163 



ALTBENATION. 



The term alternation is used to designate that phenom- 

 enon of vegetation, in which a formation recurs at different 

 places in a region, or a sr»ecies at separate points in a for- 

 mation. Although it is a fundamental feature of vegetation, 

 it has not been recognized heretofore, and has no history as 

 a concept. A degree of historical interest attaches, how- 

 ever, to one or two ideas that contain something of this 

 principle. Schouw (I823:J68), in dealing with the geograph- 

 ical areas of species, distinguished them" as interrupted or 

 continuous, interrupted distribution containing, at least by 

 inference, some idea of alternation. In his concept of substi- 

 tution (221), Schouw has expressed a kind of alternation 

 found in related species or genera, which occur in different 

 regions. Meyen (1836:89) adopted the ideas of Schouw in 

 regard to interrupted areas, and definitised them in such a 

 way that interruption comes to be identical with the 

 simplest type of alternation, i. e., that in which a given 

 soecies recurs in separated but similar stations. A. De 

 Candolle (1855:472) spoke incidentally of sparse plants, which 

 "changeiit un peu de place, par une loi g^n^rale d'alter- 

 nance", but he did not carry the idea further. In the 

 vicarious or representative forms of Dmde (I890:J24) and 

 others, there is a hint of this principle, but it was not 

 elaborated. Thornber (1901:55) discussed the general fea- 

 tures of the alternation of species in the prairie grass for- 

 mation. Jaccard (t90I:547), in his statistical studies of 

 Alpine vegetation, has laid especial stress upon the floristic 

 diversity of different examples of the same formation. He 

 has sought to use this as an index of the physical character 

 of the habitat, however, and has not developed it further. 



Alternation is the response of vegetation to the hetero- 

 geneity of the surface of the earth. It is in sharp contrast 

 to zoiiation in as much as it is directly caused by asymmetry 

 in the topography. In consequence, it deals with the sub- 

 divisions of zones, arising from physical differences within 

 the symmetrical area. It deals with vegetation areas of 



