BASES OF CLASSIFICATION. 175 



(2) the edaphic successions, in which the replacement of one type of vegetation 

 by another has resulted from changes in topography and a bio-chemically 

 diminished water-supply." 



The climatic successions are the regional successions of Cowles, and the 

 edaphic ones correspond partly to his topographic and partly to his biotic 

 successions, thus emphasizing the impossibility of distinguishing between the 

 two on the grounds proposed. Dachnowski's climatic successions would 

 include the geosere, eoseres, and cliseres and coseres in part, though deforma^ 

 tion and gradation play a profound r61e in them. His edaphic successions 

 would correspond partly to the cosere and sere. According to the definition 

 given, seres due to biotic initial causes would find no place in either group. 

 In short, the distinction proposed, like all of those based upon initial causes, 

 runs counter to the process of development, and hence is largely artificial. 



Braun and Furrer (1913 : 19) use the term phylogenetic successions for the 

 regional successions of Cowles, though this term should obviously include his 

 topographic successions as well. Contrasted with this is the ontogeny of 

 actual communities, which establish themselves imder the eyes of the observer. 

 These apparently correspond exactly to the biotic successions of Cowles, 

 though the authors ignore this fact, and distinguish artificial successions, 

 equivalent to Cowles's retrogressive biotic successions. 



Possible bases of classification. — From the preceding discussion it becomes 

 clear that development, cause, initial area, and climax must be weighed as 

 possible bases for the classification of successions. Reaction is not available, 

 since one sere is often the result of several reactions, and since widely different 

 seres may have the same sequence of reaction. Since the reaction upon water- 

 content is nearly universal in succession, classification may be based upon the 

 direction of movement, such as mesotropic, xerotropic, etc., but our present 

 knowledge hardly suffices for this. 



In a natural, i. e., a developmental system of classification, it is clear that 

 development must constitute the chief basis. This is true of the actual seres 

 of to-day, which culminate in the present climax formation. It is true of the 

 cUseres, which result from the shifting of existing climaxes, and of the coseres 

 formed by successive seres. It is even more marked in the eoseres, which are 

 major developmental series within the climatic climax of the geological eras. 

 In short, seres are related to each other by their development into the same 

 climax and by their sequence in the cosere. Climaxes, the static units of 

 to-day, are related to each other in the developmental sequence of the clisere, 

 which is produced by a change of climate, such as glaciation. These climaxes 

 of the existing flora are phylogenetically the descendants of the climaxes of a 

 preceding flora, which characterized an eosere. All eoseres have a similar 

 phylogenetic relationship, and taken together constitute the geosere, the whole 

 course of the development of vegetation from its beginning down to the present. 

 The further discussion of major developmental units is foimd in Chapter XII. 

 Developmental basis of classification. — While the unique importance of 

 development for successional analysis and classiflcation has repeatedly been 

 emphasized, it is felt that over-emphasis is impossible. Though it is easy to 

 carry analogy too far, there seems to be no question that the history of ecology 

 must repeat that of botany itself to a large degree. In morphology and tax- 

 onomy development alone is regarded as capable of furnishing basic criteria, 



