STRUGGLE FOR EXISTENCE 111 



is, I plainly saw near Farnham, in Surrey. Here there 

 are extensive heaths, with a few clumps of old Scotch 

 firs on the distant hilltops: within the last ten years 

 large spaces have been inclosed, and self-sown firs are 

 now springing up in multitudes, so close together that 

 all cannot live. When I ascertained that these young 

 trees had not been sown or planted, I was so much sur- 

 prised at their numbers that I 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 look- 

 ing 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, dur- 

 ing 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 exist- 

 ence of the Scotch fir; but in several parts of the world 

 insects determine the existence of cattle. Perhaps Para- 

 guay 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 





