434 



THE INVESTIGATION OP SUCCESSION. 



been based upon the establishment of individuals, and there has been almost 

 no attempt to determine the r61e of migration itself in succession, as one of the 

 two processes concerned in invasion. A few studies have been made of the 

 kind and number of migrules brought by long-distance carriage, such as that 

 of birds, but these have little or no effect upon succession. Local, and usually 

 mass, migration is the chief factor in the latter, and such movement permits 

 of fairly accurate measurement, even though ecesis is not taken into account. 



4 dm. wide 

 Scale 1cm.= ldm. 



V=yearlings 



s=seedlings (older than y) 



d=dead trees 



Seedlings and yearlings are 

 without exception restricted 

 to free soil areas between sod 

 patches or mossy areas. In 

 holes or shallow depressions 

 where there is an absence of 

 soil cover the yearlings are 

 most abundant. 



Fig. 51. — Transect of a migration arc, Uncompaigre Plateau, Colorado. 



