250 THE FORMATION 
phibious to mesophytic, and, in dry regions, xerophytic conditions. When 
the process of drying out occurs rapidly, as in a single summer, the original 
formation is destroyed, and the new vegetation consists largely of rudera! 
plants. A peculiar effect of climate occurs in regions with poor drainage, 
where the result of intense evaporation is to produce alkaline basins and salt 
lakes, in which the succession becomes more and more open, and is finally 
represented by a few stabilized halophytes, or disappears completely. 
Fig. 63. A typical gravel slide (talus) of the Rocky mountains, 
before invasion. 
305. Succession by animal agency. Successions of this class are alto- 
gether of secondary importance, the instances in which animals produce de- 
nudation being relatively few. Such are the heaps of dirt thrown up by 
prairie dogs and other burrowing animals, upon which ruderal plants are 
first established, to be finally crowded out by the species of the original for- 
mation. Buffalo wallows furnish examples of similar successions in which 
the initial stages are subruderal, while overstocking and overgrazing fre- 
quently produce the same result with ruderal plants. 
306. Succession by human agency. ‘The activities of man in changing 
the surface of the earth arc so diverse that it is impossible to fit the resulting 
successions in a natural system. While man does not exactly make new 
soils, he exposes soils in various operations: mining, irrigation, railroad 
