CHAPTER X.

HAVING shown in the last chapter how considerably the numerical increase or the 
extension of the geographical range of any one species must derange the numbers 
and distribution of others, let us now direct our attention to the influence 
which the inorganic causes described in our first volume are continually 
exerting on the habitations of species.
 

So great is the instability of the earth's surface, that if Nature were not 
continually engaged in the task of sowing seeds and colonizing animals, the 
depopulation of a certain portion of the habitable sea and land would in a few 
years be considerable. Whenever a river transports sediment into a lake or sea, 
the aquatic animals and plants which delight in deep water are expelled : the 
tract, however, is not allowed to remain useless, but is soon peopled by species 
which require more light and heat, and thrive where the water is shallow. Every 
addition made to the land by the encroachment of the delta of a river banishes 
many subaqueous species from their native abodes ; but the new-formed plain is 
not permitted to lie unoccupied, being instantly covered with terrestrial 
vegetation. The ocean devours continuous lines of sea-coast, and