168 



topographic form has its own pecuHar vegetation. This is due 

 to the fact that the soil conditions upon which plants depend are 

 determined by the surface geology and topography." And, 

 since the nature of the topography is constantly undergoing 

 modifications as a result of erosion and deposition, it follows 

 that "just as there is an order of succession of topographic forms 

 in the changing landscape," so "there must be an order of suc- 

 cession of plant societies. As the years pass by, one plant society 

 must necessarily be supplanted by another, though the one passes 

 into the other by imperceptible gradations." Such a classifi- 

 cation, it will be seen, is at once genetic and dynamic. It aims 

 to "group plant societies according to their relationship and 

 their evolution." 



In a more recent contribution Cowles* has defined three types 

 of vegetative succession, regional, topographic, and biotic. 

 Regional successions are due primarily to secular changes in 

 climate and m.ove with extreme slowness. In Connecticut the 

 series of changes in vegetation that have ensued since the retreat 

 of the continental ice sheet serve to illustrate this type of suc- 

 cession. Topographic successions "are associated with the 

 topographic changes which result from the activities of such 

 agencies as running water, wind, ice, gravity, and vulcanism." 

 Such successions in Connecticut are seen principally along rivers 

 and along the coast. Topographic successions take place much 

 more rapidly than regional successions. Biotic successions are 

 instituted by plant and animal agencies. On account of the 

 comparative rapidity with which these agencies operate and their 

 far-reaching influence, this type of succession must be regarded 

 as more important than either of the two preceding. "If, in 

 their operation, regional agencies are matters of eons, and topo- 

 graphic agencies matters of centuries, biotic agencies may be 

 expressed in terms of decades." 



In treating the plant societies of Connecticut and their eco- 

 logical relations, it has been found convenient to group them in 

 the following manner.* 



* Cowles, H. C. The causes of vegetative cycles. Bot. Gaz. 51: 161-183. 

 1911. 



t Compare Cowles, 1901, op. cit. 



