174 CLASSIFICATION OF SERES. 



mations, which seem to correspond roughly to climax and serai communities 

 respectively. It also serves to lend much emphasis to the fact that in the 

 study of the development of vegetation development is obviously paramount 

 and physiography quite secondary. 



Watson's (1912:213) use of the term "biotic succession" also illustrates 

 the inevitable confusion to which it leads: 



"After a fire in the Douglas spruce, the quaking aspen always takes posses- 

 sion, but it has also its natural place as a transition between the oak chaparral 

 and the Douglas spruce in the biotic succession. The biotic succession in the 

 Sandia Mountains is as follows: The bare rock first incrusted with crustose 

 lichens, then foliose Uchens, mosses, herbs, oaks, followed in some cases directly 

 by Douglas spruce, and in others by aspen and then the spruce; and then as 

 physiographic succession comes in, the poplars, pines, and box-elders in the 

 canon and pine, pinon, and cedar on the slopes, and the ultimate formation of 

 the mesa is reached." 



The aspen is a characteristic stage of the secondary succession due to man 

 as a biotic cause, while it progresses to the Douglas-spruce stage in consequence 

 of the reactions of plants as biotic agents. The last is also true of the primary 

 succession initiated on rock by crustose lichens, but as to cause, this succession 

 is essentially topographic. 



Siegrist (1913 : 145) has also distinguished topographic and biotic successions, 

 but his topographic succession is the biotic succession of Cowles. This is 

 shown by the definition of a topographic succession as one in which a topo- 

 graphic change is necessary for the initiation of a new formation. The 

 examples given on pages 158 and 159 further prove that he is concerned with 

 local unit succession or seres, and not at all with the topographic successions of 

 Cowles, which are matters of centuries and belong to far-reaching erosive 

 cycles. Biotic succession is defined as one in which no topographic change is 

 necessary, though it does not exclude the simultaneous occurrence of such 

 changes, which, however, have no influence upon the biotic succession. The 

 author's use of the term is in itself incorrect as well as misleading, as he employs 



it for parts of a unit succession or sere (1. c, 145, 158), e. g., Hippophaetmn > 



Pinetum, Hippophaetmn ^>Transition association, Pinetum >Transition 



association. As already indicated, his topographic associations are necessarily 

 biotic in reaction, and would be called biotic successions by Cowles. The 

 distinction made on page 159 is far from evident, but it seems to be based upon 

 whether colonization takes place in the water or upon a new area of sand or 

 gravel. From the standpoint of development, a pond or stream is just as 

 much a bare area due to a topographic initial cause as is a sand-bar or a 

 gravel-bank, and the succession on each proceeds as a consequence of the 

 biotic reactions of the plants. It is also difficult to understand how local 

 topographic changes can occur without initiating or affecting succession. 



Dachnowski (1912 : 259) has distinguished two kinds of successions, as 

 follows: 



"Two great, relatively wave-like and integrating phases of vegetation suc- 

 cessions define themselves rather clearly: (1) the climatic successions, asso- 

 ciated with the succession of geological periods and of which the migration of 

 plants accompanying and following the retreat of the glaciers is an example; 



