four-fifths of the birds in my own grounds; and this is a tremendous
destruction, when we remember that ten per cent. is an extraordinarily
severe mortality from epidemics with man. The action of climate seems at
first sight to be quite independent of the struggle for existence; but in
so far as climate chiefly acts in reducing food, 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
individuals, or those which have got least food through the advancing
winter, which will suffer the 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 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 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 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 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.