II 
TilK STRUGGLE FOR EXISTENCE 
17 
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 iu 1740 and 
the two following years the larva of a moth (Phalama 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 multi¬ 
tude 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. 1 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 en¬ 
closures 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 
1 The Earth as Modified by Human Action, p. 51. 
C 
