Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or
drought, drought, 1859 1860 | drought 1861 1866 1869 1872 |
I believe I believe 1859 1860 | seem 1861 1866 1869 1872 |
to be the most effective of all checks. I estimated
that that 1859 1860 |
(chiefly from the greatly reduced numbers of nests in the spring) that 1861 1866 1869 1872 |
the winter of
1854-55 1854-55 1859 1860 1861 1866 1869 | 1854-5 1872 |
destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per
cent. cent. 1859 1860 1866 | cent, 1861 | cent 1869 1872 |
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, vigorous, 1859 1860 1861 1866 1869 | vigorous 1872 |
or or 1859 1860 1861 1866 1869 | individuals, or 1872 |
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 very 1859 | very 1860 1861 1866 1869 1872 |
false
view: view: 1859 1860 1861 1866 1869 | view; 1872 |
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, numbers, 1859 1860 1861 1866 1869 | numbers; 1872 |
and, and, 1859 1860 1861 1866 1869 | and 1872 |
as each area is already fully stocked with inhabitants, the other species
will will 1859 1860 1861 1866 1869 | must 1872 |
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, northward, 1859 1860 1861 1866 1869 | northwards, 1872 |
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. |