lo INTRODUCTION. 



of preceding plant societies have passed away. In peat swamps 

 one can sometimes verify the results of a horizontal zonal study by 

 investigating the fossil remains beneath. 



We may now outline the main features of a physiographic classifi- 

 cation of plant societies. Speaking in the large, the tendency of the 

 erosive processes is to reduce the inequalities of the topography and 

 produce a base level. This base level may not soon be reached, 

 though geological history furnishes instances of extensive base level- 

 ing. Crustal movements interfere with the erosive agencies, and a 

 mature base level topography may become rejuvenated by a great up- 

 lift of the land, or sinking, on the other hand, may check the rapid 

 action of erosion. Yet even with the crustal movements there go these 

 topographic changes, and with them the plant societies must change. 

 Putting the facts of physiography in the terms of ecology, the con- 

 ditions become more and more mesophytic as the centuries pass. In 

 a young topography such as the recently glaciated areas of Michigan, 

 Wisconsin, and Minnesota, there is a great variety of topographic con- 

 ditions and of plant societies. Among these are many hydrophytic 

 lakes and swamps and many xerophytic hills. The hills are being 

 denuded and the swamps and lakes are being filled, so that the hydro- 

 phytic and xerophytic areas are becoming more and more restricted, 

 while the mesophytic areas are becoming more and more enlarged. In 

 passing from youth to old age, then, a region gradually loses its hydro- 

 phytic areas, and also its xerophytic areas, though in the latter case 

 there is usually at first an increase in the xerophytic areas, which is due 

 to the working back of the young streams into the hills. These latter 

 conditions are well shown in Iowa ; in the comparatively recent Wis- 

 consin drift of northern central Iowa the topography is much less 

 diversified and there are fewer xerophytic areas than in the older 

 lowan drift farther south, which has been greatly dissected by stream 

 erosion. Later, however, the inequalities are removed and we find 

 great mesophytic flood plain areas, such as are seen along the lower 

 Mississippi. 



From what has been stated it will be seen that the ultimate stage 

 of a region is mesophytic. The various plant societies pass in a series 

 of successive types from their original condition to the mesophytic 

 forest, which may be regarded as the climax or culminating type. 

 These stages may be slow or rapid ; some habitats may be mesophytic 

 from the start; undrained lakes and swamps fill up and become meso- 



