52 STRUGGLE FOR EXISTENCE. [CHAP. IIL 



viously and planted with Scotch fir. The change in the native 

 vegetation of the planted part of the heath was most remarkable, 

 more than is generally seen in passing from one quite different 

 soil to another : not only the proportional numbers of the heath - 

 plants were wholly changed, but twelve species of plants (not 

 counting grasses 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 com- 

 mon in the plantations, which were not to be seen on the heath ; 

 and the heath was frequented by two or three distinct insecti- 

 vorous 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 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 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 

 effectually 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 Kengger have shown that this is 

 caused by the greater number in Paraguay of a certain fly, Avhich 

 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, 



