| 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, &c., 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
but any one who has tried, knows how troublesome it is to get seed from a few wheat or other such plants in a
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
such as that of very rare plants being sometimes extremely
in the few spots where they do
and that of some social plants being social, that
abounding in individuals, even on the extreme
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
from utter destruction. I should add that the good effects of
intercrossing, and the ill effects of close interbreeding,
come into play in
of these cases; but
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I will not here
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| 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,
interested me. In Staffordshire, on the estate of a
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
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