KEGBESSION. 147 



proceeds uniformly over a large area, but progressive and regressive areas 

 alternate with each other constantly. (371) 



Nilsson's whole description agrees perfectly with the course of events in a 

 sere where local conditions bring about the destruction of a particular stage 

 in alternating spots. He makes it clear that drying-out kills the Sphagnum, 

 Eriophorum, and shrubs in certain areas, and produces conditions in which 

 lichens thrive. In turn, the accumulation of water kills the lichens, and, more 

 slowly, the relict Eriophorum and shrubs, and prepares a new area for the 

 invasion of Sphagnum and sedges. In all of this destruction there is nothing 

 whatever of an organic successional development. Wherever plants are 

 destroyed, whether quickly or slowly, over large areas or in a spot of a few 

 square centimeters, invasion becomes possible, and local development begins. 

 A general view of a moor with alternating pools and hummocks, of drier and 

 wetter places, may well give the appearance of regression. But this is an 

 appearance only, for in each pool and on every hummock development pro- 

 ceeds always in a progressive direction, though it may be interrupted again and 

 again by a change of conditions. Nilsson also regards the repeated passage 

 from progressive to regressive to progressive again as indicating a circulation 

 or cycle of development, but this view depends upon the existence of an actual 

 backward development. 



Cowles's view. — Cowles distinguishes between progressive and retrogressive 

 succession chiefly upon physiographic grounds. The distinction is drawn 

 clearly in the following statement: 



"Retrogressive phases, i. e., away from the mesophytic and toward the 

 hydrophytic or xerophytic, must be included, as well as progressive phases 

 away from the hydrophytic and toward the mesophytic." (81) 



The distinction is further elaborated as follows: 



" In flood plains, the meanderings of the river may cause retrogressions to the 

 hydrophytic condition, such as are seen in oxbow lakes, or the river may lower 

 its bed and the mesophytic flood plain become a xerophytic terrace. The 

 retrogressive phases are relatively ephemeral, while the progressive phases 

 often take long periods of time for their full development, especially in their 

 later stages. ... If a climate grows colder or more arid, we find retro- 

 gressive tendencies toward the xerophytic condition, while in a climate that 

 is getting more moist or more genial, the mesophytic tendencies of the erosive 

 processes are accelerated. . . . (82) Retrogression is almost sure to come 

 in connection with terrace formation. A river may swing quite across its flood 

 plain, destroying all that it has built, including the mesophytic forest. Not 

 only is the vegetation destroyed directly but also indirectly, since the lowering 

 of the river causes the banks to become more xerophytic. . . . (107) 

 The life history of a river shows retrogression at many points, but the progres- 

 sions outniunber the retrogressions. Thus a river system, viewed as a whole, 

 is progressive. . . . (108) A young topography is rich in xerophytic 

 hills and in hydrophytic lakes and swamps. There may be local retrogressions 

 toward xerophytic or even hydrophytic plant societies, forming eddies, as it 

 were, but the great movement is ever progressive and toward the mesophytic 

 condition. So far as plants are concerned, however, a physiographic termi- 

 nology may be still used, since all possible crustal changes are either toward or 

 away from the mesophytic, i. e., progressive or retrogressive." (178) 



