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.
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