METHODS AND PRACTICE 153 



marked by special characters, and at high altitudes, pure 

 woods, or Avoods consisting of a few species only, are 

 frequently met with. The broad-leaved forests of the 

 lowlands are, for instance, usually mixed ; those of the 

 mountains and peat bogs pure; but, of course, local 

 variations occur in all natural forests which give a mixed 

 character to large areas wherever they occur. 



When woods came to be artificially raised, foresters 

 naturally confined themselves to the species best fitted to 

 the soil, or best calculated to yield a profit when grown. 

 Pure crops are easily managed, give a high yield per acre, 

 and enable demands for special kinds of timber to be met. 

 But many distinguished foresters believe that insect and 

 fungoid pests are able to commit far greater damage in 

 the pure Avoods of modern forestry than was the case a 

 century or two ago. Caterpillars or larvae which spread 

 from tree to tree in search of food find life much easier 

 when the crowns of the trees are interlaced than when 

 they are isolated or separated by species which do not 

 serve their purpose. Parasitic fungi find a greater 

 number of fit individuals for infection amongst the 

 suppressed trees of a pure wood than when these sup- 

 pressed trees are absent, or consist of another and immune 

 species. The soil, again, may possibly get sick of a species 

 grown on it century after century, although it cannot be 

 said that much evidence of this is as yet forthcoming. 

 With light-demanding species, again, the ground becomes 

 so dirty, and covered with such injurious forms of vegeta- 

 tion as heath, bracken, wood rush, etc., that eventually pure 

 crops lead to the deterioration of the soil. For these and 

 other reasons mixed crops are probably more in harmony 

 with nature than pure woods over wide areas. 



But to produce satisfactory financial results the mix- 

 tures adopted must be formed on correct principles. 

 The orthodox mixture in most cases is that of a shade- 



