1913] COOPER—ISLE ROYALE 220 
During the early subaerial history of Isle Royale the rock shore 
succession may have differed from that of the present day. A study 
of Gull Islands indicates that birds may have been important 
agents in determining the composition of the primitive rock shore 
flora of the island. 
The beaches on account of their sheltered location usually bear 
the climax forest down to its limit of possible extension. Low 
shrubs of various kinds are the most important pioneers in the 
beach succession, and larger ones, especially Alnus crispa, intervene 
before the establishment of the climax type. 
The hydrarch successions 
The bog succession 
Physiographic development.—The depressions which now contain 
lakes or bogs owe their origin to glacial modification of the preglacial 
topography; sometimes to the cutting off of bays or channels by 
wave-built bars. 
The physiographic history of the habitat in which the bog 
succession runs its course comprises two stages: the channel-bay 
stage and the lake stage. The lakes and harbors are tending toward 
extinction through the agencies of down-cutting of outlets, sedi- 
mentation, and vegetation, of which the last is the only one of 
importance at the present time. The lake stage ends when vegeta- 
tion, aided by the other agencies, has entirely eliminated the open 
water. 
Vegetational development.—During the channel-bay stage aqua- 
tics first appear and gradually increase with increasing shelter; the 
beginnings of the sedge mat are occasionally present. 
During the physiographic lake stage all the vegetational stages 
of the succession appear in order: aquatics (usually already 
present), sedge mat, sphagnum-shrub, bog forest. All may have 
their beginnings at practically the same time. The sedge mat is 
usually the most prominent feature at this period. The sedges 
gain their first foothold in shallow water close to shore and build a 
floating mat out over the water. They are by far the most impor- 
tant agents in peat formation. 
uring the covered bog stage the plant societies are successively 
eliminated by the centripetal encroachment of the various zones. 
