CAUSES 165 



ing areas, and originally, i. e., in new or denuded soils, the 

 sequence is constant, viz., migration, ecesis in asymmetrical 

 areas, and competition. 



With respect to the different portions of an asymmetrical 

 area, migration will have one of three effects: (1) it will 

 carry disseminules into both favorable and unfavorable 

 areas: (2) into favorable ones only, or (3) into unfavorable 

 ones alone. From the radial nature of migration, the first 

 case is far the most frequent : it is typical of sporostrotes, 

 and the highly specialized spermatostrotes and carpostrotes. 

 The effect of migration is uniform here, and alternation 

 arises in consequence of the selective power of ecesis. It 

 is evident that migration does not have even an indirect 

 effect, when the disseminules are carried into none but un- 

 favorable situations. Where the movement is into favor- 

 able places alone, alternation is the immediate result. The 

 intermittent operation of migration, and the presence of 

 barriers are responsible for the absence of plants in situ- 

 ations favorable to them, and in consequence bring about a 

 certain alternation between corresponding species. 



The selective operation of physical factors upon the dis- 

 seminules carried into the different parts of an asymmet- 

 rical area is the usual cause of alternation. Asymmetry 

 alone is universal within the more conspicuous structures 

 termed zones, down to the smallest areas which a group of 

 plants can occupy. The difference between contiguous 

 areas, particularly within the same habitat, is often small. 

 It sometimes seems inefficient in the initial stages of a suc- 

 cession when a single species is pi:esent, but even in 

 extreme cases its effect will be recognisable in the size and 

 density of the individuals. Asymmetry is clearly evident 

 in vegetation where two symmetrical series cross each 

 other, or when a symmetry is interrupted by barrier-like 

 elevations or depressions. Within formations, it arises 

 from differences, often very slight, in slope, exposure, ele- 

 vation, from irregularities of surface, differences in soil 

 structure, or composition, in the amount of cover, and in the 



