30 



baseleveling of the land is due to the combined influence of water and 

 the wind. 



On land, the movements of the soil by the wind, as in the sand- 

 dune regions of true deserts, show us a characteristic condition; in a 

 more humid climate, however, the dunes would tend to become an- 

 chored by vegetation. Other soils than sand are also blown about. The 

 extreme of dry desert conditions must be looked upon as the ultimate 

 or climax condition, a condition of relative equilibrium, under present 

 climatic conditions, for certain regions. A slight departure from these 

 extreme conditions is seen in such localities as receive most abundant 

 showers during the growing season for vegetation. These are able to 

 influence the development of the drainage only in a minor way, but 

 they moisten a shallow surface layer of soil and permit the growtii 

 of short grasses, such as the buffalo-grass (Schantz, 'ii 140). 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, '13), where the evaporation 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, per- 

 mitting 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 southeastern 

 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 standing 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 activi- 

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



