78 STEUGGLE FOIi EXISTENCE. Cuap. III. 



wliatever Cise liavinji^ been clone, "vvith the cxce}7tion of the 

 huid having 1)ecn enclosed, so that cattle could not enter. But 

 how important an clement enclosure is, I plainly saw near Farn- 

 ham, in Surrc}'. Plcrc there are extensive heaths, with a few 

 clumps of old Scotch firs, on the distant hill-tops : within the 

 last ten j'ears larg'e spaces have been enclosed, and self-sown 

 firs are now sprini!:iii2^ 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 

 numbcre that I went to several points of view, whence I could 

 examine hundreds of acres of the unenclosed heath, and liter- 

 ally I could not see a single Scotch fir, except the old planted 

 clumps. But on looking closely between the st^ms of the 

 heath, I found a multitude of seedlings and little trees, which 

 had been perpetually browsed down by the cattle. In one 

 square yfu'd, at a point some himdred 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 laud 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 south- 

 ward and northwai'd in a feral state ; and ^Vzara and Pengger 

 have shown that this is caused by the greater number in Para- 

 guay of a certain fly, which lays its eggs in the navels of these 

 animals when first born. The increase of these flics, numerous 

 as they are, must be habitually checked by some mean?, proba- 

 bly l)y other parasitic insects. Hence, if certain insectivorous 

 birds were to decrease in Paraguay, the parasitic insects would 

 probably increase ; and this Avould lesson the num])er of the 

 navel-frequenting flies — then cattle and horses would become 

 f(>ral, and this would certainly greatly alter (as indeed I have 

 obseiTod in parts of South America) the vegetation : this again 

 would largely aflect the insects; and this, as Ave have just seen 

 in Staffordshire, the insectivorous birds, and so onward in 

 ever-increasing circles of complexity. We began this scries 



