=25. Cause and effect: habitat and plant.= In seeking to lay the foundation for a broad and thorough system of ecological research, it is necessary to scan the whole field, and to discriminate carefully between what is fundamental and what is merely collateral. The chief task is to discover, if possible, such a guiding principle as will furnish a basis for a permanent and logical superstructure. In ecology, the one relation which is precedent to all others is the one that exists between the habitat and the plant. This relation has long been known, but its full value has yet to be appreciated. It is precisely the relation that exists between cause and effect, and its fundamental importance lies in the fact that all questions concerning the plant lead back to it ultimately. Other relations are important, but no other is paramount, or able to serve as the basis of ecology. Ecology sums up this relation of cause and effect in a single word, and it may be that this advantage will finally cause its general acceptance as the proper name for this great field. In the further analysis of the connection between the habitat and the plant, it is evident that the causes or factors of the habitat act directly upon the plant as an individual, and at the same time upon plants as groups of individuals. The latter in no wise decreases the importance of the plant as the primary effect of the habitat, but it gives form to research by making it possible to consider two great natural groups of phenomena, each characterized by very different categories of effects. Ecology thus falls naturally into three great fundamental fields of inquiry: habitat, plant, and formation (or vegetation). To be sure, the last can be approached only through the plant, but as the latter is not an individual, but the unit of a complex from the formational standpoint, the formation itself may be regarded as a sort of multiple organism, which is in many ways at least a direct effect of the habitat. In emphasizing this fundamental relation of habitat and vegetation, it is imperative not to ignore the fact that neither plant nor formation is altogether the effect of its present habitat. A third element must always be considered, namely, the historical fact, by which is meant the ancestral structure. Upon analysis, however, this is in its turn found to be the product of antecedent habitats, and in consequence the essential connection between the habitat and the plant is seen to be absolute.