Nature seems very prodigal in her ways. She is continually creating on the earth a great multitude of living things, far more than there is room for. Each one of these, if it would live, must have a certain amount of air, sunshine, and food. As there is not enough of these things to supply every one, there arises a struggle. Those that are weakest die, because they are not able to get what they need. To us this seems hard, but it is Nature's way.

And further, since many of the animals feed on the flesh of other animals, 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 from which such pests as the gypsy moth and the San JosÉ scale have come to find some of their enemies and bring them to this country to feed on these insects.

When men came upon the earth, they soon began to upset Nature's arrangements, and from that time until now matters of this kind have been growing worse. We have killed large numbers of the beneficial animals and birds that kept the harmful ones in check. We have carried others from the homes given them by Nature, where they were doing little harm, to new homes where they have become terrible plagues.

The killing of large numbers of hawks and owls, all the species of which many people have wrongfully thought to be harmful, has been followed by a great increase in the numbers of rats and mice. We have killed off most of the coyotes, the chief food of which was rabbits and ground squirrels. The two latter animals have now become a serious pest. They do enormous damage to the crops, and we spend thousands of dollars fighting them.

The common rabbit has in most parts of its native country so many enemies which are always on the lookout for a good meal, that it cannot increase enough to do much harm. Years ago a number of rabbits were taken to Australia, where there were none. Here they found a favorable climate and few enemies. They have now increased so that they overrun much of the continent and are a terrible pest which the farmers are unable to control.

Some years ago the gypsy moth and the browntail moth were introduced by accident into the New England states. Finding there a congenial climate and few enemies, they increased rapidly. They soon began to strip the leaves from the beautiful elms which make the streets and parks of this region so attractive. Now these moths have turned their attention to the white pine and are doing an ever-increasing amount of damage; and although they are being fought by every means in our power, we are not certain that we can ever control them.

The codling moth, whose larva is the little apple worm, causes an immense loss in our fruit orchards. The cotton-boll weevil, which destroys so much of the cotton, is, like the codling moth, an insect imported from another country. The San JosÉ scale reached California from China and has now spread throughout our country. It has a special fondness for the sap of fruit trees, and, being so small, was not noticed until it had got beyond control. This scale causes more loss than any other of the tree insects.

The Hessian fly, introduced from Europe more than one hundred years ago, causes during certain seasons a very great loss to the wheat crop. The Argentine ant has been brought to us from South America and is proving a most destructive pest. The Norway rat was brought to our country on sailing vessels and causes more loss than most of us realize. The English sparrow has spread over much of the country and is driving many of the native birds from their homes, because of its quarrelsome disposition. It makes itself a nuisance on all our city streets.

The mongoose, in its home in India, is a great rat killer, but does not there increase so as to do much harm. Wherever it has been carried for the purpose of using it as a rat killer, this little four-footed animal has become a terrible scourge. After it destroys the rats it goes after the snakes. Then it attacks the other small animals and birds. Finally it begins upon the chickens, and even the vegetables in the garden are not safe from its voracious appetite.

Men are now watching at every port to see that no more dangerous insects and animals are brought into the country. They are particularly on the watch for the Mediterranean fruit fly and for the mongoose.

When we upset the balance of Nature, we start a whole chain of troubles. What can we do to escape the consequences of our ignorance and carelessness? In the first place we can protect the birds, for they eat enormous quantities of the harmful insects. In the second place we can see that no more of these dangerous pests are allowed to land on our shores. In the third place we shall have to fight, by every means that we can discover, those that are already here.