56 STRUGGLE FOR EXISTENCE. [Chap. Ill 



Dative 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 

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

 neath ; 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 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 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 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 ether parasitic insects. Hence, if certain 



