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 years before, and these enclosures were crowded with
young fir-trees growing too close together for all to live; and these
were not sown or planted, nothing having been done to the ground beyond
enclosing it so as to keep out cattle. On ascertaining this, Mr. Darwin
was so much surprised that he searched among the heather in the
unenclosed parts, and there he found multitudes of little trees and
seedlings which had been perpetually browsed down by the cattle. In one
square yard, at a point about a hundred yards from one of the old clumps
of firs, he counted thirty-two little trees, and one of them had
twenty-six rings of growth, showing that it had for many years tried to
raise its head above the stems of the heather and had failed. Yet this