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When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract,
epidemics—at
epidemics—
at
at
least, this seems generally to occur with our game
animals—often
animals—
often
often
ensue;
ensue:
and here we have a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been
disproportionally
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, &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
winter;
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;
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
nature,
such as that of very rare plants being sometimes extremely
abundant,
abundant
in the few spots where they do
exist;
occur;
and that of some social plants being social, that
is
is,
abounding in individuals, even on the extreme
verge
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
the species
from utter destruction. I should add that the good effects of
frequent
frequent
intercrossing, and