Chap. III. 
MUTUAL CHECKS TO INCKEASE. 
71 
the ill effects of close interbreeding, probably come into 
play in some of these cases; but on this intricate sub¬ 
ject I will not here enlarge. 
Many cases are on record showing how complex and 
unexpected are the checks and relations between organic 
beings, which haye 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 in¬ 
sects 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. 
