necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such as that of
very rare plants being sometimes extremely abundant in the few spots
where they do occur; and that of some social plants being social, that
is, abounding in individuals, even on the extreme confines of their
range. For in such cases, we may believe, that a plant could exist only
where the conditions of its life were so favourable that many could
exist together, and thus save each other from utter destruction. I
should add that the good effects of frequent intercrossing, and the ill
effects
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 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. 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.
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
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