EEGRESSION. 149 



"As might be expected, the influence of erosion generally is destructive to 

 vegetation, or at least retrogressive (i. e., tending to cause departure from the 

 mesophytic), while the influence of deposition is constructive or progressive 

 (t. e., tending to cause an approach to the mesophytic). On a somewhat 

 rapidly eroding clay cliff of Lake Michigan . . . a marked increase in erosive 

 intensity would destroy all vegetation, and a marked decrease in erosive 

 intensity might institute a progressive vegetative succession. Frequently 

 a growing dune is inhabited by xerophytic annuals and by a few shrubs or trees ; 

 such a place Ulustrates the extreme of topographic dynamics, but often the 

 vegetation is static. A great increase in depositional intensity results in the 

 destruction of all the plants, while a decrease in depositional intensity results 

 in progressive succession." 



Cajander's view. — Cajander (1904) has studied three moors of northern 

 Finland in connection with Nilsson's concept of progressive and regressive 

 development, and has reached the conclusion that these views are correct. 

 Moor I is regarded as in the course of primary progressive development 

 characterized by a contiauous fresh green moss layer, with low and indefinite 

 cushions of heath-moor. Moor II is assumed to be chiefly in regressive 

 development, as it is made up of areas of cyperaceous moor separated by strips 

 of heath-moor. The reasons for this view are that (1) many areas are bare 

 spots of decayed turf, (2) the sedge areas are often sharply delimited and raised 

 above the heath-moor areas, (3) the moss-layer is lacking or consists of other 

 mosses than Sphagnum. In the extensive Moor III, regressive development 

 has everywhere taken place, and cyperaceous communities occur throughout. 

 In a large part of the moor the regressive development is followed by a 

 secondary progressive development, and in small areas of the latter is found a 

 secondary regressive development. On these grounds the author regards the 

 view of Nilsson that there is a "circulation" or cycle in the development 

 as well-grounded. As already pointed out in this connection, regressive 

 development is only destruction or denudation followed by the normal devel- 

 opment, which is always and inevitably progressive. Denudation or destruc- 

 tion may recur again and again at any stage of succession in many separate 

 areas of the community and hence produce a maze of so-called "regressive" 

 and progressive areas. 



Sernander's view. — Sernander (1910 : 208) has drawn a distinction between 

 progressive and regenerative development: 



"The real cause why the Sphagnum peat is heaped up in such fashion lies 

 in the fact that the moribund parts lag behind the living Sphagnum in growth 

 and finally form hollows in the latter. These hollows fiU gradually with water' 

 while the erosion of the surrounding peat-walls increases their extent. In the 

 water arise new Sphagneta, which begin in miniature the progressive develop- 

 ment, which I term regeneration. This regenerative development of the hol- 

 lows soon culminates in CaHMno-heath or is interrupted by a new formation 

 of hollows. The latter develops in the usual way, and in this manner arises 

 one lens-shaped peat-mass above another, characterized above and below bv 

 dark streaks, usually of heath-peat." 



In discussmg the origin of the high moor of Orsmossen (1910:1296) Ser- 

 nander states that: 



