light modifications in the physical geography of the globe may exert on
the condition of organic beings.

In the first place, it is clear that when any region is stocked with as
great a variety of animals and plants as the productive powers of that
region will enable it to support, the addition of any new species, or
the _permanent_ numerical increase of one previously established, must
always be attended either by the local extermination or the numerical
decrease of some other species.

There may undoubtedly be considerable fluctuations from year to year,
and the equilibrium may be again restored without any permanent
alteration; for, in particular seasons, a greater supply of heat,
humidity, or other causes, may augment the total quantity of vegetable
produce, in which case all the animals subsisting on vegetable food, and
others which prey on them, may multiply without any one species giving
way: but whilst the aggregate quantity of vegetable produce remains
unaltered, the progressive increase of one animal or plant implies the
decline of another.

All agriculturists and gardeners are familiar with the fact that when
weeds intrude themselves into the space appropriated to cultivated
species, the latter are starved in their growth, or stifled. If we
abandon for a short time a field or garden, a host of indigenous plants,

The darnel, hemlock, and rank fumitory,

pour in and obtain the mastery, extirpating the exotics, or putting an
end to the monopoly of some native plants.

If we inclose a park, and stock it with as many deer as the herbage will
support, we cannot add sheep without lessening the number of the deer;
nor can other herbivorous species be subsequently introduced, unless the
individuals of each species in the park become fewer in proportion.

So, if there be an island where leopards are the only beasts of prey,
and the lion, tiger, and hyaena afterwards enter, the leopards, if they
stand their ground, will be reduced in number. If the locusts then
arrive and swarm greatly, they may deprive a large number of
plant-eating animals of their food, and thereby cause a famine, not only
among them, but among the beasts of prey: certain species perhaps, which
had the weakest footing in the island, may thus be annihilated.

We have seen how many distinct geographical provinces there are of
aquatic and terrestrial species, and how great are the powers of
migration conferred on different classes, whereb