Chap. III.] STRUGGLE FOR EXISTENCE. 89 



existence of the Scotch fir; but in several parts of the 

 world insects determine the existence of cattle. Per- 

 haps Paraguay offers the most curious instance of this; 

 for here neither cattle nor horses nor dogs have ever 

 run wild, though they swarm southward and northward 

 in a feral state; and 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, 

 numerous as they are, must be habitually checked by 

 some means, probably by other parasitic insects. Hence, 

 if certain insectivorous birds were to decrease in Para- 

 guay, the parasitic insects would probably increase; 

 and this would lessen the number of the navel-frequent- 

 ing flies — then cattle and horses would become feral, 

 and this would certainly greatly alter (as indeed I have 

 observed in parts of South America) the vegetation: 

 this again would largely affect the insects; and this, 

 as we have just seen in Staffordshire, the insectivorous 

 birds, and so onwards in ever-increasing circles of com- 

 plexity. Not that under nature the relations will ever 

 be as simple as this. Battle within battle must be 

 continually recurring with varying success; and yet in 

 the long-run the forces are so nicely balanced, that 

 the face of nature remains for long periods of time 

 uniform, though assuredly the merest trifle would give 

 the victory to one organic being over another. 

 Nevertheless, so profound is our ignorance, and so high 

 our presumption, that we marvel when we hear of 

 the extinction of an organic being; and as we do not 

 see the cause, we invoke cataclysms to desolate the 

 world, or invent laws on the duration of the forms of 

 life! 



