^'"iQ^^^^] Adams, Ecological Succession of Birds. 131 



surrounding Lake must be recognized. This formerly stood at a 

 level much above that of the highest ridges upon the island, as is 

 clearly evidenced by the abandoned beaches on the north shore of 

 Lake Superior. Such relations prove that Isle Royale was once a 

 rocky reef in the lake, which, as the Lake level was lowered (it is 

 quite unlikely that the island has been materially elevated) became 

 exposed as a wave-washed beach. These conditions are approxi- 

 mated to-day by the low outlying islands. The beach or shore is 

 thus the original habitat upon Isle Royale, and in general, all others 

 have been derived or developed from it. To discuss these as a truly 

 genetic series would require that these be described simultaneously, 

 as the differentiation took place. These habitats did not develop 

 as isolated phenomena, but several developed at the same time, or 

 abreast. Thus as soon as enough of the land surface had become 

 exposed so that its inequalities began to have an influence, the ridges 

 would be the parts best drained, and certain depressions would tend 

 to accumulate the drainage. This process would lead to a simul- 

 taneous development or differentiation of the well, moderately, 

 and poorly drained habitats. Almost all of the residual soil 

 formed as the region was baseleveled was probably cleared away by 

 the glaciers; or later, as the waves fell from the island, by the 

 pounding of the waves. Thus the relative absence of a soil must 

 characterize all habitats. At what period life first reached the 

 island in post-Glacial time is not definitely known; but it is likely 

 that the pioneer vegetation of lichens, mosses and low herbaceous 

 vegetation reached it soon after its exposure. If the biota reached 

 the island about the time of the formation of the Algonquin beach, 

 which, roughly speaking, may have been at about the present 

 elevation of 475 feet above the Lake surface, it has since spread up- 

 ward and downward from that level. The composition of the 

 initial societies is not liable to as much variation as the later ones. 

 Thus if the Herring Gulls returned to the region at this early period 

 of the exposure, they were probably the pioneer birds ; but if only 

 at a much later date, still other species might have accompanied 

 them. While such variations as this may be expected, and due 

 allowance must be made for them, yet there can be little reasonable 

 doubt but that waier birds and those frequenting open places tended 

 to become the pioneers, and that later, with the development of a 

 soil and forests, other associations of birds became established. 



