284 THE FORMATION 
region, or a species at separate points in a formation. Although it is a funda- 
mental feature of vegetation, it has been recognized but recently.* 
Alternation is the response of vegetation to the heterogeneity of the sur- 
face of the earth. It is in sharp contrast to zonation, inasmuch as it is directly 
caused by asymmetry in the topography. In consequence, it deals with the 
subdivisions of zones, arising from physical differences within the sym- 
metrical area. It deals with vegetation areas of every rank below that of 
major zone, with the habitat and geographical areas of species, and, in a 
certain way, with the correspondence of vicarious genera. The breaking up 
of vegetation into formations is a striking example of alternation. The same 
phenomenon occurs in every formation, producing consocies and minor 
plant groups, and everywhere giving variation to its surface and structure. 
The essential idea involved in this principle is the recurrence of like forma- 
tions, consocies, or groups, which are more or less separated by forma- 
tions, consocies, or groups differing 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 consocies. 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. 
343. Causes. The primary cause of alternation is physical asymmetry, 
which is everywhere present within the symmetrical areas which produce 
zones. This is influenced so strongly, however, by migration and plant 
competition (phyteris) that the consideration of this subject will gain in 
clearness if these are treated as separate causes. The essential relation be- 
tween them must not be lost sight of, however. Migration carries dissemi- 
nules 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 character 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 individuals 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 alternating areas, and 
originally, i. e., in new or denuded soils, the sequence is constant, viz., migra- 
tion, ecesis in asymmetrical areas, and competition. 
__» *Clements, F. E. The Development and Structure of Vegetation. Rep. Bot. Surv. 
Nebr., 7:162. 1904, 
