peds, be
let to grow, the more vigorous plants {68} gradually kill the less
vigorous, though fully grown, plants: thus out of twenty species growing on
a little plot of turf (three feet by four) nine species perished from the
other species being allowed to grow up freely.

The amount of food for each species of course gives the extreme limit to
which each can increase; but very frequently it is not the obtaining food,
but the serving as prey to other animals, which determines the average
numbers of a species. Thus, there seems to be little doubt that the stock
of partridges, grouse, and hares on any large estate depends chiefly on the
destruction of vermin. If not one head of game were shot during the next
twenty years in England, and, at the same time, if no vermin were
destroyed, there would, in all probability, be less game than at present,
although hundreds of thousands of game animals are now annually killed. On
the other hand, in some cases, as with the elephant and rhinoceros, none
are destroyed by beasts of prey: even the tiger in India most rarely dares
to attack a young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought, I believe to be
the most effective of all checks. I estimated that the winter of 1854-55
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, {69} acts directly, it will be the
least vigorous, or 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 false view: we forget that each species, even where
it most abounds, is constantly suffering enormous destruction at some
period of its life, from e