hole effect to its direct action. But this
is a false view; we forget that each species, even where it most
abounds, is constantly suffering enormous destruction at some period of
its life, from enemies or from competitors for the same place and food;
and if these enemies or competitors be in the least degree favored by
any slight change of climate, they will increase in numbers; and as each
area is already fully stocked with inhabitants, the other species must
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 favored 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 northward; 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 southward 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 favoring 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 naturalized,
for they cannot compete with our native plants nor resist destruction by
our native animals.

When a species, owing to highly favorable 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 disproportionally favored:
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 num