e destroyed, chiefly by slugs and
insects. If turf which has long been mown--and the case would be the
same with turf closely browsed by quadrupeds--be let to grow, the more
vigorous plants gradually kill the less vigorous though fully grown
plants; thus out of twenty species growing on a little plot of mown 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 in 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 shot. On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for 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 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 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 w