THE STRUGGLE FOR EXISTENCE. 



141 



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 multiplica- 

 tion 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 sur- 

 rounding conditions, and of the alternation of the sea- 

 sons, to which the body must accommodate itself. Or- 

 ganisms 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 de- 

 pendence of living beings are, however, the best and most 

 conclusive; but the vast consequences depending on cir- 

 cumstances and connections apparently insignificant, and 

 the extreme complexity of the mechanism by which equi- 

 librium 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- 



