540 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1917. 



humid regions have been better studied, the arid regions will probably 

 be the next to be carefully investigated. The plant ecologists, by their 

 studies in these regions, have already furnished imj)ortant facts, pre- 

 paring the way for the animal ecologist, because they have investi- 

 gated both the physical and vegetational conditions upon the prairies 

 and plains of the West. If the regions of progressively increasing 

 aridity are examined, there will be found to be a corresponding series 

 of changes in the animal habitats. The standing-water series of 

 habitats found in such a series, in contrast with those of humid re- 

 gions with fresh-water lakes, ponds, and swamps in addition to the 

 temporary fresh waters, are alkaline and salt waters, and we find 

 an extensive series ranging from Great Salt Lake, Salton Sea, and 

 Devil's Lake, to strong briny pools and alkaline mud flats. These 

 are, of course, as capable of a genetic treatment as are the corre- 

 sponding fresh-water bodies of humid areas. The stream series is 

 also present in the arid region, but it exists under conditions quite 

 different from those in humid areas. The through-flowing streams 

 are relatively independent of local conditions because their main sup- 

 ply of water is from the mountain; but they are nevertheless much 

 modified by the character and amount of the burden which they carry 

 during the time of high water, and they tend to become clogged at 

 low-water stages. The chemical composition of such waters is quite 

 different from that of regions continually leached by rains. The 

 small streams flowing from the mountains, whose diminishing volume 

 does not allow them to traverse the arid regions, succumb, and disap- 

 pear in the dry earth — examples of a second degree of dominance of 

 the desert or plains. But the truly characteristic streams of the arid 

 regions are those primarily dependent upon the desert conditions. 

 Such streams are well within the arid regions and are dominated 

 wholly by them. They are solely of a temporary character, and cor- 

 respond to the initial stage of stream development, the temporary 

 stream, in a humid climate. In an arid climate, however, develop- 

 ment does not proceed beyond this early stage, and the degradation 

 and base-leveling 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 

 anchored by vegetation. Other soils than sand are also blown about. 

 The extreme of diy 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 vege- 

 tation. These are able to influence the development of the drainage 



