uch plants in a garden: I have in this
case lost every single seed. This view of the 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 the species from utter
destruction. I should add that the good effects of frequent intercrossing,
and {71} 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
encl