358 PAST succession: the ceneosere. 



points. It would fall naturally into a sequence of developmental stages, 

 determined by the associes of the sere, and hence capable of exact study. 

 These habitat stages would be less distinct than the corresponding associes, 

 for the very reason that their factors are essentially invisible, and their 

 separation from each other would depend wholly or nearly so upon the limi- 

 tation of serai stages. It is this absence of sharp distinctions between the 

 physical factors of successional stages which has made the concrete applica- 

 tion of the term " habitat " so completely a matter of individual opinion. The 

 developmental concept of the habitat will remove this serious difficulty, and 

 will make it possible to determine habitats as objectively as we can climax 

 formations. Moreover, it is felt that this view of the habitat not only has the 

 advantage of putting it in complete harmony with its organic expression, the 

 formation, but that it is in the direct line of ecological progress, in its move- 

 ment away from a formal to a developmental basis. 



The developmental concept of the habitat has the further advantage in 

 that it not only brings the formation and habitat into theoretical harmony, 

 but also in that it makes it possible to obtain an actual harmony between the 

 two the world over. Both become objective, and the personal equation is 

 consequently eliminated, except in drawing the exact geographical limits of 

 a climax. The insistence upon the correspondence between formation and 

 habitat (Clements, 1905 : 292; Moss, 1910 : 35; Tansley, 1911 : 9) has played 

 its part in the development of ecology. But it was peculiarly subject to the 

 most divergent individual interpretations, for the very reason that opinion 

 has afforded the chief method of recognizing either habitat or formation. 

 Clements (1905 : 292) endeavored to remedy this condition by proposing to 

 regard "the connection between formation and habitat as so close that any 

 application of the term to a division greater or smaller than the habitat is 

 both illogical and unfortunate. The final test of a habitat is an efficient 

 difference in one or more of the direct factors, water-content, humidity, and 

 light, by virtue of which the plant covering differs in structure and in species 

 from the areas contiguous to it." While this recognized clearly the basic 

 interrelation of habitat and formation, the two criteria, an efficient difference 

 in the habitat and a difference in the vegetational structure, were still depend- 

 ent upon individual judgment. This was strikingly illustrated by the fact 

 that Clements used water-content and light to distinguish habitats and the 

 corresponding formations, while Moss and Tansley based the distinction 

 wholly or chiefly upon the soil. 



The present proposal to regard the habitat as a developmental entity, the 

 exact causal and environmental equivalent of the formation, is in complete 

 accord with the idea of the original concept (Clements, 1905 : 292) that "it is 

 inevitable that the unit of the vegetative covering, the formation, should 

 correspond to the unit of the earth's smface, the habitat." The climax for- 

 mation is the unit of the earth's vegetation in an exact and objective sense, 

 and the habitat must be conceived as its precise coimterpart. It seems clear 

 that the use of this concept is essential if ecology is to become a quantitative 

 rather than a qualitative science. No matter how detailed and accurate the 

 methods of quadrating and instrumentation may become, they can but over- 

 shoot the mark unless they are applied to definite units, essentially free from 

 the personal equation. 



