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.

CHAPTER TWENTY-FOUR

WHAT SHALL WE DO WHEN THE COAL, OIL, AND GAS ARE GONE?

If coal, oil, and gas were suddenly taken away, all the nations would
become poor and many of their industries would cease. Just think for a
moment of the amount of work these things do for us and what an effort
there would be made to find something to take their place!

Wood once formed the chief fuel. It was used only to cook our food or to
keep us warm. Now fuel is required for so many different purposes that
with the decrease of the forests wood has been found insufficient.

Peat is one of those substances that has been used in parts of Europe to
take the place of wood, but it is used so little in our own country that
many have never se