146 DIRECTION OF DEVELOPMENT. 



Course of development. — The basic and universal progression from bare 

 areas to climax is a complex correlated development of habitat, community, 

 and reaction. The general relation of these is indicated by the gradual coloni- 

 zation of a bare area, and the progression of associes until the climax is reached. 

 Beneath this, as motive forces, lie invasion and reaction. The total effect is 

 seen in four progressive changes or processes. The initial change occurs in the 

 habitat, which progresses normally from an extreme or relatively extreme 

 condition to a better or an optimum condition. This is especially true of the 

 unfavorable extremes of water-content, both as to quantity and quality, i. e., 

 the presence of alkali, acid, etc. With respect to the plant life, the progressive 

 movement is from lower to higher phyads, from algae, lichens, or mosses, to 

 grasses and woody plants. The interaction of habitat and community results 

 in a progressive increase of dominance and reaction, both in the most intimate 

 correlation. Finally, in the climax formation as a whole, there is the simul- 

 taneous progression of almost innumerable primary and secondary seres, all 

 converging toward the climax into which they merge. 



Regression and retrogression. — It has already been stated that regressive 

 development is impossible and that regressive succession does not occur. 

 Hence it becomes necessary to examine the views of several authors who dis- 

 tinguished between progressive and regressive succession and to interpret their 

 observations in terms of development. Such a distinction seems first to have 

 been made by Nilsson (1899) in the study of the development of Swedish 

 associations. Cowles (1901) used the same terms, but with a very different 

 meaning, in his physiographic treatment of the ecology of the region about 

 Chicago. Cajander (1904) adopts the distinction proposed by Nilsson, as 

 does Moss also (1910), while Hole (1911) uses progressive and regressive in 

 still another sense. As will become evident, some of these concepts are sub- 

 jective and have little relation to the organic development, while others rest 

 upon an incomplete interpretation of the facts. The existence of five conflict- 

 ing views seems to afford illuminating evidence as to the actual occurrence of 

 such a distinction in nature. 



Nllsson's view. — The regular development of vegetation about the lakes of 

 Sweden exhibits the following stages: (1) sedge moor; (2) Eriophorum moor; 



(3) scrub moor, with various consocies, chiefly Calluna, Erica and Betula; 



(4) forest moor, usually Pinus silvestris, often Picea or Betula. This is properly 

 regarded as progressive development. Regressive development is said to take 

 place when lichens, Cladina and Cladonia, appear in the Sphagnum masses of 

 the scrub moor, and come to play the dominant r61e, as the Sphagnum and 

 shrubs die off in large measure. Sphagnum, Eriophorum, and the shrubs still 

 persist, however, in scattered altemes. The cause of the regression Hes in 

 the drying-out of the upper layer, the death of the Sphagnum, and the conse- 

 quent weathering of the peat. During wet seasons the lichens perish through 

 the accumulation of water. Eriophorum and Andromeda persist longer, but 

 finally die out also as a result of the continued accumulation of water. The 

 water is colonized by Sphagnum and sedges, especially Carex limosa, Sdrpus 

 caespitosus or Scheuchzeria, and progressive development begins anew, to 

 terminate in forest moor or to be again interrupted by unfavorable conditions. 

 This continues untU progressive development prevaDs throughout the entire 

 area, and finally terminates in the climax forest. The development rarely 



