rf three feet by four, twenty distinct species of
plants were found to be growing, and no less than nine of these perished
altogether when the other species were allowed to grow up to their full
size.[5]

But besides having to protect themselves against competing plants and
against destructive animals, there is a yet deadlier enemy in the
forces of inorganic nature. Each species can sustain a certain amount of
heat and cold, each requires a certain amount of moisture at the right
season, each wants a proper amount of light or of direct sunshine, each
needs certain elements in the soil; the failure of a due proportion in
these inorganic conditions causes weakness, and thus leads to speedy
death. The struggle for existence in plants is, therefore, threefold in
character and infinite in complexity, and the result is seen in their
curiously irregular distribution over the face of the earth. Not only
has each country its distinct plants, but every valley, every hillside,
almost every hedgerow, has a different set of plants from its adjacent
valley, hillside, or hedgerow--if not always different in the actual
species yet very different in comparative abundance, some which are rare
in the one being common in the other. Hence it happens that slight
changes of conditions often produce great changes in the flora of a
country. Thus in 1740 and the two following years the larva of a moth
(Phalaena graminis) committed such destruction in many of the meadows of
Sweden that the grass was greatly diminished in quantity, and many
plants which were before choked by the grass sprang up, and the ground
became variegated with a multitude of different species of flowers. The
introduction of goats into the island of St. Helena led to the entire
destruction of the native forests, consisting of about a hundred
distinct species of trees and shrubs, the young plants being devoured by
the goats as fast as they grew up. The camel is a still greater enemy to
woody vegetation than the goat, and Mr. Marsh believes that forests
would soon cover considerable tracts of the Arabian and African deserts
if the goat and the camel were removed from them.[6] Even in many parts
of our own country the existence of trees is dependent on the absence of
cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in
Surrey, a few clumps of old Scotch firs, but no young trees over
hundreds of acres. Some portions of the heath had, however, been
enclosed a few