APPLICATIONS 15 



able to do more than guess at the physical causes which control both struc- 

 ture 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 occur- 

 ring, 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. Fur- 

 thermore, it makes it possible for him to enter a new region and to deter- 

 mine its nature and capabilities at a minimum of time and energy. 



21. Physiography. Physiographic features play an important part in de- 

 termining 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 suc- 

 cessions is to be sought in the physiographic processes which pr-^duce 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 re- 

 lation 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. 



23. Zoogeography. Since animals are free for the most part, and hence 

 not confined so strictly to one spot as plants, their dependence upon the 



