STRUGGLE FOR EXISTENCE. *>3 



and carices) flourished in the plantations, which could not 

 be found on the heath. The effect on the insects must 

 have been still greater, for six insectivorous birds were very 

 common in the plantations, which were not to be seen on 

 the heath ; and the heath was frequented by two or three 

 distinct insectivorous birds. Here we see how potent has 

 been the effect of the introduction of a single tree, nothing 

 whatever 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 

 enclosed, and self-sown firs are now springing up in multi- 

 tudes, 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 steins 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 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 effect- 

 ually 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 

 gome means, probably by other parasitic insects. Hence, if 



