38 INITIAL CAUSES. 



has furnished a basic and fertile viewpoint for physiography, but it seems to 

 possess no such value for vegetation. 



Base-leveling. — The complex topographic development of a region known 

 as base-leveling seems to present a fundamental explanation of those seres 

 initiated by topographic changes. But the relation between base-leveling and 

 the development of vegetation is apparent rather than real. The connection 

 between them appears to be incidental but not fundamental. There is no such 

 correspondence between the life-history of the Mississippi system and its 

 vegetation as an intrinsic relation between the two would demand. The serai 

 development from origin to climax is a wholly different thing in northern Min- 

 nesota from that found in Louisiana, in spite of similarly swampy habitats, 

 and must always remain so while the present climatic relations persist. This 

 seems even truer of mature streams which flow northward, such as the Mac- 

 kenzie, in which the upper and lower courses must develop in the midst of 

 very different climax formations. In the case of the great drainage basin of 

 the Mississippi, differences in climate and climax vegetation make the course 

 of succession very different in areas of the same age topographically. On the 

 other hand, the valley of the Platte is much more mature than that of the Nio- 

 brara or Running Water, but both streams flow through the same climax 

 formations with the same developmental history. 



Similar evidence is afforded by lakes and flood-plains developed at different 

 stages in the life-history of a river. According to Davis (1887), a young drain- 

 age system contains many lakes which disappear by flliing and draining as the 

 river matures. New lakes may then form by the damming back of tributaries, 

 by the cutting off of meanders to form ox-bow lakes, and by the production of 

 lakes in the delta. At any time in the course of the development lakes may also 

 arise by accidepts, such as lava^flows, ice, landsUps, work of man, etc. In the 

 same climax region the succession in all these lakes will be essentially identical, 

 regardless of their relation to the life-history of the river. It can be changed 

 only by a decisive change in climate which produces a new climax formation. In 

 the prairie region the succession in cut-off lakes of mature rivers duphcates in all 

 essentials the development in lakes belonging to the youth of tributary streams. 



The one striking connection between base-leveling and succession seems to 

 lie in the fact that bare areas for colonization are naturally most abundant 

 when erosion and deposition are most active. Since erosion is typical of hills 

 and deposition of valleys, bare areas produced by erosion tend to be drier than 

 the mean, and those produced by deposition to be wetter. In consequence, 

 just as hill and lowland tend to reach a mean in a temporary base-level, so 

 vegetation tends to a mean, which is usually mesophjrtic. That it is the 

 extremes and the climatic mean which control, however, and not the topo- 

 graphic process, is shown in semiarid and desert regions. In the Santa 

 Catalina and Santa Rita ranges of Arizona the torrential rains cut back 

 deep canyons and carry out the detritus in enormous alluvial fans known as 

 bajadas. The vegetation of the bajada, instead of being more mesophytic 

 than that of the forested slopes or the moist upper canyons, is intensely 

 xerophytic. A similar condition exists in the Uncompahgre Plateau of Colo- 

 rado, where the extensive table-land is covered with spruce and fir forest with 

 a rainfall of 30 inches or more, while the streams carry eroded material away 

 into an Artemisia-AtripUx vegetation with a rainfall of 12 inches. 



