CLIMATIC COMMUNITIES 311 



form of hills. These hills are broken up into smaller hills by the smaller 

 tributaries, and the resulting hills into still smaller ones, until the upland 

 is all removed and the country reduced to a generally level condition 

 known as a peneplain. The process of peneplanation then tends to 

 fill all low lakes and ponds and drain all high ones. It works over all the 

 materials of the upland and lays them down as alluvial deposits, which 

 process tends to make the surface materials of a uniform nature. Asso- 

 ciated with this, and more or less independent of it, the process of plant 

 succession makes the conditions converging (Diagram 8) to a still greater 

 degree (13). 



The principle of convergence, while not generally established, is 

 believed to be of wide application. It has been suggested for the tropical 

 forest of the Philippines by Whitford (198), for the coniferous forest 

 regions of North America by Adams and by Gleason, and for the arid 

 Southwest by Ruthven. Theoretically at least, in all the varied types of 

 land habitats of any large area, communities are tending toward some 

 one type which is primarily adjusted to the climate of the region when its 

 topography approaches base level. Such a climatic type of community 

 rapidly displaces the communities of all the varied kinds of soil of a 

 newly uplifted area which is only a few hundred feet above the sea. In 

 these situations the climatfc communities dominate sterile soil by process 

 of successional development extending over a few score or hundreds of 

 years. 



V. General Relation of Communities of the Same 

 Climate (13) 



In each climatic realm of the world there are relations between 

 communities of two sorts, (a) physiological relations, best defined as 

 physiological similarities, and (b) successional or evolutionary relations. 

 Diagram 9 shows both types of relations for the temperate American 

 forest border area. Single-pointed arrows show the directions of suc- 

 cession, double-pointed arrows show similarities of conditions and the 

 occurrence of several or many of the same species in considerable num- 

 bers in communities between which such arrows extend. Broken lines 

 indicate less definite relations than the solid lines. Starting with the 

 aquatic communities, we note that spring-fed and intermittent stream 

 communities converge with physiographic aging to small, permanent, 

 swift-stream communities, and permanent swift-stream communities 

 are succeeded by base-level stream communities. The characteristic 



