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 Staffordshire, 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 that the land had been enclosed, so that cattle
could not enter. But how important an element enclosure is, I plainly
saw near Farnham, in Surrey.
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