nemies or from competitors for the same place and
food; and if these enemies or competitors be in the least degree favoured
by any slight change of climate, they will increase in numbers, and, as
each area is already fully stocked with inhabitants, the other species will
decrease. When we travel southward and see a species decreasing in numbers,
we may feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel northward, but
in a somewhat lesser degree, for the number of species of all kinds, and
therefore of competitors, decreases northwards; hence in going northward,
or in ascending a mountain, we far oftener meet with stunted forms, due to
the _directly_ injurious action of climate, than we do in proceeding
southwards or in descending a mountain. When we reach the Arctic regions,
or snow-capped summits, or absolute deserts, the struggle for life is
almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species, we
may clearly see in the prodigious number of plants in our gardens which 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. {70}

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 amongst the
crowded animals, been 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: but any one who has tried, knows how troublesome it is to
get seed from a few wheat or other s