ood, it
brings on the most severe struggle between the individuals, whether of
the same or of distinct species, which subsist on the same kind of food.
Even when climate, for instance extreme cold, acts directly, it will
be the least vigorous, or those which have got least food through the
advancing winter, which will suffer most. When we travel from south to
north, or from a damp region to a dry, we invariably see some species
gradually getting rarer and rarer, and finally disappearing; and the
change of climate being conspicuous, we are tempted to attribute the
whole effect to its direct action. But this is a very 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 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.

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 fac