^°1908^^] Adams, Ecological Succession of Birds. 135 



not improbable that as the island appeared the Herring Gull was 

 one of the first species to discover it. Such a bird might even reach 

 the island under climatic conditions of the Ice Age, for the species 

 now ranges far north along the shore of the Arctic Sea. A species 

 of such extensive chronological and geographical range will tend to 

 give much stability to succession. The present breeding range of 

 the Mergansers and the Loon is not so far north, and for this reason 

 they may have arrived under milder climatic conditions. But if 

 the island became exposed under mild post-Glacial conditions, all 

 of these species may have arrived at much the same time. But even 

 with the chances for such variations the general succession seems 

 to have been initiated with the aquatic association as the pioneer 

 society. 



In following the genesis of the habitats and associations from this 

 point onward, divergence and differentiation becomes so marked 

 that it is impossible to develop all lines abreast. A linear treat- 

 ment becomes necessary, and therefore certain general relations 

 are liable to become obscured unless specifically mentioned in 

 advance. 



The aquatic and beach habitats possess a marked tendency to- 

 ward a zonal arrangement. From the Superior beach the transition 

 is through open or shrub zones into the climax forest. The topog- 

 raphy of the island with its longitudinal ridges and valleys form a 

 dominant factor in impressing this zonal structure upon the biotic 

 associations. This series, — from the water, through the beach, 

 open and shrub marginal zone, into the climax forest, — may be 

 considered as the genetic vegetative succession. They change 

 simultaneously and are due to the same general cause, — the falling 

 Lake surface, which transforms the water area into beach, the 

 beach into forest margin, and forest margin into the climax asso- 

 ciation. But as mentioned, it is manifestly impossible to discuss 

 all these transitions at once, and each ecological unit must therefore 

 receive separate genetic treatment. 



This tension line or marginal zone between the Lake and the 

 forest shows such a wonderful diversity and complexity in its con- 

 ditions, that several plant and animal associations are formed within 



