Chap. III.] STKUGGLE FOE EXISTENCE. 89 



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

 world insects determine the existence of cattle. Perhaps 

 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 Eengger 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 Paraguay, the parasitic insects would probably 

 increase ; and this would lessen the number of 

 the navel-frequenting 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 complexity. 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. Never- 

 theless, 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 ! 



