be clothed with vegetation before animals could be introduced.  A field-mouse dies and decays, and its elements are appropriated by the roots around its grave; and we can easily imagine the next generations of mice, the children and grandchildren of the deceased rodent, feasting off the tender bark which was made out of the remains of their parent.  The soil of our gardens and the atmosphere above it are full of potential tomatoes, beans, corn, potatoes, and cabbages,—­even of peaches of the finest flavor, and grapes whose aroma is transporting.

Plants, as well as animals, have their peculiar tastes.  Cut off the supply of phosphate of lime from a field of corn, and it will not grow.  You can easily do this by planting the same land with corn for three or four successive years, and your crop will dwindle away to nothing, unless you supply the ground every year with as much of the mineral as the corn takes away from it.  All plants have the power of selecting from the soil the materials necessary to their growth; and if they do not find them in the soil, they will not grow.  It is now a familiar fact, that, when an old forest of deciduous trees has been felled, evergreens will spring up in their places.  The old oaks, hickories, and beeches, as any observer would discover, pass their last years in repose, simply putting out their leaves and bearing a little fruit every year, but making hardly any new wood.  An oak may attain to nearly its full size, in spread of branches, in its first two hundred years, and live for five or six hundred years longer in a state of comparative rest.  It seems to grow no more, simply because it has exhausted too much of the material for its nourishment from the ground around its roots.  At least, we know, that, when we have cut it down, not oaks, but pines, will germinate in the same soil,—­pines, which, having other necessities and taking somewhat different food, find a supply in the ground, untouched by their predecessor.  Hence the rotation of crops, so much talked of by agriculturists.  Before the subject was so well understood, the ground was allowed to lie fallow for a year or two, when the crops began to grow small, that it might recover from the air the elements it had lost.  We now adopt the principle of rotation, and plant beans this year where last year we put corn.

It is not merely that plants deprive themselves of their future support by exhausting the neighboring earth of the elements they require.  Some of them put into the ground substances which are poisonous to themselves or other plants.  Thus, beans and peas pour out from their roots a very notable amount of a certain gum which is not at all suited to their own nourishment,—­so that, if we plant beans in the same spot several successive seasons, they thrive very poorly.  But this gum appears to be exactly the food for corn; if, therefore, we raise crops of beans and corn alternately, they assist each other.  Liebig gives the results of a series of