a species, and periodical seasons of extreme cold or drought seem to be
the most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5
destroyed 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.

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 end