That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure 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