SUCCESSION ^T\ 



to follow. For this, (he use of the permanent quadrat is imperative, as the 

 sequence and structure of the stages can be understood only by a minute 

 study of the shifting and rearrangement of the individuals. Permanent mi- 

 gration circles are indispensable for tracing movement away from the pioneer 

 areas by which each stage reaches its maximum. Denuded quadrats are a 

 material aid in that they furnish important evidence with respect to migration 

 and ccesisv By melius of them, it is possible to determine the probable devel- 



Fig. 70. Alternating gravel slides on Mounts Cameron and Pals- 

 grove, from the comparison of which the initial development of the 

 talus succession has been reconstructed. 



opment of stages which reach back a decade or more into the past. In the ex- 

 amination of successions, since cause and effect are so intimately connected in 

 each reaction, it is especially important that general and superficial observa- 

 tions upon structure and sequence be replaced by precise records, and that 

 vague conjectures as to causes and reactions be supplanted by the accurate 

 determination of the physical factors which underlie them. 



329. Method of alternating stages. The period of time through 

 which a primary succession operates is usually too great to make a complete 

 study possible within a single lifetime. Secondary successions run their 

 course much more quickly, and a decade will sometimes suffice for stabiliza- 



