Gleason (1926) describes a natural plant community as 'representing the selection of a certain environment acting on a given population of species'. In a climax community it is to the local variation in this environment itself that some at least of its internal vegetational variation, that is, variable success and incidence of its constituent species, is attributable. Further, the incidence of a definite type of plant community (which implies incidence of its characteristic species, e.g. Calluna in heathland) appears in some cases to correspond to the distribution of some environmental factor, e.g. heathland on acid sandy soils (Ashby, 1935). It appears probable, therefore, in cases where the incidence and success of a species appears to be noticeably correlated with variations in some environmental factor, that a quantitative investigation into the causes of such variations as they occur locally, together with some means of assessing the relative importance of the effects of such causes on local distribution and success of the species investigated, could be used to predict the wider incidence and success of such species in respect to this factor. Anything which enhances a knowledge of the physiological requirements of the species will increase the accuracy of any prediction of the habitat best likely to meet these requirements. For example, the wider the range of local variation in the factor investigated, and the closer the analysis of the causes of this variation, the more likely is the prediction of distribution and success, drawn from local investigations, to be correct. As for the causes which cannot be investigated locally, e.g. climatic variation, their effects can often be predicted qualitatively from experience. Generally, it is the edaphic factor which is the primary environmental factor influencing species success and distribution in a given climatic area, and since its influence can be approximately assessed from determinations of pH, organic content, and moisture contentin themselves easily determinable functions of a wide range of heterogeneous effects, such as slope, incidence of tree cover, exposure, base exchange capacity, and the like-it is the factor which will be considered in assessing the value of the above conclusions. We may point out here the essential similarity of the above concept to that which has led Boyko (1947) (of whose latest paper we were unaware at the time of our experiments) to his conclusion: 'The specific topographical distribution (micro-distribution) of an ecotypic plant species is a parallel function of its general geographical distribution (macrodistribution), since they are both determined by the same ecological amplitudes.' Boyko is dealing with the variables which together constitute climate, and while we appreciate his conclusion, we can commend a more rigorous analysis of the results. 11. The following experiment, then, is a preliminary attempt to assess the relative importance of the effects of the causes of local edaphic variation on local distribution and success of Holcus mollis and Deschampsia flexuosa. From the results we shall attempt to