344 VICTOR E. SHELFORD. 



closed forest was likewise divisible into three habitats and three 

 corresponding communities, the blackoak-whiteoak-redoak, the 

 redoak-hickory, and the beech-maple. 



(b) Dynamic Relation of Local and Climatic Conditions. There 

 are many local conditions in each climatic area. The relation of 

 local conditions to the climatic or major is closely bound up with 

 the principle of succession. In the preceding papers we have 

 noted that succession may be due chiefly to physiographic 

 changes, or to the fact that the organisms of a given stage affect 

 conditions in such a way as to make their own continued existence 

 impossible, and prepare the way for others. While in particular 

 cases, physiographic conditions may dominate, in others biological 

 conditions dominate. Both are probably always detectable 

 factors. (Cowles, 'n; Adams, '01, '08.) 



Under the head of physiographic causes of succession come 

 such processes as the uplifting and degradation of land, erosion, 

 deposition, etc. Along a coast, the processes (Gilbert, '85; 

 Gulliver, '99; Salisbury, '07) are causes of changes in physical 

 conditions, material for abode, etc., and result in ecological 

 succession. Such geological processes are treated in textbooks 

 on geology and physiography and may be only outlined here. 



When a body of land is uplifted or the level of the water into 

 which it is drained is lowered, streams begin to work their way 

 into the new land mass and cut deep valleys, with marked dif- 

 ferences in both vegetation, physical conditions, and animal 

 communities (Shelford, 'n). The formation of numerous tribu- 

 taries (see diagrams by Salisbury, Adams, '01) isolates portions 

 of the upland in the form of hills. These hills are broken up 

 into smaller hills by the smaller tributaries, and the resulting 

 hill's into still smaller, until the upland is all removed and the 

 country reduced to a generally rolling topography with very 

 little relief and known as a peneplain (Adams, '01; Salisbury, 

 '08; Chamberlain and Salisbury, '06). The process of pene- 

 planation then tends to fill all low lakes and ponds and to drain 

 all high ones. It works over all of the materials of the upland 

 and deposits them over much of the resulting surface, which tends 

 to make the surface materials of a uniform nature. The processes 

 involved go on in definite directions during longer or shorter 



