CONVERGENCE. 165 



Cowles (1901) and Hole (1911), as already mentioned, have used the final 

 condition as a basis for distinction. While both use the terms progressive and 

 retrogressive or regressive, Cowles regards all development toward amesophytic 

 condition as progressive, while Hole uses this term for movement toward a 

 hygrophilous or hydrophytic climax. The disadvantages of the use of the 

 terms progressive and retrogressive have already been discussed. 



The emphasis here laid upon the climax formation as an organic unit with a 

 characteristic development would seem to make terms based upon the course 

 of the reaction and the final condition unnecessary. In all cases the progres- 

 sive development leads to the highest life-form possible, and the tendency of 

 the reaction upon water-content is usually toward a mesophytic mean. Excep- 

 tions occur only in dry regions or in moist tropical ones. Hence the nature of 

 the climax formation indicates the direction of movement, and the terms 

 mesotropic, xerotropic, mesostatic, etc., hardly seem necessary at present. 

 To one who does not know the general conditions of a climax formation, they 

 are useful, but there is little need for them until more hydrotropic and xero- 

 tropic seres are known. This does not seem true of the terms hydrarch and 

 xerarch since they indicate the extreme condition in which the seres originate, 

 though they also indicate by inference the general course of development. 

 Since it is the kind of initial bare area which gives character to all the earlier 

 stages of a sere, hydrarch and xerarch are now of much value in introducing a 

 basic distinction into both primary and secondary succession. They suggest 

 the normal movement toward the mesophytic mean, but are hardly applicable 

 to seres which are xerotropic or hydrotropic. As a consequence, it may prove 

 desirable to employ the latter terms for the sake of completeness, even in the 

 present state of our knowledge. 



Convergence. — It is obvious that all the seres of a climax formation converge 

 to the final community, ^o matter how widely different they may be in 

 the pioneer stages, their development is marked by a steady approach to the 

 highest type of phyad possible in the climatic habitat and to a corresponding 

 water-content. The pioneer Hchens of a rocky ledge and the pioneer algae of 

 a pool both initiate seres, which are characterized by increasingly higher phyads 

 and more and more medium water-contents, until both terminate in the 

 climatic climax of both vegetation and water, as, for example, in the grassland 

 of the Great Plains. 



This fundamental convergence to a climax is developmental, and not indi- 

 vidual or local. Each sere in itself is a unit development which moves in the 

 inevitable direction from bare area to climax. Convergence is visible only in 

 a survey of the succession in the cUmax association as a whole. The actual 

 situation suggests an imaginary developmental cone formed by lines converg- 

 ing from a broad base of various primary and secondary areas through grass- 

 land and scrub to the final climax forest. Thus, while the development in 

 every bare area, e. g., rock-ledge, pond, burn, fallow field, etc., is a unit com- 

 prising the whole range from the initial extreme to the climax, the seres taken 

 collectively are identical in one or more of the final stages. Convergence may 

 be upon practically any stage in the succession, but it is usually upon a sub- 

 cUmax stage of grassland or scrub in the case of forest, for example. In addi- 

 tion there is often an earlier convergence of primary seres, especially upon 

 some medial stage. This is well shown by the graphic illustrations used to 

 represent the course of succession, such as are found in Chapters X and XI. 



