STRUGGLE FOR EXISTENCE 67 



trees ; and one of them, with twenty-six rings of 

 growth, had during many years tried to raise its head 

 above the stems of the heath, and had failed. No 

 wonder that, as soon as the land was enclosed, it be- 

 came thickly clothed with vigorously growing young firs. 

 Yet the heath was so extremely barren and so extensive 

 that no one would ever have imagined that cattle would 

 have so closely and effectually searched it for food. 



Here we see that cattle absolutely determine the 

 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 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 birds. Hence, 

 if certain insectivorous birds (whose numbers are prob- 

 ably regulated by hawks or beasts of prey) were to 

 increase in Paraguay, the flies would decrease — 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 just 

 have seen in Staffordshire, the insectivorous birds, and 

 so onwards in ever-increasing circles of complexity. 

 We began this series by insectivorous birds, and we 

 have ended with them. Not that in nature the rela- 

 tions can ever be as simple as this. Battle within 

 battle must ever be recurring with varying success ; 

 and yet in the long-run the forces are so nicely 

 balanced, that the face of nature remains uniform for 

 long periods of time, though assuredly the merest trifle 

 would often 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 



