ill touch the
nettle, but fifty different kinds of insects are fed by it.[950] Some of
these seize upon the root, others upon the stem; some eat the leaves,
others devour the seeds and flowers; but for this multitude of enemies,
the nettle (_Urtica dioica_), which is now found in all the four
quarters of the globe, would annihilate a great number of plants.
Linnaeus tells us, in his "Tour in Scania," that goats were turned into
an island which abounded with the _Agrostis arundinacea_, where they
perished by famine; but horses which followed them grew fat on the same
plant. The goat, also, he says, thrives on the meadow-sweet and
water-hemlock, plants which are injurious to cattle.[951]

_Agency of insects._--Every plant, observes Wilcke, has its proper
insect allotted to it to curb its luxuriancy, and to prevent it from
multiplying to the exclusion of others. "Thus grass in meadows sometimes
flourishes so as to exclude all other plants; here the Phalaena graminis
(_Bombyx gram._), with her numerous progeny, finds a well-spread table;
they multiply in immense numbers, and the farmer, for some years,
laments the failure of his crop; but the grass being consumed, the moths
die with hunger, or remove to another place. Now the quantity of grass
being greatly diminished, the other plants, which were before choked by
it, spring up, and the ground becomes variegated with a multitude of
different species of flowers. Had not nature given a commission to this
minister for that purpose, the grass would destroy a great number of
species of vegetables, of which the equilibrium is now kept up."[952]

In the above passage allusion is made to the ravages committed in 1740,
and the two following years, in many provinces of Sweden, by a most
destructive insect. The same moth is said never to touch the foxtail
grass, so that it may be classed as a most active ally and benefactor of
that species, and as peculiarly instrumental in preserving it in its
present abundance.[953] A discovery of Rolander, cited in the treatise
of Wilcke above mentioned, affords a good illustration of the checks and
counter-checks which nature has appointed to preserve the balance of
power among species. "The _Phalaena strobilella_ has the fir cone
assigned to it to deposit its eggs upon; the young caterpillars coming
out of the shell consume the cone and superfluous seed; but, lest the
destruction should be too general, the _Ichneumon strobilellae_ lays its
eggs