Chap. III. MUTUAL CHECKS TO INCREASE. 71 



of close interbreeding, probably come into play in some 

 of these cases ; but on this intricate subject I will not 

 here enlarge. 



Many cases are on record showing how complex 

 and unexpected are the checks and relations between 

 organic beings, which have to struggle together in the 

 same country. I will give only a single instance, which, 

 though a simple one, has 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 pre- 

 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 common in the plantations, which were 

 not to be seen on the heath ; and the heath was fre- 

 quented by two or three distinct insectivorous birds. 

 Here we see how potent has been the effect of the in- 

 troduction of a single tree, nothing whatever else having 

 been done, with the exception that the land had been 

 enclosed, so that cattle could not enter. But how im- 

 portant 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. 



