DIVERGENCE. 167 



the communities which furnish the next invaders are imable to do so, or when 

 the abundance and mobility of certain species enable them to take possession 

 before their proper turn, and to the exclusion of the regular stage. When a 

 stage foreign to the succession is inserted, replacing a normal consocies or 

 shpping in between two such, the development may be called interpolated 

 succession. 



Divergence. — Graphic representations of the development to a climax often 

 show divergence as well as convergence. This is frequently due to the ability 

 of a particular consocies to develop in one serai area but not in another. The 

 corresponding diagram often shows a divergence in such cases when none 

 actually occurs. Usually, however, apparent divergence arises from con- 

 necting the development of secondary seres with preceding primary ones, or 

 from the presence of two or more nearly equivalent communities, such as 

 Scirpus caespitosics and Eriophorum, or the alternation of consocies, such as 

 Typha, Scirpus, and Phragmites, which may occur separately or variously 

 grouped. Within the same climax formation actual divergence is rare if not 

 impossible. It can occur for a time when a foreign dominant is interpolated 

 and it would take place if climatic changes were to affect one part of a great 

 climax area and not another. On the other hand, while the initial stages on 

 rock, in water, and on dvme-sand are identical or similar throughout the 

 northern hemisphere, the final climaxes differ widely. This is a natural conse- 

 quence of the fact that relatively few species can grow in extreme conditions, 

 and that such species axe usually able to migrate widely. As a consequence, 

 a few communities form the pioneer and initial stages of the development of 

 a large number of climax associations. The result is that the corresponding 

 seres diverge just as soon as the initial extremes become modified to the point 

 where the effect of the various climates begins to be felt. Such divergence, 

 however, is a feature only in the composite picture of vegetational develop- 

 ment in North America and Eurasia. In the case of each climax formation 

 it is absent. 



