ECOLOGICAL SUCCESSION. 349 



climate will then also be different, and will more nearly approxi- 

 mate that at present prevalent at the base; therefore the erosive 

 topography at old age will have a vegetative condition not unlike 

 the Bambusa-Parkia formation. Just before the death of such 

 a topography, the whole country will be brought nearly to base 

 level (peneplain) with the ground water near the surface. The 

 vegetative conditions will not be unlike that of a delta region, of 

 which there are many fine examples in the Philippines. ... Of 

 course the above are only theoretical considerations, yet these 

 erosive stages are approximated in different parts of the island, 

 so that when logically united, the genetic relations of the different 

 vegetative formations can be made clear." 



Physiographic processes and the processes of plant succession go 

 on at varying rates under different conditions. Where the land 

 stands low, is of easily eroded material and in a rainy district, a 

 peneplain may develop over a considerable area in a few thousand 

 or even hundreds of years. The same process would require 

 infinitely more time in an arid region. With favorable soil condi- 

 tions, a shallow pond will pass through all the stages of succession 

 and into prairie or early forest in two or three decades. In 

 moist climates, young forest springs up in a comparable period. 

 On the other hand, the peneplanation of large areas, climatic 

 changes, etc., require longer periods but follow the same general 

 laws as do those changes which take place quickly, and ecological 

 succession is similar in principle no matter whether the changes 

 are slow or rapid. Animal habitats and animal communities are 

 orthogenetic and converging (Adams, '08). 



5. Primary and Secondary Conditions and Communities. 



Every system of agriculture is, ecologically speaking, the hold- 

 ing back of all natural changes due to the effect of organisms. It 

 consists in preventing the operations of all the biological laws, 

 by which changes in the character of the habitat are brought 

 about. If the organization of ecological materials is to be 

 brought about in correlation with natural laws, then agricultural 

 communities are essentially useless subjects for study. 



Plant ecologists have recognized a division into primary and 

 secondary communities (Warming, '09). The primary com- 



