STRUQaLE FOR EXISTENOB. 67 



went to several points of view, whence I could examine 

 hundreds of acres of the uninclosed 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, with twenty-six rings of growth, 

 had, during many years tried to raise its head above the 

 stems ot- 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 effec- 

 tually 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 other parasitic insects. Hence, if certain insectivorous 

 birds were to decrease in Paraguay, the parasitic insects 

 would probably increase; and this would lessen the num- 

 ber of the navel-frequenting flies — then cattle and horses 

 wonld 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 jnst seen in Staffordshire, the insectivor- 

 ous birds, and so onward 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 



