THE STRUGGLE FOR EXISTENCE. 141 



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- 



