in a life-history forming different links. The main cause of diversity of the terrestrial fauna is the diversity of the terrestrial flora. The diversity of the whole system is probably due to the greater stability of a food-web of many levels over one of few levels, as has often been realized inductively and has been demonstrated deductively by R. H. MacArthur. Limitation of diversity is due to external factors limiting biomass or growth form of the producer organisms and to absolute space available, both operating stochastically, to the degree of difference needed to separate species ecologically, apparently about 30% difference in purely metric characters influencing food choice, and to the properties of the environmental mosaic that permit more species of small than of large animals. Illustrative examples are drawn from the Palaearctic species of Corixa (Hemiptera) and from the small mammals of western Europe. A diverse community is more likely to contain species preadapted to new conditions than a less diversified community. This may have influenced overall rates of evolution during geological time. The types of emergence presented by evolutionary processes can be explained naturalistic ally, but the general problem of the possibility of emergence may be philosophically interesting. The course of evolution in small sympatrically diversified groups may be different from that in groups of large animals owing to the need in the former case to develop sharp inherent mechanisms for specific recognition.