29 



The preceding discussion is based upon the conditions of a humid 

 cUmate, but the semi-arid and the arid cHmates should also be touched 

 on. In time, as ecological studies are extended to all kinds of land 

 areas, it will be possible to formulate all of the general principles ot 

 the origin or process of development of land habitats; but at present 

 vast areas of the land have never been observed by a zoologist from a 

 modern ecological standpoint. Most of the ecological studies of ani- 

 mals have been carried on in a humid climate, only slight attention 

 having been given to the ecological relations existing in -an arid cli- 

 mate, and still less to those in alpine and polar regions. After the 

 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 important facts pre- 

 paring the way for the animal ecologist, because they investigate 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 regions 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 corresponding fresh-water bodies of hu- 

 mid 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 supply 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 be- 

 come 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 dimin- 

 ishing volume does not allow them to traverse the arid regions, suc- 

 cumb, and disappear 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 con- 

 ditions. Such streams are well within the arid regions and are domi 

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

 correspond to the initial stage of stream development, the temporary 

 stream, in a humid climate. In an arid climate, however, development 

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



