l in her way and never makes the soil poor by
growing more plants than it can support. In her own gardens she always
renews the foods in the soil which the plants require as fast as they
take them away.

The needs of men have increased so fast that the soil has often been
forced to grow more than it ought. Men have been a long time in learning
that they cannot keep on growing the same crops on the same soil year
after year without supplying to the soil extra foods, or _fertilizers_,
as we call them. The care of the soil is another thing to which we have
to give attention, but which did not worry our ancestors.

Nature clothes the earth with a carpet of grasses, bushes, or trees.
When the rain falls on the ground, their roots hold the soil so firmly
that it usually washes away only very slowly. When men first began to
cultivate the soil, they paid no attention to the fact that water washes
away the loose earth very easily. In this loose earth at the top of the
ground is stored most of the food which the plants require. Care of the
surface of the ground is, then, another thing which we have to keep in
mind.

Men at first made shelters for themselves from anything that was at
hand, such as bark, skins, rock, or earth. When they learned to make
sharp-edged tools, they began to use trees. Where it is cold, much wood
is required to build warm houses. As the numbers of men increased, they
used greater and greater quantities of wood. Wood also proved to be most
useful for many other purposes than house building. In order to plant
larger fields the trees were cut down or burned off, without thought of
doing any harm. In time trees became scarce in many parts of the world
and men began to realize that care must be used or the supply of wood
might fail them.

Coal was finally discovered and men said, "Now we have something that
will last always, for there must be an inexhaustible amount in the earth
beneath our feet. All that we shall have to do is to dig it out." When
men grew wiser they learned that coal must not be used carelessly any
more than the other gifts of Nature; otherwise the supply may give out
and leave them with nothing to take its place.

Hunting and fishing continued to be the business of many. They invented
destructive weapons with which they were able to kill such large numbers
of wild creatures that some kinds disappeared entirely. Fish, also, of
which people thought the sea and the rivers contained a neve