STRUGGLE FOR EXISTENCE 1477 



the stems of the heath, I found a multitude of seed- 

 lings and little trees which had been perpetually 

 browsed down by the cattle. In one square yard, 

 at a point some hundred yards distant from one of 

 the old clumps, I counted thirty-two little 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 inclosed, it became 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 Reng- 

 ger 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 in- 

 sectivorous 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 



