THE STRUGGLE FOR EXISTENCE. I4I 
advance of a plant is likewise a struggle with natural 
obstacles; and the conquest which it gains usually injures 
other plants in their conditions of life. If the powers 
of multiplication of any given organism were to operate 
absolutely and unrestrictedly, each being would, in a 
short series of years, claim for itself the whole surface of 
the earth, or all the waters of the sea. But each holds 
the other in check; and with the living foes of each 
creature are associated the climate and all the influences 
of the surrounding conditions, and of the alternation of 
the seasons, to which the body must accommodate itself. 
Organisms live only at the cost of, and for the profit of, 
others; and the peace and quiet of nature sung by the 
poet is resolved under the searching eye into an eternal 
disquiet and haste to assert and maintain existence, 
amid which it is only the thought of the visible and 
necessary progress that can rescue the observer from a 
pessimist view of the world. 
The simplest examples of the relations of mutual. 
dependence of living beings are, however, the best and 
most conclusive; but the vast consequences depending 
on circumstances and connections apparently insignifi- 
cant, and the extreme complexity of the mechanism by 
which equilibrium is maintained, have been exhibited by 
Darwin in some examples, which, frequently as they have 
been repeated, we shall also allow ourselves to reproduce. 
Whereas, to the South and North of Paraguay feral cattle, 
horses, and dogs abound in profusion, they are wanting 
in Paraguay itself. “Azara and Rengger have shown that 
this is caused by the greater number in Paraguay of a 
certain fly, which lays its eggs in the navels of these 
animals when first born. The increase of these flies, nu- 
