CLIMATIC COMMUNITIES ant 
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 coverging (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 climatic 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 (0) successional or evolutionary relations. 
Diagram g 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 
