72 MUTUAL CHECKS TO INCREASE. Chap. III. 



When I ascertained that these young trees had not 

 been sown or planted, I was so much surprised at their 

 numbers that I went to several points of view, whence 

 I could examine hundreds of acres of the unenclosed 

 heath, and literally I could not see a single Scotch fir, 

 except the old planted clumps. But on looking closely 

 between the stems of the heath, I found a multitude of 

 seedlings 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, 

 judging from the rings of growth, had during twenty-six 

 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 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 effectu- 

 ally 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 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 birds. Hence, if certain in- 

 sectivorous birds (whose numbers are probably 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 



