174 ALTERNATION 



This phenomenon is distinguished as numerical alternation. 

 It arises from the fact that the similar areas are sufflciently 

 different to affect the abundance, without producing com- 

 plete suppression. It is probable that this result is due 

 almost entirely to competition. Astragalus crassicarpus 

 grows on all the slopes of the prairie formation, but on some 

 it has the abundance of a facies, while on others it is repre- 

 sented by a few scattered individuals This difference is 

 much more striking in separate examples of the same for- 

 mation, particularly when a normal facies is reduced to the 

 numerical value of a secondary species This is a matter of 

 great importance in the study of formations, for it has 

 doubtless often resulted in mistaking an association for a 

 formation. 



Alternation furnishes the logical basis for what may be 

 called comparative phytogeography. The latter is of much 

 broader scope than the old subject of geographical distri- 

 bution, for it treats not only of the distribution of for- 

 mations and associations as well as of species, but it also 

 seeks to^^explain this by means of principles drawn from the 

 relation between habitat and vegetation. Jaccard (1901:547) 

 has given a fruitful trend to distributional studies. When 

 the latter come to be fully based upon physical factor inves- 

 tigations, and upon the effects of migration and competition 

 as shown Mn alternation, the comparative study of for- 

 mations will represent the highest type of phytogeographi- 

 cal!activity. 



The investigation of the physical factors which produce 

 alternation concerns itself primarily with the conditions in 

 the nucleus of the alternating areas, except in those cases 

 where the areas are rather sharply delimited by an ecotone. 

 Here, as 'elsewhere, the water content, and the physiogra- 

 phic factors which modify it, seem to be of greatest import- 

 ance, though^humidity doubtless has a more important effect 

 than is ordinarily ascribed to it. In layered formations, 

 light is of course the controlling factor. The influence of 

 migration is best determined by the use of the migration 

 circle described under invasion. The effects of competition 



