nimals, the latter have, in addition to the struggle for their food, to
watch constantly for their lives. Every organism is in one sense the
enemy of every other one. We do not mean that they often try to kill
each other because of hate, as men do, but that they are after food to
satisfy their hunger. Some of the higher animals as well as men fight
for mastery, in addition to struggling for food. We hope that among men
the unnecessary fighting will sometime cease, and that kindness and
unselfishness will rule.

The struggle for life is ceaselessly going on around us, but so quiet is
it that we are not often aware of the countless tragedies that take
place. This struggle extends from the plants and animals in the pond, so
small that we cannot see them with the unaided eye, upward through all
the larger animals.

The struggle among all living things helps us to understand the
necessity for Nature's prodigality. If the plants and animals that serve
as food for others were not produced in great numbers, they would soon
become extinct. It is seldom that any one kind of plant or animal,
because of its many enemies, has an opportunity to spread and obtain
more than its share of food and sunshine. According to Nature's
arrangements, each organism does its share in keeping down the numbers
of the others. This we call the "balance of Nature."

Sometimes the balance of Nature is disturbed and one particular kind of
animal gets the start of its enemies and increases until it becomes a
_plague_. This may be caused by a favorable season or by the decrease of
its enemies on account of disease among them. We have read of the
plagues of grasshoppers which have sometimes visited the Western states
and eaten up every green thing. Plagues of rats and field mice have been
known to do a great deal of damage. In such cases their natural enemies,
the hawks, owls, and coyotes, may be attracted to the region from far
around, because of the extra food supply. After a time they may succeed
in reducing the numbers of these pests.

This balance among the animals, which comes from one living upon
another, is a strange and wonderful thing. No one kind can long overrun
its fellows. If one does get a start and increases until it becomes a
pest or plague, some enemy is sure sooner or later to spring up to
destroy it. We use this method in fighting some of the insect pests
which are injuring our trees. Men have searched in various parts of the
world f