56 THE ORIGIN OF SPECIES 



to struggle together in the same country. I will give only a single 

 instance, which, though a simple one, interested me. In Stafford- 

 shire, on the estate of a relation, where I had ample means of 

 investigation, there was a large and extremely barren heath, 

 which had never been touched by the hand of man; but several 

 hundred acres of exactly the same nature had been enclosed 

 twenty-five years previously 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 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 hilltops : 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 un- 

 enclosed 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 



