ure our climate, but which never become naturalised,
for they cannot compete with our native plants nor resist destruction by
our native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this
seems generally to occur with our game animals--often 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 among the crowded animals, been disproportionally 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 the 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 exist; and that of some social plants being social, that is
abounding in individuals, even on the extreme verge 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 intercrossing, and the ill effects of close
interbreeding, no doubt come into play in many of these cases; but I
will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE
STRUGGLE FOR EXISTENCE.

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 o