ANIMALS AND THEIR ENVIRONMENTS ADAMS. 541 



only in a minor way, but they moisten a shallow surface layer of soil 

 and permit the growth of short grasses, such as the buffalo-grass 

 (Schantz, 1911:40). Very recently another important source of 

 water in the arid regions has come to be recognized. This, McGee has 

 shown to be the subsurface or artesian waters which come up from 

 below ; and this is an important supplementary source of moisture in 

 extensive areas in the arid West (McGee, 1913), where the evapora- 

 tion is large. It is not unlikely that even in humid regions where the 

 soils are very sandy, as upon the Coastal Plain, and where the strata 

 dip in such a manner as to favor an underflow of water, this supply 

 may be of considerable importance to the biota. With a greater 

 rainfall during the growing season, permitting a relative humidity 

 greater than on the short-grass area of the plains, a deeper-rooted 

 vegetational cover gives us the long prairie grasses of the eastern 

 prairie. 



As soon as the physical conditions permit a growth of vegetation 

 this material becomes an environmental factor which reflexly modifies 

 the physical conditions of the air, the soil, and the animal habitat. 

 This is shown to a marked degree in the humid area of the southeast- 

 ern United States, where the rainfall, greater than that on the arid 

 plains and prairies, favors the development of a forest cover. Such 

 a forest not only tends to retard evaporation but also acts as a sponge, 

 and by its vegetable debris and loose soil retards the run-off. In this 

 manner not only are land habitats influenced but this conservation of 

 moisture tends to prolong the duration of temporary streams and to 

 stabilize the flow of permanent ones ; and, further, through the same 

 influence the ground-water level declines slowly, and bodies of stand- 

 ing water are also influenced. Thus all the more important habitats 

 are to some degree regulated and made more stable by a forest cover. 



The foregoing discussion and examples, selected from the activities 

 of animals and changes in their environments, are varied enough to 

 show how diverse are the applications of the process method to inves- 

 tigation. The general idea is easily grasped, but to make the dynamic 

 method a regular habitual procedure in investigation is truly difficult, 

 so difficult, indeed, that there is reasonable ground for doubting if 

 this method can be mastered without a practical application of it to 

 a concrete problem, at the same time giving special attention to the 

 method of procedure. 



REFERENCES TO LITERATURE. 



Adams, C. C. 1904. On the analogy between the departure from optimum 

 vital conditions and departure from geographical life centers. Science, 

 n. s., 19 : 210-211. 

 1913. Guide to the Study of Animal Ecology. 183 pp. . New York. 



(This book contains numerous references to the literature bearing upon the subject 

 of this article.) 



