KINDS 123 



■with this is intermittent succession {successio intermissa), in 

 which the succession swings for a time in one direction, from 

 xerophytic to mesopbytic for example, and then moves in 

 the opposite direction, often passing through the same 

 stages. This phenomenon usually is characteristic only of 

 the less stable stages, and is generally produced by a 

 climatic swing, in which a series of hot or dry years is 

 followed by one of cold or wet years, or the reverse. The 

 same effect upon a vast scale is produced by alternate ele- 

 vation and subsidence, but these operate through such great 

 periods of time that one can not trace, but can only conjec- 

 ture their effects. A normal continuous succession fre- 

 quently changes its direction of movement, or its type, in 

 transition regions or in areas where the outposts of a new 

 flora are rapidly advancing, as in wide mesopbytic valleys 

 that run down into or traverse plains. Here the change is 

 often sudden, and grass and desert formations are replaced 

 by thickets and forests, resulting in abrupt succession 

 {successio dbrupta). Species guilds are typical examples of 

 this. More rarely, a stage foreign to the succession will be 

 interpolated, replacing a normal stage, or slipping in be- 

 tween two such, though finally disappearing before the next 

 regular formation. This may be distinguished as inter- 

 polated succession (successio interpolata). 



The apparent terminus of all stabilisation is the forest, 

 on account of the thoroughness with which it controls the 

 habitat. A close examination of vegetation, however, will 

 show that its stable terms are dependent in the first degree 

 Bpon the character of the region in which the formation is 

 indigenous. It is obviously impossible that successions in 

 desert lands, in polar barrens, or upon alpine stretches 

 should terminate in forest stages. In these, grassland must 

 be the ultimate condition, except in those extreme habitats, 

 alpine and polar, where mosses and lichens represent the 

 highest type of existing vegetation. Forests are ultimate 

 for all successions in habitats belonging to a region 

 generally wooded, while grassland represents the terminus 



