24 GENERAL HISTORICAL SUMMARY. 



dune-scrub, and then into dune-forest. In the north of Europe may be 

 encountered the following formations, which show a zonal succession to some 

 extent. It is obvious that the zonal order is essentially that of the develop- 

 mental sequence. 



1. Sand algae. 5. Stationary or gray dunes. 



2. Iron-sulphur bacteria. 6. Dune-heath and dry sand-field. 



3. Psammophilous halophs^tes. 7. Dune-scrub. 



4. Shifting or white sand-dunes. 8. Dune-forest. 



MacMillan, 1894-1896. — MacMUlan's studies of the bogs and muskeags of 

 Minnesota constitute the pioneer work upon succession in America, though 

 analysis at this early period was necessarily general. In the investigation of 

 Sphagnum atolls (1894:2), he concluded that these atolls, i. e., circular zones 

 of Sphagnum, are due to a season of gradual recession of the waters of the 

 pond, followed by a season of comparatively rapid increase in area and level. 

 This is indicated by the fact that the vegetation of the atoll differs from that 

 of the pond outside and the lagoon within it. The atoll first appeared as a 

 zone of floating bog, which was separated from the shoreward turf as a conse- 

 quence of the original zonation of the shore plants and of the rise of the water- 

 level, taken in conjunction with certain special topographic conditions. The 

 sequence of events was probably as follows: The pond, as a result of silting-up 

 and of climatic variations, slowly diminished until its shore-line coincided with 

 the inner edge of the present atoll. The size of the pond at this time is indi- 

 cated by the existing lagoon. The shore vegetation then invaded the bare 

 slopes and formed characteristic zones, the inner perhaps of Sphagnum. When 

 the pond began to fill up again, the marginal zone of turf was forced upward, 

 and finally detached to form a circular floating bog or atoll. The further rise 

 in level left the atoU well out in the pond. The atoll sank as its weight 

 increased with its growth in thickness, and it finally became anchored to the 

 bottom of the pond. While it is possible that the two atolls were formed 

 simultaneously, one is now in the stage characterized by Sarracenia, Erio- 

 phorum, and Kalmia, and the other is dominated by Ledum and Picea. 



MacMillan (1896:500) also studied the Sphagnum moors or muskeags of 

 Minnesota, in which almost every stage may be found from open lakes with 

 continuous sandy beaches to solid masses of spruce and tamarack. The latter 

 is displaced by pines or hardwood, and is finally developed into mixed wood or 

 perhaps into meadow. Typical muskeag with spruce and tamarack are 

 regarded as an intermediate type between the original open lake and the later 

 forest. The center of the muskeag is usually softer than the edges, though in 

 many, even of the small ones, the center is quite firmly filled with soil, and 

 Sphagnum predominates here. When a central pool is present, it contains 

 Utricularia and Lemna, and often Potamogeton and Nymphaea. The next zone 

 contains Kalmia and Andromeda, with Carex, Eriopharum, Sarracenia, Salix, 

 Vacdnium, etc. Ledum is found on drier peripheral portions, and is often 

 the most abundant heath when the Sphagnum has disappeared. This zone is 

 surrounded by spruces, usually Picea mariana, sometimes P. canadensis, 

 tamarack, Larix laridna,-Alnv^ incana, Betula, and Salix. An examination 

 of Sphagnum moors shows that they are characterized by zones of Larix, 

 Picea, Ledum, Andromeda, and Utricularia, from the margin to the center. 

 The tamarack and spruce zones are slowly closing in upon the others, and will 



