e, and firs
prepare the soil for oaks. Every agriculturist feels the force of this
law of the organic world, and regulates accordingly the rotation of his
crops.

_Equilibrium in the number of species, how preserved._--"All the plants
of a given country," says De Candolle, in his usual spirited style, "are
at war one with another. The first which establish themselves by chance
in a particular spot tend, by the mere occupancy of space, to exclude
other species--the greater choke the smaller; the longest livers replace
those which last for a shorter period; the more prolific gradually make
themselves masters of the ground, which species multiplying more slowly
would otherwise fill."

In this continual strife it is not always the resources of the plant
itself which enable it to maintain or extend its ground. Its success
depends, in a great measure, on the number of its foes or allies among
the animals and plants inhabiting the same region. Thus, for example, a
herb which loves the shade may multiply, if some tree with spreading
boughs and dense foliage flourish in the neighborhood. Another, which,
if unassisted, would be overpowered by the rank growth of some hardy
competitor, is secure because its leaves are unpalatable to cattle;
which, on the other hand, annually crop down its antagonist, and rarely
suffer it to ripen its seed.

Oftentimes we see some herb which has flowered in the midst of a thorny
shrub, when all the other individuals of the same species, in the open
fields around, are eaten down, and cannot bring their seed to maturity.
In this case, the shrub has lent his armor of spines and prickles to
protect the defenceless herb against the mouths of the cattle, and thus
a few individuals which occupied, perhaps, the most unfavorable station
in regard to exposure, soil, and other circumstances, may, nevertheless,
by the aid of an ally, become the principal source whereby the winds are
supplied with seeds which perpetuate the species throughout the
surrounding tract. Thus, in the New Forest in Hampshire, the young oaks
which are not consumed by the deer, or uprooted by the swine, are
indebted to the holly for their escape.

In the above examples we see one plant shielding another from the
attacks of animals; but instances are, perhaps, still more numerous,
where some animal defends a plant against the enmity of some other
subject of the vegetable kingdom.

Scarcely any beast, observes a Swedish naturalist, w