148 DIRECTION OF DEVELOPMENT. 



In connection with succession on dunes, Cowles states that: 

 "A slight change in the physical conditions may bring about the rejuvena- 

 tion of the coniferous dunes, because of their exposed situation. This rejuven- 

 ation commonly begins with the formation of a wind sweep, and the vegetation 

 on either hand is forced to succumb to sand-blast action and gravity." (174) 



Elsewhere (172) the dune complex is described "as a restless maze, advanc- 

 ing as a whole in one direction, but with individual portions advancing in 

 all directions. It shows all stages of dune development and is forever chang- 

 ing." Such destruction of existing communities and the production of a bare 

 area are essentially the same as the changes in moor which NUsson calls regres- 

 sive. Cowles nowhere applies this term to the dune sere, and appears in no 

 place to speak of the succession or development as retrogressive. Indeed, the 

 use of the word "rejuvenation" in this connection is a fortunate one, as it 

 emphasizes the essentially reproductive nature of the developmental process. 



The use of progressive and retrogressive in connection with the development 

 of seres in river valleys illustrates the undesirability of transferring physio- 

 graphic terms to the organic development of vegetation. It is evident that 

 a river system shows almost constant, though more or less local retrogression 

 throughout its general progressive development during a single cycle of erosion. 

 The bed of a river, its banks, flood-plain, and terraces are constantly reshaped 

 by erosion and deposition in conformity with a general law. The material 

 of the land is not destroyed, but merely shifted. Such is not the case with 

 the community which occupies an area of erosion or deposit. As shown 

 above, Cowles points out in such cases that the vegetation is destroyed directly 

 or indirectly. Hence there can be no such thing as retrogression in the suc- 

 cessional development. What does occur is the universal phenomenon of suc- 

 cession, in which one serai development is stopped by the destruction of a 

 particular stage, and a new sere starts on the bare area thus produced. If 

 the term "retrogressive phase" be applied solely to the usually brief period 

 when the community is being destroyed, it fits the facts, but is still misleading. 

 It impUes a backward development or devolution comparable to the progres- 

 sive evolution or development of the sere, while as a matter of fact it appUes 

 not to development but to its complete absence, i. e., to destruction (c/. 

 Crampton, 1911 : 27; Crampton and MacGregor, 1913 : 180). 



The difficulty of distinguishing progression as movement toward a meso- 

 phytic mean and retrogression as movement away from it is well illustrated 

 in succession in desert regions. The development of vegetation in a desert lake 

 or pond passes from hydrophytic to mesophytic, and then to xerophytic or 

 halophytic stages. Organically this is a unit development from a bare area 

 to a climax community. Yet the distinction just mentioned would require 

 that it be divided into progression and retrogression. The only possible retro- 

 gression is in the decreasing water-content, and yet this decrease of water- 

 content is a constant feature of the progression from ordinary water areas 

 to mesophytic conditions. Similarly, the successional development along the 

 coast of the Philippines would present a peculiar difficulty, if Whitford's 

 interpretation is correct (1906 : 679). He regards hydrophytic forest as the 

 climax, and the entire development would consequently be retrogressive. 



In a later paper (1911 : 170), Cowles appears indeed to regard retrogression 

 as little if at all different from destruction, and to interpret physiographic 

 processes chiefly in terms of destruction and development or progression. 



