One of the greatest aids which modern ecology can furnish forestry, however, is the method of determining the physical nature of the habitat. So far, foresters have been obliged to content themselves with a more or less superficial study of the structure of forest formations, without being able to do more than guess at the physical causes which control both structure and development. This handicap is especially noticeable in the case of forest plantings in non-forested regions, where it has been impossible to estimate the chances of success, or to determine the most favorable areas except by actual plantations. Equipped with the proper instruments for measuring water-content, humidity, light and temperature, the ecologist is able to determine the precise conditions under which reproduction is occurring, and to ascertain what non-forested areas offer the most nearly similar conditions. A knowledge of habitats and the means of measuring them enables the forester to discover the causes which control the vegetation with which he is already familiar, and to forecast results otherwise hidden. Furthermore, it makes it possible for him to enter a new region and to determine its nature and capabilities at a minimum of time and energy. =21. Physiography.= Physiographic features play an important part in determining the quantity of certain direct factors of the habitat. Perhaps a more important connection between physiography and ecology is to be found in succession. The beginning of all primary, and of many secondary successions is to be sought in the physiographic processes which produce new habitats, or modify old ones. On the other hand, most of the reactions which continue successions exert a direct influence upon the form of the land. The most pronounced influence of terrestrial successions is found in the stabilization which their ultimate stages exert upon land forms, even where these are highly immature. The chief effect of aquatic successions is to be found in the “silting up” and the formation of new land, which result from the action of vegetation upon silt-bearing waters. The closeness of the relation between succession and the forms of the land has led to the application of the term “physiographic ecology” to that part of the subject which deals with the development of vegetation, i. e., succession. =22. Soil physics.= This subject is as much a part of ecology as is forestry. It is intrinsically that subdivision of ecology which deals with the edaphic factors of the habitat, and their relation to the plant. Since the basis is physics, there has been a general tendency to overvalue the determination of soil properties, and to ignore the fact that these are decisive only when considered with reference to the living plant. As the soil contains the water which is the factor of greatest importance to plants, soil physics is an especially important part of ecology. Its methods are discussed under the habitat.