ually be remarked that the extraordinary increase of
some one species is immediately followed and checked by the
multiplication of another, yet this does not always happen; partly
because many species feed in common on the same kinds of food, and
partly because many kinds of food are often consumed indifferently by
one and the same species. In the former case, where a variety of
different animals have precisely the same taste, as, for example, when
many insectivorous birds and reptiles devour alike some particular fly
or beetle, the unusual numbers of these insects may cause only a slight
and almost imperceptible augmentation of each of these species of bird
and reptile. In the other instances, where one animal preys on others
of almost every class, as for example, where our English buzzards devour
not only small quadrupeds, as rabbits and field-mice, but also birds,
frogs, lizards, and insects, the profusion of any one of these last may
cause all such general feeders to subsist more exclusively upon the
species thus in excess, by which means the balance may be restored.

_Agency of omnivorous animals._--The number of species which are nearly
omnivorous is considerable; and although every animal has, perhaps, a
predilection for some one description of food rather than another, yet
some are not even confined to one of the great kingdoms of the organic
world. Thus, when the raccoon of the West Indies can procure neither
fowls, fish, snails, nor insects, it will attack the sugar-canes, and
devour various kinds of grain. The civets, when animal food is scarce,
maintain themselves on fruits and roots.

Numerous birds, which feed indiscriminately on insects and plants, are
perhaps more instrumental than any other of the terrestrial tribes in
preserving a constant equilibrium between the relative numbers of
different classes of animals and vegetables. If the insects become very
numerous and devour the plants, these birds will immediately derive a
larger portion of their subsistence from insects, just as the Arabians,
Syrians, and Hottentots feed on locusts, when the locusts devour their
crops.

_Reciprocal influence of aquatic and terrestrial species._--The intimate
relation of the inhabitants of the water to those of the land, and the
influence exerted by each on the relative number of species, must not be
overlooked amongst the complicated causes which determine the existence
of animals and plants in certain regions. A lar