86 ORIGIN OF SPECIES 



the effect of the introduction of a single tree, nothing what- 

 ever else having been done, with the exception of the land 

 having been enclosed, so that cattle could not enter. 

 But how important an element enclosure is, I plainly saw 

 near Farnham, in Surrey. Here there are extensive heaths, 

 with a few clumps of old Scotch firs on the distant hill- 

 tops: within the last ten years large spaces have been en- 

 closed, 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 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 per- 

 petually 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 en- 

 closed, 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 efifectually 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 num- 

 ber 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 number of the navel-frequenting flies — 



