ii THE 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 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 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 



