164 ALTERNATION 



every rank below that oL major zone, with the habitat and 

 geographical areas of species, and in a certain way, with the 

 correspondence of vicarious genera. The brealiing ud of 

 vegetation into formations is a striljing example of alter- 

 nation. The same phenomenon occurs in every formation, 

 producing associations and minor plant groups, and every- 

 where giving variation to its surface and structure. The 

 essential idea involved in this principle is the recurrence of 

 like formations, associations, or groups, which are more or 

 less separated by formations, associations, or groups differ- 

 ing from them. It is an exact expression of the primary 

 law of association that heterogeneity of structure varies 

 directly as the extent and complexity of the habitat, or the 

 series of habitats. Vegetation is made up of what are 

 superficially homogeneous formations, but upon analysis 

 these are seen to contain associations. The latter, though 

 more uniform than formations, break up into groups, each 

 of which still shows a characteristic heterogeneity arising 

 from the varying number and arrangement of its constituent 

 species. 



The primary cause of alternation is physical asymmetry, 

 which is everywhere present within the symmetrical areas 

 which produce zones. This is iniluenced so strongly, how- 

 ever, by migration and plant competition (phyteris') that 

 the consideration of this subject will gain in clearness, if 

 these are treated as separate causes. The essenital relation 

 between them must not be lost sight of however. Migration 

 carries disseminules into all, or only some of the different 

 areas of a formation, or into different formations, with little 

 respect to the physical nature of these. The physical char- 

 acter of these asymmetrical areas determines that some of 

 these plants shall be established in one series of places, and 

 some in another, while the competition between the indi- 

 viduals in the various areas determines the numerical value 

 of each species as well as its persistence. These three 

 causes are invariably present in the production of alternat- 



'From the Greek: <^urov, to, plant; ?pts, tSos, ij. competition. 



