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 facility
of diffusion amongst the crowded animals, been disproportionably
favoured: and here comes in a sort of struggle between the parasite
and its prey.
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