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 facility of
diffusion amongst the crowded animals, been disproportionably favoured:
and here comes in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of corn
and rape-seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during winter: but any one who has tried, knows how
troublesome it is to get seed from a few wheat or other such plants in
a garden; I have in this case lost every single seed. This view of the
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