940 ECOLOGY 



fication, that is, one which groups associations in a series in their order 

 of development. 



Succession. The basis of a genetic classification is the principle of 

 succession, namely, that in the physiographic development of a region the 

 various habitats pass through a series of more or less definite stages, 

 owing chiefly to the processes of erosion and deposition, supplemented 

 by the accumulation of humus. The primitive associations, that is, 

 those of new lands or waters, are likely to be either xerophytic or hydro- 

 phytic. In a region with a mesophytic climate, the primitive associa- 

 tions become displaced by others that are slightly more mesophytic, 

 and they in turn by others, until the series finally culminates in the 

 most mesophytic association of which the region as a whole is capable. 

 For example, in the eastern United States an upland of rock, sand, or 

 clay, whose original flora is xerophytic, becomes gradually more and 

 more mesophytic, either through land denudation or humus accumula- 

 tion or both, until it becomes clothed with the ultimate plant association 

 of the region, namely, a deciduous mesophytic forest. A pond in the 

 same region gradually becomes filled through humus accumulation or 

 through stream and shore deposition, or both, so that the original aquatic 

 vegetation becomes displaced by a swamp vegetation, and this in turn 

 through further humus accumulation becomes displaced by a forest 

 quite comparable to that which marks the final stage in an upland suc- 

 cession. In arid or semi-arid climates it is obvious that the final stage 

 could not be mesophytic, but would necessarily be an association which 

 is much nearer the primitive xerophytic association of the region. 



The scope of this book forbids any attempt at a detailed classification 

 of plant associations. The general principles enunciated above must 

 suffice. In the remaining paragraphs of this chapter there will be pre- 

 sented some of the more striking features of a few of the more impor- 

 tant plant associations, especially of those that are found in the United 

 States, but no attempt will be made to bring out genetic relationships 

 or to make exhaustive analyses. 



Pond associations. Perhaps the most representative fresh- water 

 associations are those of ponds, and these are among the most interest- 

 ing of all associations, partly because they are more likely to remain 

 natural than are most habitats in densely populated districts, but espe- 

 cially because they show obvious and rapid stages in succession between 

 the primitive aquatic associations and the various sorts of swamps. The 

 vegetation of ponds consists usually of free-floating forms (including 



