ALTERNATION AND ZONATION 291 



does not arise if the physical factors permit ecesis in all of them. 

 This is a rare occurrence, however, and in the vast majority of 

 cases ecesis is possible only in those areas closely resembling the 

 original home. In consequence the same species, together with 

 its neighbors, which necessarily possess the same ability of adjust- 

 ment, tends to recur in all similar areas of the formation. The 

 same is true on a largei ^cau of the formations of a region. Exam- 

 ples of the same formation recur more or less regularly in the 

 same kind of habitat, alternating wdth other formations which 

 occur in habitats of another kind. From this it is evident that 

 the type of alternation due to ecesis is a fundamental and universal 

 feature of vegetation. ^ 



314. Alternation due to competition. The alternation pro- 

 duced by ecesis in similar areas is often affected by competition. 

 The number and kind of individuals vary in the several areas. 

 In some competition is much more intense than in others, con- 

 sequently reducing the number of individuals of invader or occu- 

 pant, or eliminating one or the other completely. In minor areas, 

 in which the physical factors are little if at all different, plants 

 persist more readily in areas with slight competition than in those 

 where the latter is intense. As a result different groups appear, 

 each one tending to recur in those spots where competition is 

 least unfavorable to it. 



315. Kinds of alternation. Alternation involves two ideas, 

 viz., the alternation of different species or formations with each 

 other and the alternation of the same species or formation in 

 similar but separate situations. This is the result of a lack of 

 symmetry in a habitat or region, in consequence of which adja- 

 cent areas are dissimilar and remote ones often similar. The 

 same species, group, or formation is said to alternate between 

 two or more similar situations, while different species, groups, or 

 formations alternate with each other, occurring in situations differ- 

 ing in ecesis or competition. From the nature of alternation the 

 two cases always occur side by side. 



Alternation is distinguished as normal, numerical, or corre- 

 sponsive. The general nature of the first has just been indicated. 

 Numerical alternation occurs when a species varies greatly in 

 abundance and importance in the various areas in which it is 

 found. In corresponsive alternation a species, group, or for- 



' Research Methods, 283. 



