CHAPTER IX.

WE have seen that the stations of animals and plants depend not merely on the 
influence of external agents in the inanimate world, and the relations of that 
influence to the structure and habits of each species, but also on the state of 
the contemporary living beings which inhabit the same part of the globe. In 
other words, the possibility of the existence of a certain species in a given 
locality, or of its thriving more or less therein, is determined not merely by 
temperature, humidity, soil, elevation, and other circumstances of the like 
kind, but also by the existence or non-existence, the abundance or scarcity, of 
a particular assemblage of other plants and animals in the same region.
 

If we show that both these classes of circumstances, whether relating to the 
animate or inanimate creation, are perpetually changing, it will follow that 
species are subject to incessant vicissitudes ; and if the result of these 
mutations, in the course of ages, be so great as materially to affect the 
general condition of stations, it will follow that the successive destruction of 
species must now be part of the regular and constant order of Nature.
 

It will be desirable, first, to consider the effects which every extension of 
the numbers or geographical range of one species