ility of
diffusion amongst the crowded animals, been disproportionably favoured:
and here comes in a sort of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during winter: but any one who has tried, knows how
troublesome it is to get seed from a few wheat or other such 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 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